Contents lists available at ScienceDirect Journal of Archaeological Science: Reports journal homepage: www.elsevier.com/locate/jasrep Early vitrification stages identified in prehistoric earthenware ceramics from northern Chile via SEM C.A. Bland a,⁎ , A.L. Roberts a , R.S. Popelka-Filcoff b , C.M. Santoro c a College of Humanities, Arts and Social Sciences, Flinders University, Adelaide, South Australia, Australia b College of Science and Engineering, Flinders University, Adelaide, South Australia, Australia c Instituto de Alta Investigación, Universidad de Tarapacá, Arica, Chile ARTICLE INFO Keywords: Scanning electron microscopy (SEM) Earthenware ceramics Firing technology Vitrification stages Northern Chile ABSTRACT Scanning electron microscopy (SEM) has been used to investigate ceramic vitrification stages since the 1970s, however its application has been primarily restricted to stoneware and high-fired ceramics. The purpose of this study was to determine whether stages of vitrification could also be identified in prehistoric low-fired archae- ological earthenware ceramics via SEM using northern Chilen samples covering a period c. 2000 to 500 years BP as a case study. This was achieved by visually comparing microstructural changes between original and re-fired ceramic sherds. The microstructural changes identified in this study indicate that the potters who made these vessels achieved early stages of vitrification. This result demonstrates that SEM is a useful technique to in- vestigate the development of firing technology in earthenware manufacture. 1. Introduction Ceramics are one of the most common materials recovered from archaeological sites. The production of ceramics involves a number of stages including: 1) raw material procurement (clay and tempering material [if required]); 2) processing of raw materials (e.g., the removal of unwanted materials and crushing); 3) manufacturing processes (e.g., mixing of the raw materials, forming the vessel, surface treatment, application of decorative or iconographic features); and 4) firing. The firing process is a key stage to analyse when investigating ceramic technology, as the physical properties of clays change depending on four main factors: the temperature, soak (how long the vessel is exposed to the maximum temperature), the total duration of the firing and the atmosphere. Ceramics go through different phases during firing. First, organic material (if present) and structural water are burnt off, the initial phase results in a ‘stickiness’ of the clay particles and partial deconstruction of the mineral structure (sintering), secondly the air spaces collapse, re- sulting in a liquid phase and, finally, total vitrification occurs (Shepard, 1956:83). When clay is subjected to sufficient firing atmosphere, tem- peratures, soak and duration it undergoes vitrification (ranging be- tween 800 and 1000 °C depending on the raw materials), which means that the microstructure of the clay body, including clay and tempers, begin to soften and fuse together (Rice, 2005). The firing technology used by ancient potters will have a significant impact on the maximum firing temperature and soak achieved. Kiln firing is able to achieve and sustain higher temperatures (up to 1200 °C) whilst open firing systems reach lower and more varied temperatures and cool more quickly (Chatfield, 2010; Rice, 2005; Tite et al., 1982). In order to determine the maximum temperature to which a sherd was exposed during ori- ginal firing the clay microstructure must be microscopically analysed to identify the stage of vitrification. Understanding vitrification stages allows archaeologists to infer the type of firing technology employed by potters in the past, which may vary depending on the function and purpose of the vessels. As noted in Sutton and Arkush (2009:119), total vitrification gen- erally occurs around 1200 °C. However, it should be noted that the temperature required for vitrification to begin is dependent on the clay properties. These differences in properties, as well as the presence of fluxing impurities (such as feldspar which lowers the maturing tem- perature and promotes vitrification), result in some clays having a lower threshold for the maximum temperature required to begin vi- trification (Sutton and Arkush, 2009:119). Further, Shepard (1956:81) and Sutton and Arkush (2009:119) note that variation in the tem- perature required for vitrification is common in ‘low-grade’ clays due to clay properties and fluxing impurities (Rice, 2005). For example, clays commonly used by traditional potters worldwide can produce ceramics which reach the initial stages of vitrification at around 800–900 °C (Rice, 2005; Sutton and Arkush, 2009). One technique that has been employed to investigate the http://dx.doi.org/10.1016/j.jasrep.2017.09.011 Received 9 June 2017; Received in revised form 12 September 2017; Accepted 12 September 2017 ⁎ Corresponding author. E-mail address: catherine.bland@flinders.edu.au (C.A. Bland). Journal of Archaeological Science: Reports 16 (2017) 309–315 Available online 23 October 2017 2352-409X/ © 2017 Elsevier Ltd. All rights reserved. T