Geological Contributions to the History of the Artist’s Iron-Based Natural Earth Pigments and the Case Study of Terra d’Ombra (Umber)

Abstract

The correct identification of historical artists’ earth pigments is mandatory for cultural, scholarly, and historical applications. This paper focuses on the definition of the distinctive mineralogical, geological, and geochemical properties and the discussion of the geological genesis and place of origin of the natural Fe-Mn-based earth pigment named terra d’ombra (umber). It one of the dark-brown earth pigment most widely used by Italian and European painters from the Renaissance to the nineteenth century. The terra d’ombra earth pigment is a primary chemical sediment mainly composed of Fe (oxy)hydroxide and Mn oxide, produced by the authigenic precipitation from oceanic or lacustrine waters rich in metal solutes of volcanic hydrothermal origin. The principal areas of provenance are the island of Cyprus and the Monte Amiata volcano (southern Tuscany, Italy). Its peculiar properties in painting derive from this specific mineralogical composition and genetic process, which also exclude its definition as a particular type of ochre and as a clay pigment. Further misinterpretations include confusion with pigments composed of organic materials and the erroneous attribution of the name and area of origin to the Italian region of Umbria.

1. Introduction

The Latin term terra (earth) was widely used—until the advent of modern mineralogy and petrography—to indicate all those natural mineral materials that show fine grain size, crumbly consistency, poorly coherent texture compared to a rock, and disintegrate in water. The natural “earth” materials have been widely employed, since ancient times, in cultural and artistic expressions [1], and as medical and magic remedies [2,3,4].

A “pigment” (from the Latin verb pingere = to paint) is a substance used to change the color of a material [5]. Pigments are distinguished from dyes [6] (1, p. 3); [7] (pp. 5–6); [8] (p. 74) because they are insoluble in common solvents (such as water), in the medium (or vehicle) in which they are dispersed, and in the surface on which they are applied [9].

A “natural earth pigment” is a natural mineral material with an earthy texture, used for coloring.

The definition of the distinctive mineralogical, geological, and chemical properties and the recognition of the geological genesis and place of origin of a natural earth pigment are essential to detect the specific descriptors needed for its identification in cultural, scholarly, and historical applications.

The focus of this paper is on historical artists’ iron-based natural earth pigments—such as terra di Siena (sienna), ocher, bolar earth, and, in more detail, terra d’ombra (umber). In the first part, the issue of the nomenclature of iron-based natural earth pigments and a brief history of these materials during the pre-scientific era are addressed. In the second part, the composition and application of the pigment terra d’ombra (umber) are discussed and its places of origin are geologically contextualized, with particular regard to Italian sources.

Historical information on raw mineral materials is dispersed in different types of textual documents, because their study and application have been performed over time in different disciplines. For the purpose of this paper, several reference sources have been combined and compared. They consist of (i) art history treatises, manuals on painting technique, and on the preparation and use of colors; (ii) ancient lapidaries, lists of collections, and catalogs of private museums; (iii) treatises on medicine and pharmacopeias; (iv) treatises of mineralogy with theories on the origin and classification of earths; and (v) modern chemical and physical analyses of pigments in situ and on art works. Moreover, the discussed data and interpretations have the support of the original geological observations carried out on the middle Pleistocene Monte Amiata volcano (southern Tuscany, Italy); its mineral resources comprise some historical natural earth pigments such as terra di Siena and terra d’ombra [10,11,12,13,14,15].

2. Geo-Mineralogical Background and Nomenclature

Natural earth pigments are more valuable, the higher the concentration of the coloring agent. In most cases, the coloring agent is diluted with other components, reflecting its geological source and genesis. This fact affects the final color and physical–chemical characteristics of the earth pigment. Therefore, to produce a pure earth pigment, it is necessary to mechanically and chemically treat the natural raw material to decrease its grain size, and separate and remove the impurities. The most common and earliest-used natural earth pigments are those in which the coloring agent is composed of iron oxides and hydroxides that produce a spectrum of colors including red, orange, yellow, and brown.

The main minerals of iron-based natural earth pigments are limonite, goethite, and hematite [16,17]. Limonite is an (oxy)hydroxide of Fe³⁺, which is represented by the non-definitive chemical formula FeO(OH) · nH₂O. It lacks a crystal structure and is found in amorphous, earthy aggregates of yellow-to-brown color. Goethite is an (oxy)hydroxide of Fe³⁺ with the chemical formula α-FeO(OH), showing an acicular crystal habit forming radial aggregates in typically yellow–brown massive encrustations. Hematite is an Fe³⁺ oxide with the chemical formula α-Fe₂O₃. It can be found in reddish earthy masses or in dark-gray crystals with iridescent metallic luster. The grinding of crystalline hematite results in a brilliant red powder. The variable amounts of other mineral components—such as manganese, arsenic, aluminum silicates, gypsum, silica, barite, and carbonate—produce various types of natural iron-based earth pigments.

The heating process of iron-based natural earth pigments, called calcination, leads to a change in color from natural yellow and brown to orange, red, and brown–red, due to de-hydroxylation of the iron (oxy)hydroxides, which are transformed from limonite/goethite into hematite at temperatures of about 270–300 °C [18,19,20]. This thermal transformation is a complex process that depends on the composition of the starting raw material and on the physical–chemical ambient conditions present during the transformation, such as the temperature to which the earth is calcined [19]. The resulting product has the same color and chemical formula Fe₂O₃ of hematite but shows a disordered nano-crystalline structure which makes it distinct from naturally occurring hematite. The crystalline structure of hematite can be reached with a temperature exceeding 800–900 °C [18,19,20].

The principal artists’ iron-based natural earth pigments are ocher (or ochre; yellow), terra di Siena (sienna; light yellow to yellow–brown), terra d’ombra (umber; dark-brown), and bole (bolus, bolar earth; yellow–red). From the technical–scientific point of view, each of them represents a natural material that is not distinguished only by its color but, above all, by its peculiar mineralogical structure, chemical composition [21,22], and, in some cases, exclusive geological genesis and place of occurrence.

In disciplines other than mineralogy and geology, and with the advent of manufactured pigments, the intrinsic meaning and physical-chemical characteristics of historical natural earth pigments have been dismissed in favor of their identification only with the name of color perception. To clarify the nomenclature and definition of these iron-based natural earth pigments, in the following, their main characteristics and the related concepts adopted in this work are described [12]

“Ocher” (ochre, oker, ocra, ochria, ocur, ocre) is an earth pigment with hues ranging from yellow–gold to pale red. It is one of the oldest pigments known and is found worldwide. From a mineralogical and geochemical point of view, it is poorly defined because its composition is a natural complex mixture of different mineral phases and varies greatly according to the geographical area and geological context of provenance. The coloring agents are iron (oxy)hydroxides—mainly limonite/goethite, possibly with minor amounts of hematite in reddish samples [23]. The proportion of goethite to hematite is the major factor influencing the color of ochers. The coloring minerals, however, represent only the minority part of ochers (generally <10%, maximum 25% in weight, wt%; [21]) into host phases such as carbonate, siliceous sand, and aluminum silicate (clay), and with accessory minerals such as gypsum, barite, feldspar, micas, and organic matter. However, although it is fine-grained and may contain aluminum silicates, “ocher” cannot be mineralogically defined as a clay, such as in [9,24,25]. Overall, primary ocher deposits are genetically associated with iron ores, of which they constitute the in situ weathered facies. Secondary ocher deposits are constituted by loose unconsolidated and semi-consolidated sediments enriched in iron minerals (placer ores) that were transformed into (oxy)hydroxides by weathering [26].

“Red ocher” (red earth, ocre rouge) is a red pigment composed almost exclusively of the earthy facies of the mineral hematite or derived directly from the grinding of hematite crystals. It should not be confused with reddish ocher, containing a small amount of hematite and with red pigments made by the calcination of ocher, sienna, and umber [27]. Also, red ochre is one of the oldest pigments known and is found in numerous countries. Miltos, sinopia, amatisto, sanguigna, and rubrica (Pliny, 35.31 and 35.33; [28]; Dioscorides, 5.111, [29]) were the names of red ochres used in ancient times. Amatisto or lapis amatito, from which it derives the Italian word matita (pencil), is a distortion of the name hematite, which in turn means blood-like, from the Greek aima, aimatos (blood), because of its bright red color. It is one of the oldest drawing instruments, composed of pulverized hematite that was reduced to sticks and appropriately pointed to form a pencil [30] (pp. 36, 40); [31,32].

“Terra di Siena” (terra gialla, Sienna, earth of Sienna, raw sienna, terre de Sienne, Sienne naturelle) is a pigment of pale yellow to pale brown color, darker and brighter than that of ocher. Due to its genesis from authigenic chemical precipitate in fresh waters [12], the original Terra di Siena is composed almost exclusively of limonite/goethite (65–85%) and <5% manganese oxide [21], with very small particle size [19]. It may contain a minority proportion of aluminum silicates [21] but is not a clay (such as it was defined in [9,24]). It is often also incorrectly referred to as a natural variety of yellow ochre. It was named as terra gialla (yellow earth) by the Romans and remained as such until the Italian Renaissance. The name “terra di Siena” comes from the Italian city of Siena in whose territory (Monte Amiata volcano) it was exclusively produced from the fifteenth century [30] (p. 38) until the 1970s [12].

“Terra di Siena bruciata” (burnt sienna, terre de Sienne calcinée) is a reddish-brown pigment that is obtained by heating and dehydrating the natural sienna. The disordered hematite structure of the burnt sienna is distinguishable from the crystalline hematite of natural red ocher.

“Bole” (terra bolare, bolar earth, bolus) is an iron-based natural pigment with a different composition both from sienna, as it consists mainly of kaolin clay with the addition of metal oxides, and from ocher, because it does not contain carbonates. The color varies from yellow to red and green, being more or less intense depending on the quantity and type of metal oxides involved in its composition. It is a pigment that is typically greasy to the touch because it is mainly in the colloidal state, is rich in aluminum oxide, and has astringent properties. Overall, bolar earths were used in antiquity in medical practices, such as the Armenian bole and Lemnian earth [33]. In the fine arts, bole was particularly suited to the preparation of mordant and substrate for water gilding [34]. The water gilding technique was described in Cennini’s treatises [30]. The term “bole” is applied to both the natural mineral material (clay with metal oxides), and the mixture prepared for the ground layer of the water gilding. In this latter case, following local technological approaches, raw material availability, and type of support of gilding, the natural mineral bole was combined with protein-based binders (i.e., animal glues, egg white, and mixed with water) and other natural (i.e., graphite and gypsum) or artificially prepared (i.e., minium Pb₃O₄) pigments. In early gilding artworks (thirteenth and fourteenth century), bole substrate was absent, the metal leaf was applied onto a well-polished grounding composed of crystalline non-clay admixtures such as chalk, gypsum, lead-based pigments (minium), and natural earth pigments (yellow ochre and dark-brown umber). Between the end of the fourteenth and the eighteenth century (i.e., late Medieval and Baroque periods) was the time of extensive usage of bole in painting and of the research and exploitation of new important sources of raw mineral materials. During the same period, following the requests of the time, the fame of Monte Amiata (Italy) earth pigments (named bole and bolar earth) has grown (see Section 6).

“Terra d’ombra” (umber) is composed of iron (oxy)hydroxides with an important amount of manganese oxides which give it a dark-brown color. This pigment is described in detail in the second part of the paper (see Section 4).

3. Facts, Information, and Skills in the Pre-Scientific Era

3.1. Textual Documents on Sumerian Knowledge on Iron-Based Earths and Minerals

The first available textual documents that express the concept of earthy mineral materials distinguished from coherent stones and record the use of iron-based earth pigments, are Sumerian words of the third millennium BC. These texts were found in multilingual cuneiform clay tablets of seventh century BC—forming part of the so-called Royal Library of Ashurbanipal (668–626 BC) at Nineveh. In these tablets, Assyrians compiled extensive lists of signs and vocabularies of older Sumerian and Akkadian words; among them there are natural history lists of animals, vegetables, and minerals. Their translation and comment by the British archeologist Reginald Campbell Thompson (1876–1941) [35] highlighted that Sumerians, since the third millennium BC, had an advanced knowledge and elaborated a complex nomenclature of mineral materials, based on color, consistency, thermal processes, geological source, and geographical provenance.

Firstly, Sumerians distinguished stones from earthy materials. Among the earthy materials, they distinguished those of secondary (i.e., sedimentary) origin forming the Mesopotamian alluvial plains (i.e., the valley floor and delta of Tigris and Euphrates rivers; [36,37]), from those linked primarily to metal ores and coming from the mountains (i.e., Zagros Mountains and Anatolia Plateau) (Figure 1).

The earths of sedimentary origin are further differentiated—on the basis of their properties of agglutination and coherence—in SAḪAR and IM (translated in the modern terms “dust” and “clay”, respectively, by [35] (p. 17), following the mineralogical knowledge and nomenclature available in the English lexicon of his time).

Among the iron ores mentioned by Sumerians, a dark-gray and heavy iron-bearing mineral (KA.GI.NA; literary “mountain stone”)—which was used as a material for seals and weights (Figure 2) [38,40]—can be identified with hematite. The softer and soluble iron ores of a blood-red color (KA = iron oxide, blood) and yellow color (KA.SIG)—which were used as a coloring material, in the palatial wall painting, for example [41]—can be identified with red and yellow earth pigments, respectively (translated generically as “ocher” by [35] (pp. 81–83)). Furthermore, Sumerians were familiar with the color’s transformation of iron oxides with heat, and distinguished the red-colored earth (IM.MAL.LI.GUG) obtained by heating yellow iron ores (IM.MAL.LI) from the natural, red-colored earth composed of ferric oxide (IM.DIR, IM.GUŠKIN) [35] (pp. xx and 31–32).

Iron oxides and hydroxides were exploited as pigments in the clayey sediments of the Mesopotamian plain (Figure 1) [36,37] and were identified by color: IM.DIR = red; IM.GÍG = black; IM.PAR = white, gypsum; and IM.SIG = yellow [35]. However, Sumerians and Assyrians had to import many of their luxury goods and raw mineral materials from outside Mesopotamia, developing a large-scale trading network [40,41,42]. To obtain access to mineral sources (e.g., iron and manganese minerals), the major trade routes reached the volcanic region of eastern Anatolia from the upper valley of Euphrates (Figure 1) [43,44] and the highlands of Persia and southern Caucasus crossing the Zagros Mountains trough passes, such as the one formed by the valley of the Diyala River (Figure 1) [39,45].

The Sumerian acquaintance is actually comparable to that applied by coeval Egyptian technology [23,46]. It was later acquired by Greek (i.e., Homer and Theophrastus) and Roman (i.e., Plinius, Vitruvius, and Dioscorides) classical scholars.

3.2. Textual Introduction and Meaning of the Term ὠχρός (Ocher)

It is noteworthy that Sumerians and Assyrians regarded a specific kind of yellow as a flesh color, and, in particular, a greenish-yellow as the pallor of the face in terror [35] (pp. xxxviii and 32). In early times, the Greek word ὠχρός (ōchrós)—from which the Latin term “ochra” and the modern term “ocher” come—also had the same Sumerian meaning of face-paleness. Indeed, the first literary record of the word ὠχρός was to express the pallor of the face from fear in battle in the Greek epic poems Iliad (3.35, ὦχρός τέ μιν εἷλε παρειάς = pallor layeth hold of his cheeks) [47] and Odyssey (11.529, ὠχρήσαντα χρόα = his fair skin pale) [48], traditionally attributed to Homer (c. 800–c. 701 BC) and probably written in the eighth century BC [49], i.e., at the same time of the Neo-Assyrian Empire (911–609 BC).

The same meaning of the word ὠχρός is recorded in texts of Greek authors of the fifth and fourth centuries BC, such as Aristophanes (c. 446–c. 386 BC) in the play Nephelai (103, ὠχράω = to be pallid) [50]; Euripides (c. 480–c. 406 BC) in the tragedy Bacchae (438, οὐδ᾽ ὠχρός, οὐδ᾽ ἤλλαξεν οἰνωπὸν γένυν = he did not turn pale, he did not change the ruddy complexion of his cheek) [51]; and Aristotle (384–322 BC) in the treatise Categoriae (9^b32, ὠχρίας = of a pale complexion) [52].

The first evidence of the use of the word ὠχρός for an earth pigment used in painting is found in the dialog Timaeus, written by Plato (c. 428/423–c. 348/347 BC) around 360 BC (68c, τὸ δὲ ὠχρὸν λευκοῦ ξανθῷ μειγνυμένου = and ocher from white mixed with yellow) [53]. Along this sentence, it is important to note the difference between the two distinct terms that express the simple color yellow attributed to fire and sun (ξανθώ, xanthós) and the compound color ocher (ὠχρὸν, ocròn) [54].

Earthy iron ores used as red-coloring pigments were named μίλτος (miltos) by classical Greek authors, such as in Homer describing the painting of the Greek ship’s hulls during the Trojan War (Iliad 2.637) [47]. Aristotle considered ὤχρα and μίλτος as earth pigments in the treatise Meteorologica (3.78^a24) [55,56].

3.3. Ancient Greek and Roman Heritage

The treatise Peri Lithon (Περὶ λίθων, On Stones) [57] written c. 315 BC by the Greek philosopher Theophrastus (c. 371–287 BC) (Figure 3), pupil and successor of Aristotle, is considered by most scholars the first systematic description of mineral materials. As a matter of fact, it is largely based on the knowledge inherited from the Sumerians and Assyrians about the type, practical use, physical properties, and provenance of rocks, earths, and minerals [35]. However, Theophrastus’s method of distinguishing and classifying ωχρά and μίλτος according to color and area of origin represents a problematic aspect which had significant repercussions on scholarly thought and the interpretation of mineral earth materials during the following centuries.

Firstly, regressing with respect to the rich nomenclature on the basis of the physical—chemical properties of natural mineral materials proposed by Sumerians, Theophrastus applied the term μίλτος to all variants of red pigments containing ferric oxide, i.e., the natural iron-based red-colored earth, the red pigment artificially produced by roasting yellow iron oxide minerals, and the hematite coming from iron mines, irrespective of their genesis and composition (Peri Lithon, 51, 52 and 53) [57]. Furthermore, Theophrastus introduced the practice of distinguishing minerals based on their place of origin. For example, he described red Lemnian earth (coming from the Lemnos Island) and red Sinopic earth (coming from Cappadocia and brought down to the city of Sinope located on the southern coast of Black Sea) as different and separate lithotypes (Peri Lithon, 52) [57] based only on the locality of provenance.

Because of the authority attributed to Theophrastus’s work, these concepts were accepted and applied uncritically in the subsequent centuries for “earth” nomenclature, producing, on the one hand, the inclusion of all iron-based earth pigments in the two categories of red and yellow ochre, and, on the other, the proliferation of a myriad of different names for earthy mineral materials based only on the site of origin [55,57].

Theophrastus’s treatise was a reference source for classical Roman authors. For example, Theophrastus’s description of the process of the calcination of yellow earth pigments to produce red pigments (Peri Lithon, 45 and 53) [57] was reported by the Roman architect Marcus Pollio Vitruvius (c. 80–15 BC) in the treatise De architectura libri decem written in 29–23 BC (7.9.2 and 7.11.2) [59]; by the Greek–Roman botanist and physician Pedanius Dioscorides (c. 40–90 AD) in the treatise De materia medica (5.112) [29]; and by the Roman naturalist Pliny the Elder (23/24–79 AD) in the Naturalis historia (35.35), which, however, erroneously reversed the results of the calcination process) [28].

During the intervening centuries, from the fall of the Western Roman Empire until at least the eighteenth century, a plethora of natural philosophers, art makers, encyclopedists, and physicians produced treatises and lapidaries with compilations of mineral earthy materials, merely copying or elaborating on the classical texts of Theophrastus, Vitruvius, Pliny, and Dioscorides.

Moreover, what is surprising is that the seminal legacy of the Sumerian’s knowledge of mineral materials was found to be intact up to the early Middle Ages in Isidore of Seville (Isidorus Hispalensis, 560–636) [60], who presented the same Sumerian’s classification of earthy mineral materials (SAḪAR and IM) describing: pulvis (XVI.1.1, Pulvis dictus quod vi venti pellatur = dust is what can be moved up by the wind) and glebis terrae (XVI.1.3, Gleba pulveris collectione conpingitur et in uno glomere adunatur = a clod of earth is many dust particles glued and held together).

4. The “Terra d’Ombra” Natural Earth Pigment

Among the artists’ iron-based natural earth pigments, with great value was the one named “terra d’ombra” (Latin and old English: umbra, umbre; modern English: umber, raw umber; Spanish: ocra bruna; French: terre d’ombre; German: umbraun), which is greenish brown to dark-brown in color. As with other iron-based earth pigments, natural terra d’ombra also changes its color to a reddish dark-brown when subjected to the calcination process (Italian: terra d’ombra bruciata; English: burnt umber, French: terre d’ombre brulée).

4.1. Geochemical and Mineralogical Composition

The earth pigment terra d’ombra is a natural mineral material of chemical–sedimentary origin. The application of this name implies a definite genetic origin from the authigenic in situ precipitation of Fe-Mn (oxy)hydroxides of hydrothermal origin in an aqueous medium. It is composed of iron (hematite Fe₂O₃ and goethite α-FeO(OH); 30–65 wt%) and manganese (MnO₂ and Mn₃O_4; 5–20 wt%) (oxy)hydroxides. Other components are silica (<15 wt%), alumina (<5 wt%), and water (c. 14 wt%) [61]. Umber is a mudstone very fine in grainsize and may contain a small proportion of aluminum silicates, but, mineralogically, it is not a clay [24]. In numerous papers of art history, painting technique, and conservation, it is often incorrectly described as an ochre variant.

The eminent German mineralogist and chemist Martin Heinrich Klaproth (1743–1817) provided the first modern and reliable chemical analysis of the natural earth pigment terra d’ombra (umber) coming from the island of Cyprus [61], resulting in the following composition: Fe oxide 48 wt%, Mn oxide 20 wt%, silica 15 wt%, alumina 5 wt%, and water 14 wt%. The high concentration of iron and manganese was also confirmed by [62] on a commercial sample of Cypriot umber pigment (ferric oxide Fe₂O₃ 36.5 wt%; manganese oxide MnO₂ 12.3 wt%) and [63] on a sample of raw material coming from Cyprus (ferric oxide Fe₂O₃ 48.5 wt%; manganese oxide MnO₂ 19 wt%).

Recent and more detailed chemical analyses on raw umber ores of Cyprus were performed by [64,65,66,67]. These authors concord with a concentration of Fe₂O₃ in the range 20–70 wt% and MnO₂ in the range 9–23 wt%. Analyzing the umber raw material exposed in association to the ophiolite units of Oman, [68] recorded that umbers are depleted in aluminum, magnesium, sodium and potassium, due to the lower proportion of clay minerals present, whereas they are enriched in manganese, calcium strontium, and all rare-earth elements (REEs).

4.2. History and Use

In the first instance, terra d’ombra was mainly used as pigment for flesh and shadow painting (terra d’ombra … serve per le ombre delle carni = umber … serves for the shadows of the flesh) [69] (p. 155); see also [70,71]. Natural terra d’ombra (raw umber) was highly appreciated for its properties of facilitating the drying of oil paints and of blending with any color which, when mixed, is transformed into a darker shade of the original shade, precisely in its shade of shadow, without becoming overwhelmed [71].

From the Late Bronze Age onwards, Mn-based pigments coming from local ores in Cyprus—subsequently identified as umber—were employed for the black and dark-brown surface decoration and coating in Cypriot ceramics [72,73], which were widely distributed throughout the eastern Mediterranean [74] and southern Italy [75].

The first implicit literary meaning related to the Cypriot pigment terra d’ombra may be in Theophrastus (On Stones, 315 BC), who described an earth mixed with copper which “has the remarkable power of improving the beauty of the color” [57] (p. 49). Pigments used by painters to make shadows were mentioned by Pliny the Elder (sil quo utuntur ad picturae umbras = earth which is used to paint shadows; Naturalis Historia, 33.56) [28] as coming from the regions of Achaia, Skyros (Greece), and Lydia (Western Turkey).

After the Pliny reference, textual documents related to the earth pigment terra d’ombra were not found in the following centuries. Apparently, this pigment was not used in the Middle Ages. Indeed, it is not mentioned in the manuscripts of the eleventh–fourteenth centuries that collected recipes for making colors (e.g., De diversis artibus by Theophilus, circa 1120) [76]; moreover, it is not described in the treatise on painting by Cennino Cennini (c. 1370–1440) at the end of fourteenth century [30,77]. During the early Renaissance, Giotto di Bondone (c. 1270–1337) used a brown pigment composed of Fe and Mn (oxy)hydroxide that was generically defined as ochre [78].

At the beginning of the sixteenth century, the pigment terra d’ombra was introduced unequivocally in the painting palettes of Leonardo da Vinci (1452–1519) [79], Raphael (1483–1520) [80], and the Netherlander master Hieronymus Bosch (c. 1450–1516) [81]. Textual quotations on the use of terra d’ombra are testified in Italy, starting from the middle sixteenth century in the art treatises by Giorgio Vasari (1511–1574) [82] (p. 90), [83] (pp. 55, 198); Raffaello Borghini (1537–1588) [31] (p. 174); Giovanni Paolo Lomazzo (1538–1600) “terra d’ombra detta Falzalo” [84] (p. 191); and Giovanni Battista Armenini (1530–1609) [69] (p. 155). However, its composition and provenance remain obscure; also taking into account the fact that it is not recorded in coeval natural history treatises, which, nevertheless, reported the description of the “earths” as mineral materials (such as Agricola, Gesner, and Kentmann).

At the end of the sixteenth century, the Neapolitan apothecary Ferrante Imperato (1525?–1615?), in his Historia Naturale [85], after having faithfully reported the sentences of Pliny and Vitruvius regarding the Terre appartenenenti all’uso di Pittura (earths belonging to the use of painting), added the description of pigments that were considered a novelty in his time (Varie spetie de colori c’hoggi si adoprano = various kinds of colors that are employed today), comprising the pigment named terra di ombra:

“La terra detta di ombra è di color affumato, di sostanza leggiera, sottilissima, ben ligata, and adherente gagliardamente alla lingua, mentre con essa si tocca. Si rompe in schieggie, e si alliscia nel modo de boli: onde alcuni l’han stimata bolo armeno. S’indura al fuoco, come il bolo, e l’argille. Simile al suo colore si vede la terra nelle vene putri dell’oro. Serve per adombratura de carni, e de gialli.”

[85] (4.44, p. 122)

“The earth called umber is of a smoky color, of light substance, very fine grain size, well coherent, and adherent vigorously to the tongue, while it is touched with it. It breaks into splinters and smooths in the manner of the boles: hence, some have considered it as an Armenian bole. It hardens in the fire, like the bole and clay. Similarly to its color is the earth in the veins of gold. It is used for shading of flesh, and of yellows”.

The late Renaissance and the following Baroque periods were the era of maximum success of the pigment terra d’ombra [86]. It was included in the palette of, among others, Titian (1488/1490–1576) [87], Polidoro da Caravaggio (c. 1495–1543) [88], Michelangelo Merisi da Caravaggio (1571–1610) [86], Peter Paul Rubens (1577–1640) [89], Diego Velázquez (1599–1660) [90], Rembrandt (1606–1669) [91], and Johannes Vermeer (1632–1675) [92].

The first textual quotation of the English translation from terra d’ombra to “umber” is found in the manuscript The Art of Limning written between 1598 and 1603 by the English miniaturist Nicholas Hilliard (c. 1547–1619) [93] (p. 34). This pigment was probably already known and used by English painters before the Hilliard treatise, because of the translation and diffusion of the books of Vasari and Lomazzo [94]. From the acquaintance and mutual influence with Hilliard, William Shakespeare (1564–1616) possibly derived the quotation of the term “umber” in the text of the comedy As You Like It, written at the end of sixteenth century, where the character of Celia says “and with a kind of umber smirch my face” (1.3.118). Umber was also documented as pigment in England in 1627/1628 by the miniature painter Edward Norgate (1581–1650) [95,96].

Since the seventeenth century, the pigments terra d’ombra and terra d’ombra bruciata were often described as the essential components of painting palettes in France: terre d’ombre and terre d’ombre brulée [97] (pp. XV–XVI), Pierre Le Brun, 1635, [98] (pp. 770, 810), and [99] (pp. 12, 15); in Italy: [32] (p. 167), Giovanni Battista Volpato 1633–1706, translated by [98] (pp. 744–746), the Paduan manuscript of mid-end seventeenth century translated by [98] (pp. 650–651), and [100]; and in England: umber and burnt umber [101] (p. 77), [102] (p. 26).

During the ensuing eighteenth and nineteenth centuries, “umbra” is definitively quoted in texts of painting technique [6,103]. Famed painters such as John Constable (1776–1837) and Joseph Mallord William Turner (1775–1851) employed the brown pigment umber for their Romantic landscape paintings [104,105].

5. Pending Issues on the Identification, Source, and Genesis of the Natural Earth Pigment Terra d’Ombra

The name “terra d’ombra” (umber) most probably derives from the Latin term umbra (Italian = ombra; English = shade) [70] (2, p. 319), [9] (p. 58), [86] (p. 135), and was related to the peculiarity of this pigment to make shadows in paintings. Conversely, some authors attribute a geographical derivation to the name of this pigment [27,106], assuming that the Umbria region in central Italy (see Figure 4 for the location) was its place of origin and its patrionymic. This statement is controversial, because it is not supported by literary and historical documents, or by geological evidence. Moreover, it implies the correlation of the earth pigment terra d’ombra with some other natural mineral materials having very different compositions, colors, uses, geographical sources, and geological geneses.

Because confusion persists, even in the art history literature and pigment compilations of the last decades, with the acritical reproduction of the theory proposing the origin of the terra d’ombra from the Umbria region, the next sections of this work are dedicated to the discussion and refutation of this interpretation and to a definitive clarification on the mineralogical and geological identification of this natural earth pigment.

5.1. The Umbria Region (Italy) in History

The Umbria region in central Italy derived its name from the Greek όμβριος, meaning rainy or flooding (Pliny the Elder, Historia Naturalis, III.XIV.112) [107]. It was firstly named by Aristotle as Oμβρικοϊς (Meteorologica, II.359b) [23]. Aristotle quoted this region for the production of salt extracted from the ashes of some saltwort plants. In fact, along the eastern foothills of the Umbria–Marche Apennines, halophyte succulent shrubs (i.e., Salsola soda; Figure 5a) typically grow near mud volcanoes that are associated with saline water and gaseous hydrocarbon seepages (Figure 5b) [108]. Until the nineteenth century, these plants were used by the inhabitants of Umbria to obtain soda salt [109]. A solution of water and burned plant ashes was boiled dry to produce the soda ash that is sodium carbonate. This is one of the alkali substances that has been in great demand since ancient times for glass and soap making.

Umbria was inhabited by the Italic people of Umbrians whose Greek lineage and Roman ancestry are shrouded in myths (see The Roman Antiquities by the Greek historian Diodorus of Halicarnassus, c.60–c.7 BC) [111]. The present-day political borders of the Italian “Regione Umbria”, established in AD1948, are different from those of Roman Umbria (Figure 4), corresponding to the Sixth Region of Augustus described by Pliny the Elder (Historia Naturalis, III.XIV.112–115) [107]. The Roman Umbria extended from the eastern banks of the river Tiber to the coast of Adriatic Sea and was crossed by the consular road via Flaminia, which connects Rome to Rimini (Ariminum) (Figure 4). This road was built by the censor Gaius Flaminius in 220 BC and was the most convenient way to cross the Apennines mountain range from Rome to northern Italy.

The same linguistic root is at the basis of both the names Umbria and Ombrone. The latter is a river that flows in Tuscany up to the Tyrrhenian Sea (Figure 4), and was described by Pliny the Elder in the Seventh Region Etruria: Hinc amnes Prile, mox Umbro, navigiorum capax, et ab eo tractus Umbriae portusque Telamo, … (after these is the river Prile, and then the navigable river Ombrone, at which begins the district of Umbria and the port of Talamone, … Historia Naturalis, III.V.51) [104]. This apparent dualism in the location of the Umbria region can be explained by Pliny’s opinion, based on ancient sources, especially Hellanicus of Lesbos (490–c.405 BC) and Dionisius of Halicarnassus [111], that Umbrians originally inhabited a great part of central Italy, from which, in the course of time, they were dispossessed by other peoples.

During the fall of the Western Roman Empire, Roman Umbria was conquered by Longboards in AD 575 and was named Ducky of Spoleto. This name remained in the following centuries, during which the region was disputed between the Holy Roman Empire and the State of the Church. As a result of this conflict, the borders of this region of central Italy were often uncertain and changed rapidly, and its territory was divided into numerous minor lordships. Only in the seventeenth century, when the region became a de facto province of the State of the Church, was the name Umbria used again, but it had lost its previous territorial identity.

5.2. Earthy Mineral Materials Historically Originating from Umbria

The natural earthy mineral materials that have been historically referred to as originating in the Umbria region are as follows: creta Umbrica or terra di Nocera, and terra ampelite. During the seventeenth and eighteenth centuries, these mineral materials were variously mistaken with the earth pigment terra d’ombra.

5.2.1. Creta Umbrica—Terra di Nocera

Creta Umbrica was firstly quoted by Pliny the Elder among the types of creta Cimolia (Greek κιμωλία γῆ, from Kimolos in the Cyclades) or creta fullonia (= fuller’s earth, used for textile laundering) [112,113]:

“…Est et alius Cimoliae usus in vestibus. nam Sarda quae adfertur e Sardinia, candidis tantum adsumitur, inutilis versicoloribus, et est vilissima omnium Cimoliae generum; pretiosior Umbrica et quam vocant saxum. proprietas saxi quod crescit in macerando; itaque pondere emitur, illa mensura. Umbrica non nisi poliendis vestibus adsumitur. …”

(Naturalis Historia, XXXV.LVII.196–197)

“… Cimoliae creta has another use, too, the whitening of clothes. The kind which comes from Sardinia, and is called Sarda, being employed only for white garments and useless for colored ones, is the least valuable of all kinds in Cimoliae. The Umbrica sort and the one they call Saxum are more highly prized. Saxum has the property of increasing in bulk when soaked in water and is purchased by weight, whereas Umbrica is sold measure. The only use of Umbrica is for imparting luster to garments…” [28].

The quotation by Pliny is followed by fifteen hundred years of silence. Further references to creta Umbrica were only recorded again starting from the middle sixteenth century, in treatises on mineralogical subjects that actually elaborated upon the classical texts in compilation lists. Among these, Georgius Agricola (1546) [114] (pp. 27–28), Johannes Kentmann [115], Ferrante Imperato [85] (4.47), Ulisse Aldovrandi [116] (II.V and VI), Olav Worm [106], and Walter Charleton [117] reproduced Pliny’s description of creta Umbrica as a type of fuller earth (Cimolia).

Since the beginning of the sixteenth century, numerous medical treatises appeared to essentially consist of collections of recipes for treating various diseases with natural remedies. The therapeutic properties of the mineral earths (i.e., the so-called terra sigillata—sealed earth) and waters were considered. The attention of physicians was focused on the “miniera”, a term used to mean the mineral content of a soil in reference to the mineral salts dissolved in the water flowing through it. In this historical and medical context, the creta Umbrica of classical authors was rediscovered in Umbria, near the village of Nocera Umbra (Figure 4), where it was renamed “terra di Nocera”. The mention of the healing properties—especially for treating venomous bites—of the water of Nocera, and of the earth associated with it as its “miniera”, is presented in the treatises of eminent physicians of the time, such as Amato Lusitano [118] (p. 21), Alexis Pedemontani [119], Gabriele Falloppio [120] (XIIII, p. 48), Andrea Bacci [121] (III.3, VI.15), and Annibale Camilli [122] (Figure 6a).

“Descrittione di molte virtù della terra medicinale del Bagno di Nocera—si deve notare, che essendo ella di più spetie, e colori, quella deve essere più stimata, e lodata, che è di color bianco, molle, leggiera, succosa, friabile, untuosa, e che tocca con la lingua s’aderisca, and attacchi, come colla, e finalmente posta nell’acqua mandi fuori ampolle con bollimento à modo di calce”

[122] (pp. 46–47)

“Description of many virtues of the medicinal earth of the Bath of Nocera—it should be noted that since it is of several kinds and colors, the one that is to be most valued and praised is white, soft, light, juicy, crumbly, greasy, and adhering and stick to the tongue, like glue, and finally placed in the water, it let out bubbles with boiling like lime.”

By the late seventeenth century, the spread of “earths” throughout Europe as a medical remedy and the increasing difficulty and cost of obtaining them from the original sources in the ancient Asia Minor prompted the local search for these mineral materials, and led to attempts to correlate the various regional “earths” by referring to those described by the classical authors (i.e., Lemnian earth, Samian earth, and Armenian bole, etc.). This was a transitional period in which scholars were trying to reconcile increasingly modern scientific observations with sometimes scarce and incorrect descriptions of the classics, which still imposed their authority. Even for terra di Nocera, attempts were made to associate it with one of the classical medical “earths” (e.g., Paolo Boccone: Terra Bezoarti di Nocera species Terra Lemnia; [123] (pp. 63–64)).

Figure 6. (a) The frontispiece of the book on water and terra di Nocera written in the first half of the seventeenth century by the Italian physician Annibale Camilli [122]. (b) A modern bar of terra di Nocera for cosmetic purposes. (c) Table VI “De Terris Sigillatis” devoted to the illustration of the Terrae Nucerinae from the German physician and botanist Christian Gottlieb Ludwig (1709–1773) [124].

Figure 6. (a) The frontispiece of the book on water and terra di Nocera written in the first half of the seventeenth century by the Italian physician Annibale Camilli [122]. (b) A modern bar of terra di Nocera for cosmetic purposes. (c) Table VI “De Terris Sigillatis” devoted to the illustration of the Terrae Nucerinae from the German physician and botanist Christian Gottlieb Ludwig (1709–1773) [124].

The fame of terra di Nocera extends throughout the eighteenth century as Bolus alba or Terra Noceriana [2] (p. 3), terra nuceriana [124] (Figure 6c), Terra Noceriana alba [125] (p. 4), and Terra Noceriana or earth of Nocera in the first edition of the Encyclopedia Britannica [126]:

“Terra NOCERIANA, earth of Nocera, in the materia medica, a species of bole, remarkably heavy, of a grayish-white color, of an insipid taste, and generally with some particles in it which grit between the teeth. It is much esteemed by the Italians, as a remedy for venomous bites, and in fevers; but, except its astringent quality, little dependence is to be had on the other virtues ascribed to it.”

[126] (volume 3, p. 402)

Archeological evidence on the knowledge and use of the medical and ritual purposes of creta Umbrica—terra di Nocera since ancient times comes from the necropolis of Colfiorito—is located about 8 km NE of Nocera Umbra. In this site, the burial equipment of several tombs—dating between the ends of sixth century BC and the second half of fifth century BC—comprises several white mudstone cakes. The results of spectroscopic analysis confirmed the identification of the archeological samples with the natural terra di Nocera [127]. Moreover, supporting evidence for the correlation of Pliny’s creta Umbrica with the modern terra di Nocera is the location of the outcropping and quarrying area of this mineral material along the Roman Via Flaminia (Figure 4). Indeed, both the ancient creta Umbrica and modern terra di Nocera have been quarried in the same locations and are used in powder and soap form (Figure 6b) as fuller’s earth, cosmetics, and medicine, based on its alkaline and smectic properties. The lithological sources of the creta Umbrica—terra di Nocera—are whitish fissile calcareous marlstone and clay marlstone, exposed in the Umbria–Marche Apennines near Nocera Umbra and comprised in the geological units Scaglia Cinerea and Scaglia Variegata, Eocene–Oligocene in age [128].

5.2.2. Terra Ampelite

The second natural mineral material quoted to be originated from the Umbria region [116,129], and wrongly confused with the earth pigment terra d’ombra, is a black-brown earth, rich in organic substances and resembling bitumen, which was called terra ampelite (from the Greek ἅμπελος, ampelos = vine) or pharmacitis, because it has been used since ancient times against vine’s parasites: Theophrastus (On Stones, 49) [57]; Strabo (Geographica, VII.V) [130]; Vitruvius (De architectura, VIII, 8.3.8) [131]; Pliny the Elder (Naturalis Historia, XXXV.56) [107]; Dioscorides (5.181) [29]; and Aelius Galenus (12.9.1) [132]. The quotation of terra ampelite in the following centuries was borrowed from the description of classical authors, such as in Aetius Amidenus (fifth–sixth century AD) [133] (2.IX.70b); Paulus Aegineta (c. 625–c. 690) [134]; Pietro Andrea Mattioli [135] (CXXXVIII); Georgius Agricola (de natura fossilium, 1546, IV) [114]; Michele Mercato, Metallotheca Vaticana, probably written 1570–1593 but published in 1719 [136] (pp. 81–82, 87); Ferrante Imperato [85] (V.8, V.22, V.33), and Bernardo Cesi [137]. In the Germanic area, descriptions of the terra ampelite are in [124,138,139].

During the classical times, the geographic source of ampelite was identified in the ancient Cilicia (Figure 7) (Theophrastus, On Stones, 49; [57]). In particular, Strabo (Geographica, VII.V.8, citing the Greek geographer and historian Posidonius, c. 135–c. 51 BC) [130] and Dioscorides (5.181) [29] quoted a source of ampelite near the city of Seleucia in Pieria, the seaport of Antioch on the boundary of Cilicia and Syria. Vitruvius (De architectura, VIII, 8.3.8) [131] described ampelite near the Roman Cilicia town of Soloi-Pompeiopolis, where the waters of the river Liparis (the present river Mezitli in Mersin Province, Turkey) were forth impregnated with mineral oil.

The lithological nature of ampelite is defined as an asphaltic earth by Strabo (Geographica, VII.V.8) [130], as black earth-like elongated pieces of coal, slightly flaky and shiny by Dioscorides (CXXXVIII) [135], and as a black bituminous earth by Pliny the Elder (Naturalis Historia, XXXV.56) [107]. The occurrence in Cilicia of bituminous materials, including ampelite, is geologically controlled.

The classical region of Cilicia corresponds to the lowland area of the Adana basin, Gulf of Mersin–Iskenderun (eastern Mediterranean), between the Taurus Mountains to the west and north, and the Amanos Mountains to the east (Figure 7). Geologically, it is a Neogene tectonic basin corresponding to the deformational area at the junction between three of the major active tectonic structures of the region: the Central Anatolian fault zone to the west (Ecemiş–Kozan fault system), the East Anatolian fault zone to the east, and the northern extension of the Dead Sea fault zone to the south [140,141] (Figure 7). For this reason, the region is highly seismic, with very large (Mw > 7) historical and recent earthquakes [142]. The Cilicia region is also one of the most important oil districts in Turkey. Numerous natural surface hydrocarbon manifestations are present, especially in correspondence with active faults. These include bituminous rock’s impregnation, liquid or gaseous hydrocarbons seepage, solid asphalt leakage, and mud volcanoes with burning gas [143,144] (Figure 7).

Figure 7. Map of ancient Cilicia (Turkey) showing the natural surface hydrocarbon seepages and simplified tectonic structure of the region. Location of the hydrocarbon seeps from [143,144,145]. Major sinistral strike–slip faults from [140,141]. EAFZ: East Anatolian fault zone; DSFZ: Dead Sea fault zone. Black arrows on the fault traces indicate the relative motion of the faults. White square: modern city; red square: the ancient cities of Soloi/Pompeiopolis and Seleucia in Pieria, named in classical text as sites of terra ampelite ores. Inset shows the locations of the Cilicia map (red box) into the regional geographic setting. Shaded topography from Google maps.

Figure 7. Map of ancient Cilicia (Turkey) showing the natural surface hydrocarbon seepages and simplified tectonic structure of the region. Location of the hydrocarbon seeps from [143,144,145]. Major sinistral strike–slip faults from [140,141]. EAFZ: East Anatolian fault zone; DSFZ: Dead Sea fault zone. Black arrows on the fault traces indicate the relative motion of the faults. White square: modern city; red square: the ancient cities of Soloi/Pompeiopolis and Seleucia in Pieria, named in classical text as sites of terra ampelite ores. Inset shows the locations of the Cilicia map (red box) into the regional geographic setting. Shaded topography from Google maps.

The strange case of water mixed with oil (petroleum) described by Vitruvius has a present-day eyewitness. Sometimes, during earthquakes, methane and liquid hydrocarbons, migrating from cracks and fractures of rocks in the subsurface, mix with the groundwater and bubble up to the surface through wells and springs, such as the seismic event occurring on 27 June 1998 [145].

Ulisse Aldovrandi [116] (XVI, pp. 260–262) was the first scholar to indicate the presence and exploitation of terra ampelite (locally named Terrae nigrae = black earths) in Umbria. The detailed geological description of the terra ampelite coming from Umbria is by Lodovico Moscardo: “Roccia sedimentaria clastica pelitica, appartenente al gruppo degli scisti argillosi, ricca di materie carboniose, di prodotti di alterazione delle piriti e di allume solubile” [129] (89, p. 164) (Pelitic clastic sedimentary rock, belonging to the group of shales, rich in carbonaceous materials, products of the pyrite alteration and soluble alum).

We are unable to indicate which of the Umbria’s bituminous and C-organic-rich materials were used in the past as terra ampelite due to the lack of any detailed data on the lithological composition and provenance area from the ancient authors. Moreover, there were probably several local sources of supply.

In Umbria and in the conterminous regions, mineral materials that can be identified and used as terra ampelite are present in several Mesozoic–Cenozoic geological units [108], even along the Roman Via Flaminia, therefore, are in easily accessible locations. They are both sedimentary rocks rich in organic matter (black shales) and spontaneous hydrocarbon seepages. Black shales are dark-colored organic-rich mudstones that are deposited in anoxic marine environments and are potential rock sources of hydrocarbons and economically significant metals. The black shale levels exposed in the central Apennines can be correlated to coeval global oceanic anoxic events recorded in the Tethys paleo-ocean (Sicily, central–southern Apennines, Southern Alps, and Carpathians) and in the Atlantic Ocean [146]. Descriptions of fluid seepages occurring in the northern–central Apennines have been compiled since the eighteenth century [147,148]. The candidate geological units to be identified as Umbrian terra ampelite are described in the following, from the oldest to the youngest.

(i) Numerous, small (from few millimeters up to 40 cm in thickness), and discontinuous lenses and laminae of organicrich bituminous marls (black shales) are present in the geological unit Marne a Fucoidi of the Aptian–Albian age [149]. Three thickest and highly C-organic-rich of these black shale levels are marker beds throughout the Umbria–Marche basin.

(ii) A bituminous stratum (black shale) is interstratified between the geological unit Scaglia bianca of the Cenomanian age at the base, and the geological unit Scaglia rossa of the Turonian age at the top (Figure 8) [150]. This level is composed of mudstone and shale widely impregnated with insoluble bitumen. It shows an intense black color if fresh or is instead grayish or yellowish (due to efflorescence of sulfur) or brown (due to hydrated iron oxides) if it has been exposed to weathering. It is compact and opaque, has a conchoidal and polyhedral fracture, and tends to break into 2–3 cm-thick tiles according to the well-layering planes. It frequently contains fossil remains of fish. Its thickness is relatively constant throughout the outcrop area, ranging between 0.80 m and 1.50 m. It has a very extensive regional development and occurs in all the Mesozoic reliefs of the Umbria–Marche Apennines along a distance of about 75 km.

(iii) Gray marl with solid hydrocarbons bands or diffuse impregnations, often accompanied by natural gas emissions, oil seepages, bitumen dripping, mud volcanoes and saline waters, occurring along the eastern foothill of the Umbria–Marche Apennines, near Macerata and Urbino (Figure 4 and Figure 5b) [108,109]. They are related to the geological unit Gessoso-solfifera of upper Miocene (Messinian) age.

5.3. The Main Sources and Geological Genesis of the Earth Pigment Terra d’Ombra

The Danish physician Ole Worm (1588–1654) was the scholar who first suggested that the earth pigment named terra d’ombra originated from the Umbria region of Italy and owed its name to this place of provenance [106], mistaking the pigment “terra d’ombra” with the fuller earth “terra di Umbria” (Creta Umbrica). Indeed, Worm quoted the sentences of Pliny to describe Creta Umbrica and those of Imperato to describe terra di ombra. Because of his leading authority, Worm’s interpretation was widespread to all his disciples and followers, such as the English naturalists Walter Charleton (1619–1707) [117] (pp. 218–219) and Samuel Dale (1659–1739) [151] (p. 50), who copied the same sentences as Worm. John Hill [2] quoted the text of Charleton. Emanuel Mendes da Costa [125] (p. 101) mixed the descriptions from all previous authors.

Although the groundlessness of the Worm’s geographical derivation of the term “terra d’ombra” was instead suggested by the English encyclopedist Ephraim Chambers [70], the German chemist Johann Heinrich Pott [152] (p. 75) and the French painter and chemist Jean-François-Léonor Mérimée [153], the misattribution persisted in the technical painting literature of the following centuries [71,154], and until the twenty-first century [155,156].

The attribution of the origin of the pigment terra d’ombra to the Umbria region is not very suitable because (a) its geographical derivation is never described in textual documents by Italian painters and natural philosophers, and (b) it is not supported by the recognition in the Umbria region of geological units with the lithological and mineralogical characteristics of this earth pigment.

The first quotation in Pliny of umber pigments indicated that the classical source areas were in Greece and Turkey. Some scholars have referred to an origin from the eastern Mediterranean [102] (p. 22), [157]. The prominent source of the terra d’ombra pigment sold in Italy and Europe was the island of Cyprus [61], from which it was probably introduced by the trade routes of the Republic of Venice under the synonym “sombra de Venezia” [98] (p. ccxxi). The Island of Cyprus was part of the Republic of Venice in the period 1489–1571—that is, during the same period in which the pigment terra d’ombra appears in the palette of Renaissance painters as Leonardo da Vinci, Raphael, and Hieronymus Bosch. In 1571, Cyprus was conquered by the Ottoman Empire, from which the denomination such as “Turkey umber” or “terre d’ombre de Turquie” derived [61].

Umber is an iron-based earth pigment with a high concentration of manganese [61,66]. In the island of Cyprus, the mines on Fe-Mn ores, from which terra d’ombra or umber pigment originate, are still visible around the Troodos massif (Figure 9a).

At present, Fe-Mn ore deposits originate mainly from submarine hydrothermal springs (black smokers) rich in metals (mainly iron, manganese, copper, and zinc) on the ocean floor near active mid-ocean volcanic ridges [158]. Some of this metallic material is dispersed into the ambient seawater and precipitated in the surrounding area, forming fine-grained authigenic metalliferous sediment of Fe and Mn (oxy)hydroxides [159,160]. Fossil analogs of these modern Fe-Mn ores have been recovered in association to paleo-oceanic volcanic rocks named ophiolite [68,161], among which the middle–late Cretaceous Troodos ophiolite massif of Cyprus is considered as one of the most well-preserved [162].

The Troodos ophiolite sequence comprises, from the deepest to the most superficial unit forming the oceanic crust, the Earth’s ultramafic mantle rocks (peridotite and gabbro), sheeted dykes, basaltic pillow lavas, and marine pelagic sediments. These sediments are fine-grained, massive, brown mudstones (Figure 9b) rich in amorphous iron hydroxide (goethite) and manganese oxide (approximately 19–23% MnO and 20–24% Fe₂O₃) [64,66]. They generally occur as small lenticular bodies, 2 to 3 m in thickness, within and immediately on top of the pillow lavas [159]. All of the field evidence is consistent with the origin of these metalliferous sediments as a chemical precipitate from submarine hydrothermal vents during oceanic volcanism [61,159]. They were mined in the areas around the Troodos massif under the Latin name of “umbras” [162] and were used as black and brown pigments in Cyprus and eastern Mediterranean regions from the end of the Late Bronze Age onwards (1050–325 BC) [66,72]. Ophiolite complexes similar to the Troodos massif are present also in the sites quoted by Pliny, i.e., the Island of Skyros, continental Greece, and Western Turkey [163].

Starting in the mid-eighteenth century, a dark-brown Fe-Mn-rich earth pigment with the same characteristics as the Cypriot umber was discovered in Italy, near the volcano of Monte Amiata, which is near but not coincident with the Umbria region (Figure 4, see Section 6). Both the time of the discovery and the location at a depth of approximately 10 m in the subsoil exclude the possibility that this pigment could be the terra d’ombra that Worm [106] and his followers attributed to the Umbria region.

5.4. Other Material Confusion

In addition to the already mentioned confusion between umber and Creta Umbrica, numerous authors have erroneously correlated true terra d’ombra (umber) earth pigment with brown pigments of a completely different origin, composition, and paint properties, such as the “Cologne earth” (synonymous with Cassel earth, earth of Cullen, and Van Dyke brown) which originated from decomposed vegetables with bituminous matter (organic peat bogs).

The eminent German chemist Klaproth [61] explained the crux of the matter:

“The ancient mineralogists cite under the name of umber an earthy dust of a brown color, which ordinarily has only the color of this substance, but which does not approach it in any way in its properties. This earthy dust is usually a very recognizable fossil wood, in that it burns in the fire, and leaves a little ash; this is also why Cronstedt named it mumia vegetabilis, and Wallerino, humus umbra. True umber is incombustible.”

At the same time, in a few lines of a celebrated treatise of the Swiss professor of chemistry applied to the arts, Pierre François Tingry (1743–1821), some major mistakes on the umber earth pigment are concentrated [164]. Indeed, umber’s composition is wrongly defined as a clay, the manganese component is not considered, the three different types of earth corresponding to the “terra di Nocera”, “terra ampelite”, and “terra d’ombra” are considered to be the same mineral material, and umber is also confused with the “Cologne earth”:

“Umber earth is a kind of clay mixed with a little oxide of iron, which renders it dry, and is rather a bituminous earth slightly ferruginous than a brown ochre. It is brought from Nocera in Umbria, a district of Italy, whence it derives its name. Sometimes it is called brown ochre. Umber earth is very much employed in painting for browns. … The bishopric of Cologne produces a kind of umber earth which is heavier, as well as browner.”

[164] (p. 367)

It is astonishing that this book was reprinted without corrections or critical comments for more than two centuries, until 2018 for purely commercial purposes.

6. The Terra d’Ombra from Monte Amiata Volcano

In Italy, the main ore of the natural earth pigment terra d’ombra was recognized and exploited in lacustrine sediments exposed near the middle Pleistocene (305–231 ka BP) Monte Amiata volcano in southern Tuscany (Italy; Figure 4).

The volcanic activity of Monte Amiata was characterized by the effusion of lava flows and exogenous lava domes with a silicic (mainly trachydacite) composition [11]. After the end of the volcanic activity, several small freshwater lakes (no longer extant) developed at the periphery of the volcanic edifice (Figure 10). At present, these paleo-lakes are recorded by geomorphologic features and lake sediment occurrences [10,14].

Both paleo-environmental and paleo-climatic results and the ²³⁰Th²³⁸U age of 140–130 ka BP on Monte Amiata lacustrine sediments suggest that the Monte Amiata paleo-lakes occurred during a period of warm–temperate climate corresponding to the last interglacial period (the Eemian terrestrial or the Tarentian marine oxygen isotope stage MIS 5e), which developed between 140 and 130 ka and 115–110 ka BP [10,14] and is referenced therein. The duration of the lake’s existence has been estimated at 20,000 years. The Monte Amiata paleo-lakes were simultaneously extinguished by events of alluvial sand and gravel filling. This sharp sedimentological shift was probably the result of regionally extensive climate change that established cold and rainy conditions during the last glacial period, starting at about 115–110 ka BP [10,14].

Different environmental, chemical, physical, and biological conditions during the life of the paleo-lakes of Monte Amiata have led to the deposition of two very different types of sediment; (1) biogenic siliceous diatomaceous earth (diatomite), consisting of microscopic unicellular fossil algae of the Bacillariophyceae class [14] and (2) chemical precipitates of authigenic Fe and Fe-Mn (oxy)hydroxides with scarce terrigenous components from low-temperature hydrothermal waters rich in metallic solute, assisted by the presence of iron-oxidizing bacteria [12].

Since Fe and Fe-Mn (oxy)hydroxides make the lacustrine authigenic sediments of Monte Amiata assume hues ranging from light yellow to red and dark-brown, they were recognized and exploited as the natural earth pigments terra di Siena (sienna) and terra d’ombra (umber) [12].

During the eighteenth and into the twentieth centuries, the historical names attributed to the Monte Amiata earth pigments comprised different denominations following a commercial language, essentially based on color, consistency (in clods or powder), and application of the raw material, rather than on their mineral–chemical composition or the artistic nomenclature. The terms inclusive of all qualities have been terra gialla (yellow earth), terre coloranti (colorant earths), and terre bolari (bolar earths). In particular, the earth pigment of dark-brown color, corresponding with terra d’ombra (umber), was first named as terra gialla scura (dark-yellow earth) [165] or bole (bolus). The names bolar earths and bole are related to its application in painting technology as a base for water gilding as well as a pigment. For the first time, Santi [166] (p. 93) applied the name terra d’ombra at the dark-brown bole of Monte Amiata.

The geological and environmental characteristics that allowed for the genesis of the Fe-Mn-based earth pigments in the paleo-lakes of Monte Amiata are outstanding. In general conditions, iron oxides at the base of earth pigments are products of the primary weathering of iron ores or secondary terrigenous deposition from their erosion. On the contrary, at Monte Amiata, favorable geologic and climatic conditions allowed for earth pigments that are quasi-pure authigenic Fe and Fe-Mn minerals. Authigenic minerals precipitate in situ on the sea or lake floor from the water’s column with supersaturation of mineral elements that are commonly attributed to hydrothermal activity associated with volcanism. The accumulation is in the form of amorphous or poorly crystalline minerals. The resulting sediments are prevalent only where the accumulation rates of terrigenous (clay) and biogenic (organic matter) colloids of detrital origin are nearly zero. If the environmental conditions remain favorable to mineral precipitation long enough, large amounts of minerals may be deposited at a site. The geological and geochemical processes that allowed for the genesis of the authigenic Fe and Fe-Mn (oxy)hydroxides at Monte Amiata are similar to those applied to the genesis of umbers in the volcanic ocean ridge such as at Troodos (Cyprus). The presence of Fe-Mn (oxy)hydroxides in the lacustrine sediments of Monte Amiata is supported by a similar occurrence of sedimentary manganese deposits in the shallow continental basins (e.g., lakes) reported by [167] and attributed to the emission of hydrothermal solutions. In the case of Monte Amiata, the manganese solutes probably derived from the leaking of the geological unit named “Manganesiferous varicolored shales formation” of the early Cretaceous age (Aptian–Albian) [10,168]. This unit is composed of multi-colored shales with widespread manganese impregnations; it was deposited in a deep marine environment and forms the substrate of the volcanic rocks in the area of Arcidosso–Castel del Piano, where lake basins with Fe-Mn (oxy)hydroxide-rich sediments developed.

The main ore of terra d’ombra at Monte Amiata was located in the locality “Mazzarelle”, near the village of Castel del Piano (Figure 10). The quarry was a funnel-shaped open pit with a diameter of about 100 m and a maximum depth of 12–15 m. The stratigraphy of the sedimentary sequence that completely filled the paleo-lake basin comprised, from top to bottom, vegetable soil, alluvial sand, and peat (1–4 m in total thickness); yellow earth (terra di Siena) stratified in plane-parallel sub-horizontal laminae; bolar earth with various shades of yellow, from light to dark, alternating in irregular laminae; and dark-brown bole (terra d’ombra) (11–13 m in total thickness) [12]. Bole was classified into different qualities based on its hues. First-quality bole was the darkest (true umber) and occupied the deepest strata of the lacustrine deposits. Above it, the lighter-in-color, second- and third-quality bole had settled.

The approximate chemical composition of the Monte Amiata earth pigment terra d’ombra was provided by Santi [166]. Manasse [169] performed the first modern geochemical analysis, finding the amorphous colloidal structure of the mineral material and the presence, in addition to Fe oxides, of Mn oxide and Fe arsenate.

The Florentine botanist Pier Antonio Micheli (1679–1737) was the author of the first description of the lacustrine sediments of Monte Amiata used as earth pigments [170,171]. During his journey through southern Tuscany in 1733, this author recorded the presence of quarries of “terra gialla” in the surroundings of Arcidosso, for occasional and local use. Already in the mid-eighteenth century, the Amiata’s “yellow earths” were subjected to systematic extraction, and industrial plants were active for the processing of the raw materials. Baldassarri [165] and Santi [166] inform that the terra d’ombra of Monte Amiata was quarried in large quantities for export to England and The Netherlands and its price was very high compared to the others earth pigments produced in the same area. Starting from the same period, several collections and catalogs of earth pigments (terre coloranti) coming from Monte Amiata [171] were included in the private museum of Tuscan physicians and naturalists, such as Giuseppe Baldassarri (1705–1785) [165] (“sample n. 6: Dark yellow earth, located in Castel del Piano, is a kind of Bole; Painters use it, and even Physician in Dysentery”), and Giovanni Targioni Tozzetti (1712–1783) [172].

The expansion of the industrial exploitation of the earth pigment ores of Monte Amiata occurred from the mid-nineteenth century until the mid-twentieth century [173]. To this period, we owe the systematic collections devoted to rocks and earth pigments of Monte Amiata, such as those of Giovanni Campani [174], Antonio Pantanelli [175], and Cesare Tommi [176]. The current location of Tommi’s collection is unknown, whereas the Campani and Pantanelli collections containing earth pigments from Monte Amiata are still preserved at the Accademia dei Fisiocritici in Siena (Italy) (Figure 11). The Campani collection [174] comprises five samples of a dark-brown earth pigment named terra d’ombra (inventory numbers 3609, 3611, 3612, 3613, and 3615), all coming from the Mazzarelle quarry in Castel del Piano (Figure 11a,b). The Pantanelli collection [175] is exclusively dedicated to the “Terre bolari naturali e manufatte del Monte Amiata” (natural and manufactured bolar earths of Monte Amiata) and is composed of 60 samples that are representative of all the varieties (natural and calcined, in lumps, and in powder, etc.) of earth pigments (named terre bolari) extracted during the nineteenth century from the Monte Amiata quarries (Figure 11c,d).

The Monte Amiata lacustrine earth pigment deposits were completely exhausted by the mining activity. Due to prolonged abandonment and subsequent urbanization, the quarry areas of the earth pigment deposits are currently no longer accessible. The historical mineralogical collections of earth pigments of Monte Amiata are now particularly valuable because they are the only physical remains of the original natural earth pigments terra di Siena and terra d’ombra.

7. Discussion and Concluding Remarks

7.1. Bias on the Nomenclature of Natural Earth Pigments

The analysis of historical textual documents of the seventeenth to nineteenth centuries on natural earth pigments is certainly a source of significant information, but in them, there can be many ambiguous and incorrect references. The lack of scientific chemical and mineralogical knowledge of rocks and minerals has limited the nomenclature and description of materials to aspects only accessible to the human senses (color, flavor, and texture, etc.), having as a model the classifications of classical Greek and Roman texts. The poor geographical knowledge applied to the origin of the mineral materials created misinterpretations. Indeed, most scholars had no firsthand knowledge of the geological and geographical situation of mineral materials outside their region of residence but instead drew on the not always accurate descriptions of travelers who brought back reports and samples of rocks and minerals. The repeated and uncritical copying from previous authors resulted in the production of distorted and inaccurate texts.

Current mineralogical, chemical, and geological knowledge allows us to distinguish very different natural earth pigments from each other, making it no longer justifiable to define them with generic and incorrect nomenclature.

Unfortunately, it must be noted that there is a recurrent improper use of the names of natural earth pigments, both in treatises on painting techniques, e.g., [154], in papers on archeology, e.g., [177], in reference books, e.g., [25], and in electronic resources, e.g., [178].

Foremost, the use of words synonymous to the words “dye” and “pigment” is incorrect, such as in [177] (pp. 2; 3; 11); see [7,8]. The main inaccuracies are the applications of the terms “ocher”, e.g., [24,25,27,177,179], and “clay”, e.g., [7,9,19,24,25,27,178], as generic synonymous with iron-based natural earth pigment as a whole. From a geological point of view, there is no rock or mineral defined as ochre, because this name includes lithologic and mineral materials that are completely different in composition and genesis.

The first issue is that some scholars have attempted to translate the ancient terms into modern categories of geological materials. Regarding this, it is worth noting the cultural disparity between the rich ancient (Sumerian, Greek, and Latin) nomenclature indicating the various types of natural earths with different names based on their composition, source, and application (see Section 3) and the poor modern lexicon and the generic translation just on the two English terms “ocher” for all yellow and red earthy materials, and “clay” (that is a group of aluminum silicate minerals) for all materials with a fine grain size. Another cause of carelessness in the definition of natural earth pigments is the full commercial practice of re-editing books from the eighteenth–early twentieth centuries. They were landmarks at the time of their release, but currently they only have historical significance. Consulting and quoting these books as reference treatises without critical comparison with modern scientific knowledge can lead to the reiteration of misinterpretations by new compilers of reviews, dictionaries and catalogs on colors and pigments. Moreover, the historical coincidence between the meaning of the names of a color and the original natural earth pigment that produced it, such as terra d’ombra (umber) and terra di Siena (sienna), has been corrupted over time because of the commercial application of the sensory colors [1], abstracting from the geological genesis, geographical source, and physical–chemical characteristics of the original natural raw material (

Table 1. Summary of the geo-mineralogical characteristics of the natural Fe and Fe-Mn-based earth pigments and other earthy compounds discussed in the text. Results of color manipulation through heating were not considered in this list, as it is possibly applicable to all the Fe and Fe-Mn-based natural earth pigments.

Use	Color/Coloring Agent Percent *	Lithology	Composition	Geological Genesis	Geographical Area of Origin
Natural Fe and Fe-Mn-based earth pigments
Ochre
Artistic, ritual, magic, and medical	Yellow gold–pale red/ <10–25%	Mixture of host phases (carbonate, siliceous sand, and aluminum silicate) and accessory minerals (quartz, feldspars, carbonate minerals, gypsum, barite, micas, and organic matter)	Limonite–goethite; subordinate hematite in reddish samples; the darker ochres contain <5 wt% of various phases of Mn oxides	Oxidation/reduction processes in Fe-ores and weathering of Fe-rich sediments	Ubiquitous; in each geographical area, it has different geological, mineralogical, and geochemical properties
Red ochre
Artistic, ritual, magic, medical	Red/100%	Reddish earthy masses or dark-gray crystals	Hematite	Fe ores	Ubiquitous
Terra di Siena/Sienna
Artistic	Pale yellow–pale brown/65–85%	Mudstone with extremely fine grain size	Limonite-goethite, and <5 wt% Mn oxide; in a colloidal state	Authigenic in situ chemical precipitate in fresh waters	Monte Amiata volcano (Italy)
Terra d’ombra/Umber
Artistic	Greenish brown–dark-brown/35–90%	Mudstone with extremely fine grain size	Hematite-goethite 30–70 wt%, Mn oxides 5–20 wt%	Authigenic in situ chemical precipitate of metal oxides of hydrothermal origin in an aqueous medium	Cyprus Island, Eastern Mediterranean region (Greece and Turkey), and Monte Amiata volcano (Italy)
Bole
Artistic	Red, yellow, and brown	Mudstone with extremely fine grain size	Kaolin clay with metal oxides	Authigenic in situ chemical precipitate in fresh waters	Ubiquitous; Monte Amiata volcano (Italy)
Other earthy mineral materials
Creta Umbrica/Terra di Nocera
Fuller earth, medical, and cosmetic	Whitish	Fissile calcareous marlstone and clay marlstone	Carbonate and clay	Marine sedimentary rock	Umbria (Italy)
Terra ampelite
Medical and agricultural	Black–brown	Bituminous sedimentary mudstone and shale	Rich in organic matter, sulfur, and Fe oxides are also present	Natural surface hydrocarbon manifestations; black shales sedimentation on ocean floor.	Cilicia (Turkey) and Umbria (Italy)
Cologne earth (Cassel earth, earth of Cullen, Van Dyke brown)
Artistic	Black–brown	Earthy lignite	Rich in organic and bituminous matter	Decomposition of vegetables (organic peat bogs)	Central Europe

* Percent of the total colorant matter within the pigment.

).

7.2. New Light on Old Knowledge About the Terra d’Ombra (Umber) Pigment

This work provides historical and geo-mineralogical information on the famous artists’ Fe-Mn-based natural earth pigment—terra d’ombra (umber)—forming part of the palette of the great masters of Italian Renaissance and European Baroque painting. The nomenclature, composition, geographical areas of origin, and genesis of terra d’ombra were discussed in the light of a critical analysis of both historical textual documents, modern works on art history and painting materials, and geological observations on the source area of the Monte Amiata volcano (southern Tuscany, Italy). The results of this study can be summarized in the following observations.

The historical natural earth pigment named terra d’ombra (umber) is almost entirely composed of nanocrystals of iron (oxy)hydroxides and manganese oxides. It is a primary chemical sediment derived from the authigenic precipitation of Fe and Mn ions from an aqueous solution fed by thermo-mineral springs rich in metal elements. The latter are produced by volcanic activity, both along active and fossil (ophiolite) oceanic volcanic ridges and near lake basins

The terra d’ombra (umber) natural earth pigment owes its characteristics to its exclusive mineralogical and chemical properties. The simple color similarity with other natural or artificial pigments cannot be taken as a valid element for a correlation. The terra d’ombra (umber) earth pigment is neither an ocher nor a clay. It is not even identifiable with the brown pigments derived from hydrocarbons or from decomposed plant materials.

The places of historical origin of the terra d’ombra (umber) earth pigment were mainly the island of Cyprus, where sediments rich in Fe and Mn oxides were formed during the Late Cretaceous period under environmental and geochemical condition similar to that present at the volcanic ocean ridges. Comparable conditions occurred during the sedimentation in the late Pleistocene paleo-lakes of the Mount Amiata volcano (southern Tuscany, Italy). The hypothesis that terra d’ombra comes from the region Umbria (central Italy), and derives its name from this supposed origin, is unfounded.