On the subject of creating a passable keratin substitute for genuine tortoiseshell, the eminent French lawyer and aristocratic handyman, Louis Georges Isaac Salivet, wrote in 1792 (under the allonym of Louis-Eloy Bergernon – in order to obfuscate any associations with his avid readers and clientele at the Royal Court and thus hopefully evade the guillotine):
Horn is very extensively used, and especially the horn of cattle. The best kinds come from Ireland. It is worked and soldered in much the same way as tortoise-shell. The following is the method adopted.
Select a good piece of cow’s horn and saw it about two to four inches from its solid end, and cut it in the direction of its length with a back-saw. Then hold it for some time in front of a moderate fire, seize it with pincers and dress it until it takes the form of a flat-board. Next remove by means of a tool called a scraper, the surface which has been exposed to the fire, and finish the process by trimming it by means of a rasp-file. The plates now made in France are at least equal if not superior in transparency to those made in England.
Dissolve in a pint of boiling water about three ounces of potash. Allow this mixture to boil for about a quarter of an hour, and then pour it out into a vessel of double the capacity, and containing about half a pound of quick-lime. Stir the whole well up. When the lime has become thoroughly slacked and cool, add to it about three ounces of red lead, and one ounce of cinnabar or vermilion, and agitate the whole again until all the elements which compose it are perfectly united. When they are, the mixture should have the consistency of thick soup and be of a soft red colour.
The mode of using this composition is very simple. Take a small portion of it on the end of a spatula and apply it to those parts of the horn which are to be coloured, avoiding those parts which are to remain transparent in order to imitate as much as possible the caprices which nature displays in distributing the colours of the tortoise-shell. The shell must remain covered with this paste till the whole has completely dried. Then wipe the piece of horn with a moist sponge; it will be found to be so well coloured in some places and transparent in others that it might easily be taken for tortoise-shell. The thicker the patches of plate the richer will be the different colours. It is then very easy to vary the tints so as to increase still further the resemblance of the piece of horn to tortoise-shell.