Editorial for Special Issue “Archaeological Ceramic Diagenesis”

In their seminal publication on neutron activation analysis of archaeological ceramics, Sayre, Dodson, and Burr Thompson [1] (page 40) wrote that “Among the possible difficulties and interferences which may come into attempts to apply the method one may note the following: lack of sufficient uniformity of clays in a given region or of sufficient differences between those from different regions of interest; differences in fabrication affecting the impurity pattern; effects of weathering. The importance of these must be assessed by further experiment”. As with any newly introduced technique [2], initial optimism that ceramic compositional studies could readily solve long-standing questions about the past gave way to a healthy skepticism about the complications inherent in using compositional analysis to determine how and where archaeological ceramics might have been produced. More than six decades of work have clarified the impacts of different geological environments, production choices, uses, and firing methods on the mineralogical and chemical composition of ceramic vessels.

The last effect mentioned by Sayre and colleagues, what they term “weathering”, has been explored by a number of means since the 1970s at the latest. By this, I mean the alterations that may occur to the composition of a ceramic after it has ceased use and has entered the archaeological record. A number of terms have been proposed to encompass these effects on archaeological ceramics. Weathering seems overly restrictive, as it implies the removal of materials or content relative to the initial pre-burial condition of a ceramic body. Other terms including post-depositional or post-burial alteration have also been utilized. In the current issue, the term “diagenesis” is employed as a blanket term for all such effects that can occur to a ceramic matrix after it enters the archaeological record. Some might find the term “diagenesis” to be overly broadly applied in this context, given the use for specific processes in other disciplines. For instance, in bioarchaeology, this term typically refers to the mineralogical alteration and remodeling of bone in the burial environment, while in geology, “diagenesis” formally refers to the alteration of sediments through water–rock interaction, microbial activity, and compaction. As ceramics may be described as “artificially metamorphosed sedimentary rock with clays as the main ingredient” [3] (page 11), it seems that the term can be aptly utilized in referring to ceramics, which are subject to the same potential processes as any other sediment, including long-term exposure to groundwater, soil microbial activity, and compaction from overlaying sediment. In addition to the papers in this Special Issue, excellent recent reviews by Maritan [4] and Schneider [5] provide more details on some of the diagenetic pathways that have been documented in prior studies.

Particularly for lower-fired ceramics (under ~800 °C), which retain a significant cation exchange capacity, interactions with groundwater can result in elemental enrichment. These effects are most pronounced for alkaline and alkaline earth elements such as Na, Ca, Sr, Ba, and Cs, which are mobile in water and can readily bond with clay minerals. Similar elemental mobility may occur in ceramics fired at higher temperatures, but for short periods of time, as the inner sections of these objects may reach relatively low temperatures and retain the ability to absorb and bond mobile elements. Vessel texture plays a role in elemental enrichment, as larger or deeper voids or gaps around inclusions provide pathways through which water can enter into the ceramic matrix, as well as larger surface area for reactions [4,5].

In higher-fired ceramics, loss of elemental content (leaching) may occur instead. In ceramics fired above ~900 °C, sintering and vitrification can result in the formation of new phases as Si and alkaline elements begin to form glassy phases while Al and other transition metals separate into high-temperature mineral phases, including gehlenite and rutile. Some of these phases can be unstable in the presence of groundwater and can break down and leach out into the surrounding burial environment [4,5]. High-temperature ceramics may, on the other hand, absorb elemental content from the surrounding environment during the neoformation of zeolites (analcime, for instance) and attendant absorption of mobile alkaline elements (Na, Ba, and others) from the environment [6]. The fixation of environmental elemental content can also occur via biological activity. This is particularly the case for Mn and Fe, which may be deposited onto vessel surfaces or within pores and voids via microbial activity [4,5].

In addition to the breakdown of high-temperature phases, mineral grains and phases present in lower-fired ceramics may also undergo alteration. Perhaps the most common of these changes concerns calcite, which can be easily dissolved, redeposited, or neoformed within the ceramic matrix in the presence of groundwater, as can phosphate-rich phases. Other unstable volcanic or metamorphic grains (amphibole, for instance) may also deteriorate in the presence of groundwater in some environments [5,6]. Lower-fired ceramics may undergo rehydroxylation such that clay mineral structure may be altered from the original source clay for the ceramic [6]. A recent study of kiln wasters suggests that overfired ceramics may also undergo the neoformation of clay minerals [7].

Specific burial environments may lead to particular diagenetic impacts. In marine environments, ceramics may be subjected to a buildup of calcium carbonate from marine organisms and leaching of Na, Mg, B, Li, Sr, and other elements that are abundant in seawater. Limestone-rich environments also represent a potential issue, as Ca may be easily dissolved and introduced into ceramic pastes. Burial in rubbish pits may result in substantial uptake of elemental content from proximity to material rich in phosphate (e.g., ash and charcoal or decomposing food wastes, or large amounts of animal bone). Other studies have documented leaching of silver associated with the storage of ancient coinage [5]. In some of these cases, it may be difficult to distinguish use-related alteration (e.g., from cooking or storage of organic materials) from post-burial alterations. In any case, the initial composition of ceramics and specifics of the burial environment play a large role in determining how likely diagenesis is to occur. Ca-rich pastes are generally more susceptible to alteration than low-Ca pastes, but the degree of alteration is also dependent on geological substrate, moisture content (and variability), localized redox conditions, and pH [4,5].

Prior studies have relied on several primary approaches to identify the presence of diagenesis of archaeological ceramics and assess the relative impact of post-burial alteration. These include mineralogical examination in thin-section [8] or via XRD to identify the dissolution of mineral phases or the redeposition or neoformation of calcite and other phases. These methods can also be used to identify the presence of high-temperature phases or the neoformation of clay minerals [5]. Other studies have relied on simulated experiments [9] to explore the potential for alterations under different temperature, redox, or pH conditions. The leaching and absorption of elemental content have been principally investigated using profile mapping across vessel cross-sections [6]. Each method has advantages and disadvantages [6]. Mineralogical examination is useful for identifying larger-scale mineral alteration or changes to clay mineral structure at smaller scales. However, this method cannot always identify chemical leaching or absorption. Simulated experiments cannot accurately assess the impacts of ceramic–environment interactions over hundreds or thousands of years, but they are useful for gaining a controlled understanding of particular diagenetic pathways that may account for changes observed in archaeological materials. Profile mapping may identify leaching or deposition in cases where these effects result in a pronounced compositional gradient from core to surface, but if alteration proceeds to the extent that the entire internal chemistry of a sherd is impacted, it may prove difficult to identify alteration using profile mapping.

The five papers in this Special Issue explore a variety of diagenetic effects using a number of these different approaches. Gilstrap et al. [10] explore the diverse ways in which calcium-containing minerals can be altered in ceramics, including case studies from terrestrial and marine environments using petrographic and elemental analysis. Stoner and Shaulis [11] examine alterations in low-fired ceramics from the Valley of Mexico using profile mapping by LA-ICP-MS, demonstrating substantial Ba enrichment in the outer margins of some of the ceramics they study, as well as the uptake of a number of metallic elements. Golitko et al. [12] examine the composition of dense surface encrustations found on Bronze Age ceramics from eastern Hungary using a combination of mineralogical and chemical methods of characterization, identifying soil carbonates and phosphates as the likely source of surface concretions. They also utilize LA-ICP-MS for profile mapping, identifying a buildup of mobile elements within the ceramic matrix.

The remaining two papers in this issue specifically deal with the impacts of marine environments on ceramics. Sanchez-Garmendia and colleagues [13] use an experimental approach to simulate the impacts of underwater deposition on ceramics using test briquettes of variable composition (low-Ca, high-Ca, and micaceous clays) and firing temperature, exposed to either tap- or seawater for up to 18 months. Mineralogical and chemical analysis documents the formation of crystalline phases containing Na, Ca, F, S, O, Mn, and other elements in briquettes exposed to seawater. Renson and Glascock [14] use lead isotopic analyses to examine the alteration of ceramics that were deposited near lead sheathing from the Kyrenia shipwreck (Cyprus). They find evidence for the mobility of lead both into ceramics and in the shells of marine organisms that have built up on the wreck in the intervening 2200 years, which has altered both the chemical and isotopic composition of the studied ceramics.

The papers in this issue document some of the variable ways in which different burial or depositional environments can impact archaeological ceramics. These studies also demonstrate the utility of applying multiple analytical approaches to address the issue of ceramic diagenesis in archaeological contexts. Most of these papers use some combination of bulk chemical measurements, petrographic and mineralogical examination, and targeted microanalysis of chemical composition to identify the degree to which alteration has impacted the ceramics in question. Such multimethod approaches are becoming the norm in archaeometric studies of ceramics as instrumentation becomes more widely available and the costs of analysis continue to fall. These studies highlight the probability that in almost any study of ceramic composition, there is likely to be some degree of chemical and/or mineralogical alteration from the original composition. The identification of these diagenetic effects should be a standard part of any archaeometric ceramic study that utilizes compositional data to infer either production technology or production source.