One of the problems we face as historians studying and reconstructing recipes is that the names describing ingredients, tools, and materials change over time, and that the meaning of terms itself changes over time. This is even the case with relatively recent recipes and materials that are in theory unchanged as I recently discovered. As part of my research for the ARTECHNE project, I recently looked at instructions for making anatomical casts from plaster from 1791.
The creation of anatomical casts and models using plaster of Paris became increasingly popular towards the end of the eighteenth century, fuelled by the omnipresence of plaster in the visual arts and interior decoration, and the increasing importance of pathology and later physiognomy within the study of medicine. The latter meant that medical men were looking for durable three-dimensional ways to preserve diseased bodies and body parts that could not be preserved otherwise (e.g. in a preparation), either because decay could not be stopped or because the patient was still alive.
In his 1790 book The Anatomical Instructor, physician Thomas Pole (1753-1829) not only gave advice on how to make anatomical preparations and drawing, but also included over fifty pages on how to create, colour, repair and maintain plaster casts and models. Pole started the chapter on modelling with outlining the relevance of the quality of the plaster of Paris, or calcined alabaster, that was to be used. He explained that
“…that of a middling price is used for making of moulds; the finer sort is for casts, to be poured first into the mould, when properly prepared; after it has formed a layer of about half an inch, more or less, according to circumstances, then the coarser sort is to be used to fill up the mould, or to give it sufficient thickness.”
But exactly what were the various qualities of plaster for sale in London in the 1790s made of? The term ‘calcined alabaster’ tells us little, as alabaster was and is a collective noun that designates both various kinds of light-coloured, translucent and soft stone used mainly for carving decorative artefacts (often the minerals gypsum or calcite – the former much softer than the latter), and a specific compact and fine-grained variety of gypsum. In the decades after Pole’s publication, the French chemist Antoine François de Fourcroy (1755 –1809) would distinguish nine kinds of calcareous sulfate, one of which was sulfate of lime or common gypsum. In an 1810 ‘dictionary of the arts’ Fourcroy’s sulfate of lime or common gypsum was described as follows:
“Sulphat of lime, or common gypsum, or plaster-stone. This substance is white, more or less inclining to grey, interspersed with small brilliant crystals, easily cut with a knife. it is found disposed in Paris. We shall hereafter find, that it is not pure selenite, but owes its most valuable property, as plaster, to the admixture of another kind of earth. (…) Calcareous sulphat is likewise found dissolved in waters, as in the well-waters of Paris; it is never pure, but always combined with some other earthy salt, with base lime or magnesia. This salt has no apparent degree of taste. It decrepitates if a sudden heat be applied to it; it is then of an opaque white, in which state it is called fine plaster, or plaster of Paris: by this calcination it loses about twenty in one hundred.”
As this fragment suggests, plaster of Paris indeed derives its name from a large and very pure gypsum deposit at the Montmartre and Menilmontant hills in Paris – there were plaster quarries at this site at least as early as the year 500. This led “calcined gypsum” (roasted gypsum or gypsum plaster) to be commonly known as “plaster of Paris”, even after the exhausted quarries were converted into Montmartre cemetery and the Buttes de Chaumot gardens respectively in the mid-nineteenth century. Although not all plaster came from Paris at the time Pole was writing, there is a fair chance that much high-quality plaster was indeed plaster from Paris.
Today, gypsum plaster, or plaster of Paris, no longer comes from Paris, but is still produced by heating powdered gypsum to about 150 °C. When mixed with water, this forms a paste that will harden within minutes, producing an exothermic reaction, which means it warms up. You can easily buy ‘plaster of Paris’ from artist’s supplies shops and online retailers, but none of these mention the exact chemical composition. Yet before I (or anyone else) can try my hand at reconstructing Pole’s instructions I will need to find out whether the best, finest plaster of Paris still contains a percentage of lime or magnesia, what the ‘coarser varieties’ that Pole described contained, and whether these are still available.
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 648718), and was supported by the Wellcome Trust (grant number 203403/Z/16/Z).
 Pole, Thomas. The Anatomical Instructor ; or an Illustration of the Modern and Most Approved Methods of Preparing and Preserving the Different Parts of the Human Body and of Quadrupeds by Injection, Corrosion, Maceration, Distention, Articulation, Modelling, &C. London: Couchman & Fry, 1790: p. 202-3.
 Wilkes, John. Encyclopaedia Londinensis, Or, Universal Dictionary of Arts. Vol. 4. London: J. Adlard, 1810: p. 230.