What kind of calcite? Disclosing the origin of sparry calcite temper in ancient ceramics

Highlights

• Use of exogenous temper in local pottery production during Late Roman and Medieval times.

• O and C stable isotope analysis disclosed the geological origin of sparry calcite and marble temper.

• Firing experiments allowed definition of the influence of temperature on calcite O and C stable isotopes.

• archaeometric analysis revealed the reuse of white marbles and calcite alabasters.

Abstract

The addition of temper in the pottery manufacturing process is attested since Prehistoric Times and is still a production choice adopted in the ceramic industry. When the temper is composed of minerals and rocks which outcrop in regions distant from each other, new questions about the production technology arise. Such situations can be explained by considering the recycling of imported rocks, including those used for architectonic elements or sculptures, mainly coming from contemporary or earlier buildings, a practice that was widely diffused during the Roman and successive periods. This study presents evidence of the deliberate addition of recycled white marbles and sparry calcite (probably from calcareous sinters/calcite alabasters) within the long-lived production (between the 4th and 14th century CE) of coarse and cooking ware in north-eastern Italy. The petrographic analysis of about 200 potsherds attested the use of marble as unusual kind of temper, in addition to fragments of sparry calcite, in about half of the repertoire. The occurrence of different types of marbles, associated with rocks and minerals typical of the alluvial deposits of the eastern Po plain as well as locally available rocks (Euganean Hills trachyte), clearly pointed to the intentional addition of recycled marble fragments from ancient spolia, excluding the hypothesis that the pottery was imported from other regions. Detailed petrographic and microstructural analysis, including maximum grain size (MGS), accessory minerals (when observed) and grain boundary shapes allowed us to limit the provenance of these marbles to the most important Mediterranean classical source regions. These conclusions have been confirmed by the oxygen and carbon stable isotope data derived from marbles and calcite fragments mechanically separated from the ceramic paste. Some fragments of sparry calcite were characterised by very negative δ¹³C values, significantly different from known marble varieties, and typical of calcite crystallised in superficial geological environments, consistent with calcareous sinters, such as calcite alabasters. Moreover, a series of firing experiments were carried out in the temperature interval between 450 °C and 800 °C, both reproducing oxidising and reducing conditions, on clay pastes tempered with Carrara marble, and fired, to evaluate whether these anomalous δ¹³C values observed in the ancient ceramic inclusions could also be related to the firing process.

Introduction

The addition of temper (i.e. non-plastic materials) represents the main clay paste preparation procedure used to reduce the plasticity in excessively plastic (“fat”) clay materials, preventing shrinkage during drying and firing (Eramo 2020 and citations therein). According to the accepted definition, clay is “a naturally occurring material composed primarily of fine-grained minerals, which is generally plastic at appropriate water content and hardens when dried or fired” (Guggenheim and Martin 1995). The plasticity of a clay (measured according to Atterberg 1911) can be very variable (Gualtieri 2020), depending on the clay grain-size, quantity and type of clay minerals it contains, percentage of organic matter and of salts, as well as moisture content (Andrade et al., 2011 and citations therein). It can be reduced, therefore, by adding a sand/silt fraction, in order to decrease the quantity of clay minerals.

On the one hand, the deliberate addition of temper improves the workability of the clay paste, on the other, it changes the microstructural features of the ceramic product. According to the type of temper and the firing conditions, tempering a base clay increases the thermal shock resistance and/or toughness of the obtained ceramic material (Bronitsky and Hamer 1986; Feathers 1989, 2006; Skibo et al., 1989; Hoard et al., 1995; Kilikoglou et al., 1998; Tite et al., 2001; Feathers et al., 2003; Maritan et al., 2005a, b, c; Allegretta et al. 2014, 2016; Tenconi et al., 2013; Hein et al., 2013; Müller 2017; Bebber 2017).

The addition of temper is widely attested throughout human history, and recorded in ceramic artefacts by the presence of various types of material, mainly represented by:

• mineral temper: fragments of rocks and/or minerals, added in the form of naturally available sediments such as sand (e.g. Dal Sasso et al., 2014; Eramo et al., 2014, Tenconi et al., 2016; Maritan et al., 2019), or obtained by crushing and grinding rocks (e.g. Maritan et al., 2005a, b, c; Palumbi et al., 2014; Tenconi et al., 2013; Borowski et al., 2015; Antonelli et al., 2018) and minerals (e.g. Maritan et al., 2005a, b, c; Fabbri et al., 2014; Dal Sasso et al., 2014; De Bonis et al., 2017);

• temper of organic origin: plants and plant parts, such as straw, grass and rice (e.g. Reid 1984; Skibo et al., 1989; Stilborg 2001; van Doosselaere et al., 2014), wood-ash (e.g. O'Brian 1989), herbivore dung (D'Ercole et al., 2017), and hard parts of animal remains, such as bones, horns, shells, and hair (e.g. Stilborg 2001; Perttula et al., 2011; Kiryushin et al., 2012, Mariotti Lippi and Pallecchi, 2017; Všianský et al., 2019);

• artificial temper: grog or chamotte, consisting of previously-fired clay materials (e.g. Maritan et al., 2009; Tenconi et al., 2013; Spataro 2014; Eramo and Mangone 2019; Holmqvist et al., 2018; del Pino Curbelo et al., 2019).

A peculiar type of temper is represented by sparry calcite (e.g. Maritan et al., 2005a, b, c; Teoh et al., 2014; Travé Allepuz et al., 2014; Whitbread and Mari 2014; Fabbri et al., 2014, and citations therein; Albero Santacreu and Cau Ontiveros, 2017; Albero Santacreu et al., 2019; Cau Ontiveros et al., 2019), occurring with crystals of rhombohedral shape defined by cleavage planes. In such a form, calcite is not a commonly found as loose large crystals in nature, since it tends to be easily reduced into very small crystals, due to its low hardness (3 on the Mohs scale) and perfect rhombohedral cleavage on $\left\{10\bar{1}4\right\}$ (Deer et al., 2013). Therefore, the occurrence of sand-sized rhombohedral crystals of sparry calcite in ancient pottery has been mainly interpreted as deliberately added temper, supplied from crystalline calcite veins occurring in carbonate rocks (see Fabbri et al., 2014 and citations therein). However, from a geological point of view, large crystals of calcite can be found also in medium- to coarse-grained pure and impure marbles, in calcite alabasters (also defined as calcareous sinters), in sparite limestone, as well as in coarse crystalline speleothems. For the latter, literature data indicates that speleothems have been used as temper in ancient pottery productions (Santoro et al., 1996; Gómez-Gras and Risch 1999; Shoval et al., 2006; Capelli et al., 2008; Muntoni et al., 2009; Marlasca et al., 2013; Tenconi et al. 2013, 2016; Fabbri et al., 2014), in which the typical sparry calcite crystals are associated with a peculiar saw-tooth pattern (alternating white and thin dark brown laminae) (Frisia et al., 2000; Frisia 2003; Baker et al. 1993, 1998). Marbles have been recognised as temper in ancient ceramic productions (Flügel et al. 1997a, 1997b, 2004; Flügel 1999; Peloschek 2018), but mainly related to the use of locally available rock materials, outcropping in the vicinity of the production sites (Peloschek 2018), or to the use of debris from the working of imported stones often from hundreds of kilometres away (Flügel et al., 2004).

The archaeometric study of coarse and cooking ware dated mainly from the 4th to 14th century CE from the Padova province in Veneto region (north-eastern Italy) revealed the occurrence of white marble fragments associated with sparry calcite crystal as tempers. These latter could have been supplied from calcite veins in limestones outcropping in the Mesozoic and Tertiary carbonate sedimentary sequence of the Southern Alps. In contrast, white marbles are absent from the region, nor, equally, are they present as pebbles in the alluvial or moraine deposits. Therefore, the possible supply region of the white marble used for the production of this pottery must be sought outside the Veneto region.

This study therefore aims to define the provenance of the calcite and marble materials occurring in potsherds to disclose possible importation from other regions or specific technological choices during production. Further, the deliberate addition of rocks originating geologically from outside the region could reflect various different scenarios: i) importation of vessels produced elsewhere; ii) locally produced pottery with temper from external geo-resources that were imported for various reasons, such as for use in architecture and sculpture. For this reason, potsherds were subjected to petrographic analysis for fabric definition and stable isotope measurements of the calcite inclusions occurring in the ceramic body.