In the Medieval-Renaissance period, the city of Ferrara was located at the end of the navigable Po river, a safe and fast waterway compared to land routes. The city was one of the main crossroads between the Adriatic coast and the internal areas of the Po valley. It was an area of attraction for traders and artisans who, by settling, transformed the city into one of the technologically advanced areas where ceramic, glass, and metallurgical workshops developed [1
Different archaeological excavations in medieval Renaissance sites in the urban center of the city of Ferrara have discovered significant quantities and types of fragments of glass and ceramics as evidence of the flourishing glass activity of the period [4
Glass, defined as the product of fusion of inorganic materials cooled to a hard condition without crystallizing, is generally characterized by high chemical stability. Often transparent or translucent, glass is made by heating a mixture of materials such as sand or soda and lime at a high temperature to form a liquid. When this liquid is taken from the furnace, it stiffens rapidly as it cools. Glass has been extensively used, thanks to its unique mechanical and chemical-physical properties, from ancient until modern times. It is therefore frequently discovered during archaeological excavations. The object of this study was to collect data on glass used in northern Italy in the Middle Ages, and in more detail in the city of Ferrara, an area where ancient glass was probably produced and processed. Scientific studies on glass produced in the second half of the first millennium AD mostly concern fragments from northern Europe [10
], whereas more limited attention has been paid to glass produced in Italy [12
]. Gruppioni [19
] pointed out that raw materials and processing techniques independent from Venice were used in the glass of Ferrara, and this suggests that there were unique processing methods for the creation of glass products in Ferrara [20
In this paper, glass fragments had been characterized after finding them in the Piazza Municipale of Ferrara (northern Italy) following repaving works. Many fragments had been found in a garbage tank free of subsequent contamination and archaeologically well characterized. During the excavation, 8198 fragments were found, currently kept at the National Archaeological Museum of Ferrara. The garbage tank, from where the studied fragments came, was in the southern side of the current Piazza Municipale, and was leaned against the foundations of the load-bearing walls of the medieval buildings. In this area, two waste discharge tanks were found, which supplied a large quantity of ceramic, glass, and metal objects and fragments. The fill, divided into stratigraphic units, was well preserved and entirely ascribed to a short period of use based on pottery and glass analyses. The period was related to the second half of the 15th century AD, but before 1479, when Eleonora d’Aragona, the wife of Duke Ercole I d’Este, wanted to demolish this part of the ducal palace to make way for a large inner courtyard, currently the Piazza Municipale [22
]. The garbage tanks were made of brick and were quadrangular underground rooms covered by a barrel vault whose top was placed at the level of the floor of the Via Nova, which overlooked the ancient Palazzo del Duca, in the current Piazza Municipale, demolished in 1479 by the will of Eleonora D’Aragona [22
]. The discharge chambers began to appear in Ferrara around the mid-fourteenth century and were used at least until the seventeenth century in aristocratic living contexts, such as Palazzo Paradiso and Palazzo Schifanoia, or in monastic complexes such as S. Paolo and S. Antonio in Polesine [23
], or in residences belonging to the wealthy class, exemplified by the house found in Via Vaspergolo/Corso Porta Reno [7
]. The waste tanks represented an ancient waste management system, in which the wet was separated from the solid and discharged into special wells and tanks equipped at the top with a disposal system that allowed the daily management of waste [8
]. This innovative system was also adopted in the Middle Ages in other historical centers of the Emilia Romagna region [22
]. Once filled, the structure was sealed and then abandoned; in the case of the tanks in Piazza Municipale, it was possible to refer them to the second half of the fifteenth century, and no subsequent contaminations were found. In the two tanks, the excavation operations allowed for the identification of six stratigraphic units, from which 8198 fragments of glass were found. Only the Stratigraphic Unit US1050 was investigated in this work to verify the compositional characteristics of the Ferrara glass. In the US1050 unit, the tank was sealed; this probably means that the glass fragments discovered in it were not contaminated by objects from later periods. This allows us to characterize a short chronological period more accurately. In addition, about 73% of the total fragments of glass belonging to numerous glasses, bottles, and other artefacts were found in this stratigraphic unit.
Quantitative information on the major, minor, and trace elements in archaeological artefacts including glass, is important in resolving problems connected with manufacturing technology, raw materials, and the origin of these objects. X-Ray Fluorescence analysis (XRF) allowed to us know the compositions of the glass fragments analyzed and to hypothesize the raw materials used [25
]. In addition, Scanning Electron Microscopy (SEM) observations allow us to describe the surface in order to detect deterioration phenomena on glass objects and to characterize the phenomena of atmospheric attack observed on some fragments.
Raw materials used for the manufacture of glass are divided into formers, fluxes, and stabilizers. Wide literature about chemical composition of glass findings, throughout the Roman Empire, emphasizes a roughly identical major elements composition, typically with low Mg and K content, suggesting traditional and conservative technology [25
]. On the other hand, during the Medieval and post-Medieval periods, widely different compositional features were documented, such as high-Mg high-K glass [27
], green forest glass (with about 22% CaO and 8% K2
O), yellowish green fern glass (with different K/Ca concentration ratio), and mixed-alkali glasses (with high Na and K content) [11
], suggesting a continual evolution of technology. Moreover, as suggest by [18
], the large differences in major element composition are attributable to a different choice of raw materials, so the chemical characterization just in terms of major and minor components can be conveniently used as a descriptive tool to identify the raw materials used for making ancient glass. Comparatively, alumina and lime content in soda-lime-silica glass can be used to trace back the raw material source of the former used [28
], being this latter either obtainable from crushed quartz pebbles and quartz sand [29
]. The use of quartz sand instead of quartz pebbles can be recognized by a higher amount of alumina and lime [12
]. In glass production, the use of fluxes (or melting raw materials) is fundamental to lower the melting point of the mixture and to improve the rheological characteristics of the glass (viscosity, workability). Three fluxes could be used: Natron (leading to a glass with low Mg and K around 1% wt oxide), halophytic plant ashes (leading to a glass with Mg and K around 3%) [12
], and continental plant ashes (leading to a potash glass). Potash and magnesia contents can be used to trace back the possible sources of flux, natron, or ashes, since the usage of natron as a flux turns out in a low Mg and K content, while plant ashes can significantly increase the potash and magnesia content [28
]. Consequently, some relationships between the major elements, such as Na and K, can provide useful information to recognize the nature of the flux used and to reconstruct the supply areas and commercial exchanges of the period.
This paper aims to investigate glass production technology in the Middle Age city of Ferrara throughout the analyses of fragments collected during the excavation of the Piazza Municipale and their comparison with a previous dataset.
Considering the MgO and K2
O contents, it was possible to infer the flux selected for the glass paste. The binary diagram MgO vs. K2
O (Figure 6
) shows two dotted areas regarding plant ash-based alkali and natron-based alkali [26
], and samples 154 and 177A fell in the “plant ash-based alkali zone”, while other fragments showed slightly low K2
O content, but high magnesia content.
The analyses of the 10 glass fragments were compared with the analyses of ashes of various plants typical of the Mediterranean area. Most of these plants belonged to the Chenipodiaceae
family, imported into Europe from the Middle Eastern countries [51
]. In general, the flux used was obtained from the combustion of several plants from the Chenopodiacee
family, where Salsola soda was the predominant plant.
The chemical analyses of these fragments have been compared with those of different plants taken from scientific articles concerning the trade relations between East and West [53
], and which have been used to obtain chemical parameters useful to discriminate the various fluxes. The plants taken into consideration were: Salsola Soda and Hammada Scoparia from the herbarium of the botany department of the University of Jerusalem; Salsola Kali from the areas around the city of Acre in northern Israel; a particular type of fern that grows on the Lido of Venice (northern Italy); Catania Soda that grows near Palermo (Sicily, south of Italy), and the "Vareque" type of seaweed from the Mediterranean coast of France.
The chemical composition of the individual plants was a function of the place of origin, climate, microclimate, and age, and this allowed us to distinguish the various supply areas.
To characterize the single fluxes, the ash analyses studied by Ashtor-Cevidalli [55
] were plotted in Na2
O (Figure 7
a) and CaO/MgO diagrams (Figure 7
b). The slopes of the various straight lines defined each individual plant based on the ratio indicated. The stars indicated the positioning of the glass fragments analyzed in this work according to the ratio shown in Table 1
In detail, Figure 7
a shows that the ash was rich in alkali carbonates of potassium and sodium, therefore it could not be used for a good quality glass. This was probably the reason why the Venetian government prohibited its use [52
]. Salsola Soda ash contained about 80% alkaline carbonates, mainly sodium carbonate, which made it an excellent material for glass [53
]. The difference between Salsola Kali and Catania Soda was slight. Catania Soda was richer in NaCl, and the amount of alkali was around 41%, slightly lower than Salsola Kali (45%). On the other hand, the ratio of Na2
O was higher in Catania Soda (2.3) than in Salsola Kali (0.9). Hammada Scoparia had intermediate characteristics to the two previous plants, and the Na2
O ratio was 1.3. The amount of alkaline carbonates was 27%, about half that in Salsola Kali. This means that a double quantity had to be used to obtain the same glass [52
]. Finally, the ash produced by the "Vareque" algae was characterized by a Na2
O ratio equal to 1.13.
Both sands and ashes of plants could contain calcium and magnesium compounds. In the ashes of plants, the CaO/MgO ratio was variable, but generally it was 2:1. In old soda lime glasses, the content of calcium oxide was consistent, but normally did not exceed 8%, as also evidenced by the analyses of Figure 7
b, except for samples 179 and 181. We must not forget, however, that a contribution of magnesium was also provided by the flux used, in fact, even in the ashes of the plants used, such as Salsola Soda, there was a percentage of Mg, which will join the total percentage in the relationship with calcium.
Even the 15th century glass fragments from Ferrara analyzed by Gruppioni [19
] were subjected to the same comparison (Figure 7
a—square symbols in the green circle), and also for them the predominantly flux used was obtained by burning Salsola Soda, perhaps with the addition of ashes from other plants.
The data analysis obtained on the glass fragments were normalized with the averages of the major elements obtained on those from Venice dating between the 7th–12th century and from the 13th–14th century [18
] to assess whether there could be affinities between the composition of glass produced in the same period of time between Venice and Ferrara.
From the analyzed glasses, it was noted that the vitrifying agent used was purer than that used by Venetian glass in the two different periods, being poorer in TiO2
). All of these elements did not exceed 1%. Slightly less for glass dating from the 13th–14th century. Instead, the concentration of manganese was higher than in the Venetian glasses, probably because it was inserted into the glass as a glass soap. Manganese was greater in the mixture of the Ferrara glass, except for the 177A sample, compared to Venice glass from the 7th–2th century (Figure 7
a), while it was in similar relationships with the Venetian glass from the 13th–14th century (Figure 7
The concentrations of Na2O and K2O, although being less abundant in percentage terms, shows ratios more or less similar to Venetian glass from the seventh to the fourteenth century. This highlighted the use in the analyzed glass of vitrifying raw materials other than those used in the Murano workshops; sands of better quality than those used in Venice between the 7th–12th century, but slightly poorer than those used in the following centuries, between the 13th–14th century. Na2O/K2O and CaO/MgO have been compared with the Venetian glasses of the various periods, the first ratio was more similar, even if the K2O level was lower in the Ferrara glass, instead the second ratio had a positive anomaly, probably due to the use of weakly dolomitic limestone as stabilizer.
In addition, the presence of phosphate in Ferrara glass was less than that present in Venetian glass (Table 9
), and this is probably dependent on the melting conditions (temperature, time) and the treatments (crushing the glass, washing and melting again) used: Venice obtained the vitrifying from the grinding of quartz arenites, rock rich in silica, while the analyzed glass of local production from Ferrara had a lower content of phosphate because river pebbles were used, which contained few percentages.
This paper allowed us to classify different fragments of glass of a historical period between 1450 and 1479. It was possible to divide the fragments by shapes, colors, and objects they belonged to. There were problems to better define the shades of the fragments since, apart from a few pieces, they did not have a single color, but had variations in colors, probably based on the purity of the raw materials used.
Chemical analyses of the fragments collected show that the glasses are made up of fairly homogeneous mixtures for all the fragments examined. The only difference was found for the blue glass fragments, where the percentages of cobalt and copper were greater than the percentages obtained for the green or yellow fragments. Considering that cobalt and copper levels were very low compared to iron and manganese in the fragments analyzed, they do not impart a visible color to the glass. Probably the blue, green, or yellow color was mainly due to the balance between Fe2+, Fe3+, and Mn2+, Mn3+. Manganese not only acts as a dye, but also as a glass soap: It was able to oxidize the ferrous ions present in the compound, neutralizing the green color given by the iron.
The XRF data were compared to other glass fragments from excavations in the city center of Ferrara of the 15th century. The data confirmed the same origin.
In addition to the chemical composition, the state of the surface of the fragments was observed through the scanning electron microscope analyses. The glass surface had no particular deterioration, except in some fragments in which Na level is lower, probably indicating surface corroded layer.
The data on the study of the flux used show that a particular plant belonging to the Chenopodiacee family, Salsola Soda, was used for the production of glass in the Ferrara area. The data were also comparable with the Venetian glasses of the various periods; this confirms the use of the same melting raw materials.
The compositions of the glass analyzed were also compared with analyses carried out on glass of Venetian origin, to observe any analogies of the Ferrara glass with the Murano glass (Venice) of the same period and previous periods to evaluate any relevance with the raw materials used for the production of the glass. It has been shown that the materials used for the Ferrara glass were purer than those of the Venetian glass dating from the 7th–12th century and 13th–14th century. From the comparison, it is possible to highlight that the analyzed glass, dating from 1450 and 1479, were obtained from vitrifying raw materials other than those used in the Murano workshops (Venice); sands of better quality than those used in Venice between the 7th–12th century, but slightly poorer than those used in the following centuries, between the 13th–14th century.
This paper, however, could confirm the hypothesis of the presence of glassworks also in the Ferrara area, capable of producing for the most part common use glasses with semi-pure raw materials and with the presence of elements added voluntarily to imbue the glass with better rheological characteristics (stability, resistance, workability).