The devil is in the details: turpentine varnish

Corrosion cast of bronchi and trachea, possibly from a rabbit, sheep, or dog, 1880-1890
Likely prepared by Harvard anatomist Samuel J. Mixter.
The Warren Anatomical Museum in the Francis A. Countway Library of Medicine

By Marieke Hendriksen

One of the first things you learn when you do reconstruction research is that the tiniest detail can make a difference.

Recently, I wanted to prepare an injection wax for corrosion preparations according to a 1790 recipe. Corrosion preparations are anatomical preparations created by injecting an organ with a fluid coloured wax that hardens. The organ is then lowered into a container with a corrosive substance, such as a hydrochloric acid solution, which corrodes the tissue, leaving a negative image of the veins and arteries of the organ. These preparations were made from at least the mid-eighteenth century, but because of their fragility, very few remain. As they were supposedly difficult to make, corrosion preparations were not only a way of studying anatomy, but also a tool for self-fashioning and establishing one’s status as an anatomist.

I have tried to create an injected preparation in the past.[1]It was my first attempt at reconstruction research ever, and although it served me well at the time, now I do things differently.

Most importantly, I want to stay much closer to the original recipe if possible. When we made the injected preparations in 2012, we used modern substitutes for some historical ingredients for economic reasons, and we did not have the time to study every ingredient in detail, substituting those we could not find directly with something we thought would have pretty much the same effect.

The recipe I want to use, Thomas Pole’s 1790 instruction for making a corrosion preparation, calls for a coarse red wax, made from fifteen ounces of yellow bees wax, eight ounces of white resin, six ounces of turpentine varnish, and three ounces of vermillion or carmine red.[2]The wax, resin, and pigment are fairly straightforward.

What is turpentine varnish though? Back in 2012, we ended up using just turpentine rather than turpentine varnish, and although those injections were not meant to be corroded, we ran into numerous problems. For example, it turned out to be almost impossible to keep the wax and the organs at a temperature at which we could both handle it and have it fluid enough to inject. It made me wonder whether sticking with the original recipe could solve that problem, so I set out to recreate it.

This turned out to be more complicated than expected, as there is not one standard recipe for turpentine varnish. Eventually I found a Dutch recipe from 1832 listing a turpentine varnish to finish display cabinets for natural history collections.[3] The ingredients are a pound of oil of turpentine, 8 ‘loot’ (a loot being 1/32 Dutch pound) of white resin, four loot of Venice turpentine, and ½ loot of aloe or kolokwint. Raw larch turpentine has a high concentration of volatile oils that can be distilled. The fluid part is known as oil of turpentine, whereas the residue left in the retort is usually called resin, rosin, or colophony. Oil of turpentine is the essential oil that remains after distilling raw larch turpentine. Venice turpentine is a thick, viscous exudation from the Austrian larch tree, which is not used as a varnish on its own as it becomes dark and brittle when exposed to oxygen and light. Aloe vera is widely known; kolokwint (the Dutch name for Citrullus colocynthisor bitter apple) less so. It is a plant with yellow fruits that resemble small pumpkins, which are very bitter and poisonous. That quality might explain its presence in a recipe for a varnish that is meant to ward off insects. Powdered aloe is readily available from artist’s material suppliers, so I went with that.

The varnish after 10 hours in the sun. The Aloe is the clearly visible murkiness on the bottom. Photograph: author.

The preparation of the varnish was pretty straightforward: put all ingredients in a bottle, cover, and leave in the sun for a day. The only problem was that I had to wait a week for a sunny day. When it came, I put in the ingredients and just left the bottle out in the sun for a couple of hours, which allowed me to stir the ingredients together. The aloe however did not resolve properly, and just sits at the bottom of the jar. While this might not be much of a problem when the varnish is applied to a cabinet, it makes this particular turpentine varnish unsuitable for use in my injection wax. Next time, I will make another batch without aloe and use that instead.

Why do I recount this–admittedly not very exciting–story? It shows how difficult it can be to follow a historical recipe to the letter. It also shows how much you learn from reconstruction research, even if it does not always yield the results you’d like, or as fast as you’d like.

[1]Marieke Hendriksen, Elegant Anatomy, (Leiden: Brill 2015), pp. 1-9.

[2]Thomas Pole, 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), pp. 21-5, 122-42.

[3] S. de Grebber, Over de schadelijke huisinsekten, als de huisvliegen, wespen, muggen, weegluizen, vlooijen, luizen, motten, pels-, boek- en kruidkevers en wormen, hout-, blad- en schildluizen, plantmijten enz., met aanwijzing van voldoende en proefhoudende middelen, om dezelve geheel uit te roeijen, Volume 1,(Amsterdam, 1832), pp. 52-3.

What’s in a name: Plaster of Paris

By Marieke Hendriksen

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.

Painted plaster cast of a large fibroma of the jaw, 1830s. Courtesy Surgeons' Hall Museums, RCSEd.
Painted plaster cast of a large fibroma of the jaw, 1830s. Courtesy Surgeons’ Hall Museums, RCSEd.

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.

Johan Zoffany, The Portraits of the Academicians of the Royal Academy, 1771-72
Johan Zoffany, The Portraits of the Academicians of
the Royal Academy, 1771-72. Note the plaster models of antique statues around the room.

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

Illustration of how to make a cast of a diseased bone from Pole's 1790 'Anatomical Instructor'
Illustration of how to make a mould of a diseased bone from Pole’s 1790 ‘Anatomical Instructor’

“…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.”[1]

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.”[2]

Entrance to the Montmartre gypsum quarry. Probably early 19th century, artist unknown.
Entrance to the Montmartre gypsum quarry. Probably early 19th century, artist unknown.

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).

[1] 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.

[2] Wilkes, John. Encyclopaedia Londinensis, Or, Universal Dictionary of Arts. Vol. 4. London: J. Adlard, 1810: p. 230.