Colour Transformation and Textural Change in Biotite: Some Remarks for the Interpretation of Firing Technology in Greyware Pottery Thin-Sections

Esther Travé Allepuz

doi:10.3390/min11040428

Abstract

Firing is a crucial step in the production of pottery, as it irreversibly transforms the clay into ceramic. Clay sintering and subsequent vitrification occur during firing, together with other transformations undergone by specific minerals and rock inclusions according to their optical and physical properties, including their colour. Some of these are visible in thin-sections and might be interpreted as technological markers or contribute to the estimation of firing temperatures, although most of them are poorly documented. In this paper, we approach the transformations in colour, texture and optical properties that occurred in biotite inclusions from medieval greyware pottery. Our study considers a batch of 40 pottery samples from medieval Catalonia analysed by XRD. According to the estimated firing temperature ranges and atmospheres, we examined the behaviour of biotite at different temperature ranges from 700 °C to 1000 °C by means of optical microscopy, considering its size, shape and abundance, and compared these features to a wider assemblage of thin-sections from medieval earthenware. The results obtained are interesting, as they offer a valuable reference for petrographic studies on pottery. We discuss the potential of ceramic petrography as a way to perform more precise and refined sample selection for further analysis on archaeothermometry.

Highlights

According to the estimated firing temperature ranges and atmospheres, we examined the behaviour of biotite by means of optical microscopy, considering its colour, birefringence, size, shape and abundance, according to the temperature range for each sample
We aimed to determine the difference in colour, birefringence and texture from samples included within one temperature range to another, and to compare whether the changes occurred in biotite and other transformations of clay matrix, inclusions or voids
All 40 samples are coarse; they contain a percentage of inclusions counting for 20–40% and were fired under reducing conditions, which was the most-common practice in medieval Catalonia attested by a considerable number of examples from the archaeological record [53,54] and the ethnographic approach [10,11,13]

Summary

Introduction

The study of these plain undecorated vessels, with their simple and homogeneous shape of boiling pots or ‘ollae’, revealed the main circuits of production and distribution in Catalonia and their implications in social and economic terms [1,2]

Objectives

Methods

Results

Discussion

Conclusion