A forgotten technology: The production of artificial millstones for windmills in Sistan, southeastern Iran
Introduction
Pyrotechnologies for the production of metal, ceramics, glass, and mortar have fundamentally impacted the lifestyle of humanity (Wertime and Wertime 1982:22; Artioli, 2010:204; Rehder, 2000:3). It is often claimed that controlling heat through associated technologies has been the greatest continuously developing skill of human beings (Rehder, 2000:3). Most of these technologies have been extensively investigated because of an interest in tracing their development from their origins to the present. This is not the case for those pyrotechnologies whose products have not yet been identified or where direct connections to present industries remain unclear. The present paper draws attention to one of these forgotten pyrotechnologies and the role it played in the making of large synthetic millstones for Persian windmills or smaller querns during the Islamic period in Sistan in southeastern Iran.
During archaeological surveys in Sistan, researchers discovered ceramics that had been fired at high temperatures (Tosi, 1969; Mousavi Haji and Mehrafarin, 2009). In general, this is not astonishing, as Mesopotamia and surrounding regions have a long tradition of highly-developed pyrotechnologies and a record of technologically sophisticated pottery production (Simpson, 1997). The principal author of this paper carried out surveys in 2008 and 2009 to investigate the role of pyrotechnologies in Sistan. A third survey season in 2012 resulted in the discovery of materials that seemed at first sight to be blackish vesicular basalt. They looked like the usual material in the region for the production of millstones and hand querns. However, closer inspection showed that these fragments were artificial products because some of the items displayed two exterior flat surfaces, one of which is a distinctive greenish/grayish thin layer, unlike any natural basalt. Another remarkable find consisted of a ceramic disk with an unknown function in the Zabol museum which closely resembles a half-disk quern made of the pseudo-basaltic material found during the survey. The absence of any similar vesicular basalt in the region and the well-formed shape of the pseudo-basaltic millstones, with two flat surfaces, gave rise to the hypothesis that the ancient craftsmen in Sistan had likely developed a technology that allowed them to produce a ceramic material that was suited to be used for millstones. However, not one complete millstone for a windmill has yet been found, only several large broken fragments that were discovered inside and around the ruins of windmills in Sistan. The nature of the finds and their find context motivated the first author to investigate this technology in the laboratory.
Besides the basic questions of the chemical and mineralogical composition of artificial millstones, an essential element for an understanding of their manufacture is an investigation of the temperature regime used to manufacture them. For archaeological ceramics, this estimation is usually based upon the occurrence of thermometamorphic minerals or the microstructure (often the level of vitrification) of the pottery caused by the firing process (Tite and Maniatis, 1975:122–123; Heimann and Franklin 1979:27–28; Tite, 1995:37; Duminuco et al., 1998:185; Gliozzo, 2020). The artificial millstones studied here are a unique class of material that in its production process bears similarities to the production of various pyrotechnologies such as metal, glass or ceramics, but in other respects remains different. The composition of the millstone fragments is similar to ceramics, but the millstones were also melted as in glass making. Rather, similar to slags – and in contrast to glass – crystallization took place inside the matrix. Practical aspects of the production process can be derived from material and thermodynamical analysis. An important characteristic of these materials is their production via unfinished reactions. Therefore, it is not only the thermodynamics of the chosen starting material but also the reaction kinetics that control the properties of the final product, and therefore the maximum temperature as well as the temperature-time-path play a decisive role in the manufacturing of artificial millstones. We propose that these variables were carefully controlled, which makes Persian artificial millstones not only the precursors of modern glass-ceramics, but also the first ceramic-based material used as an integral part of a much larger machinery.
Section snippets
Persian windmills and the Sistan region
Persian windmills are frequently referred to as the starting point of an extensive application of wind power for technical purposes on land (Derry and Williams, 1960:253–255; Wade, 1974). This type of windmill with a vertical axle is typically situated in the path of a strong summer wind, the so-called “120-days wind”, which blows down the present-day border region of Iran and Afghanistan in a north-south direction from mid-June until mid-September (Ferdinand, 1963:73; Harverson, 1991:3;
Archaeological material
Archaeological samples and raw material (local clay/silt and sand) were collected from three sites where the existence of a highly specialized pyrotechnology for the production of artificial millstones was first recognized during the 2012 survey (see above and Mishmastnehi, 2015:386). In the field, it was difficult to distinguish with the naked eye small fragments of artificial millstones from natural volcanic materials such as pumice, vesicular basalt, or from slag. Only in the case of large
Results
Petrographic microscopy reveals two different textures for most of the historical samples: a blackish bubbly body and a greenish/grayish edge (Fig. 5a). The body of the millstones consists of a vesicular structure surrounded by a glassy matrix full of acicular crystals of Cpx with various amounts (between 1% and 5%) of rounded and/or sub-rounded, usually cracked quartz grains, where all of these shapes and cracks are consequences of high temperatures. Usually, large needle-like crystals of Cpx
Discussion
There are plenty of over-fired clay materials from the early periods of ancient Mesopotamia, starting with the 5th millennium up to the 15th century BCE. Intentional over-firing is evident in a variety of objects, for instance, clay sickles from the 5th and 4th millennia BCE (Benco, 1992:121) or ceramic axes from the Ubaid period (Pollock, 1999:80). Clinkers and over-fired waste materials, as well as high-temperature fire installations are other examples of such materials that are reported from
Conclusion
How did these two peculiar innovations, the Persian windmill and the artificial millstones, joined in Sistan to form a complex technological ensemble? This puzzle is still unsolved; but what is clear from this study is that artificial millstones are akin to over-fired ceramics. They also closely resemble the “synthetic basalt” from the Mesopotamian early second millennium BCE city of Mashkan-Shapir. We tend to think that there is no hidden continuation of this technology somewhere in Greater
Acknowledgments
The authors are thankful to Prof. Susan Pollock, Freie Universität Berlin, Dr. Parviz Holakooei from Art University of Isfahan, and Rashed Rahdari for their valuable help and support during different stages of this project. The authors also are grateful to Prof. Ian Freestone, UCL Institute of Archaeology, for his helpful discussions on different stages of this study, and his constructive comments on an earlier version of this article. We are also grateful to Prof. Elizabeth Stone for her
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