Gesso = gypsum
a prepared surface of plaster as a ground for painting.
Gypsum
Hydrous calcium sulphate, the mineral from which plaster of Paris is made.
Whiting - a preparation of finely powdered chalk
Chalk consisting of carbonate of lime with some impurities.
Gesso.
Several specimens of Gesso have been examined: all consisted of carbonate
of lime (i.e. whiting, chalk) mixed with blue and in no case was there any
admixture of sulphate of lime (gypsum). Carbonate of lime in this form
possesses no adhesive property whatever and it was in order to make it
cohere and adhere that glue was added.
The specimens of gesso examined varied in thickness from 0.1mm to 1.0 cm.
In many cases a layer of coarse woven fabric is used underneath gesso to
enable it better to adhere to the material on which it is placed which is
generally wood, but occasionally faience.
A.L.
Skin found in gesso from shrines
Answered Feb. 12th 1931
34, Upper Hamilton Terrace, N.W.8.
December 29th 1930.
My dear Carter,
All the best wishes possible for the New Year and for the continued success of your work. I had intended to write in time for Christmas but did not get it done. I have been busy with one thing and another especially as our Laboratory is to be taken over entirely by the Trustees of the British Museum on 1st April, so I have been distributing to my staff formal notices from the Department of Scientific and Industrial Research that their appointments under it will cease on March 1st. I have not seen the new conditions yet but I do not think they will necessitate much change in any way. We are also to have a new Director of the Museum on 1st January, Dr G. F. Hill, whom I know to be very well disposed to our Laboratory and work.
I see from The "Times" of Wednesday last that you are finishing your work especially on the shrines and I believe I have made quite an interesting discovery concerning the overlying gold leaf and gesso work. I had some very small pieces of this which I brought away in 1924 and I did observe then when detaching the gold leaf by means of dilute acid in order to measure its thickness that there was a semi-transparent residue which was practically of the same shape and size as the piece of gesso. At this time I thought it was only gelatine or albumen which had been rendered insoluble by time, etc., and put it down
the sink. Repeating this work about 3 weeks ago I made some further experiments with another fragment and found this semi-transparent residue to be leather or at least the skin of an animal. This skin had been embedded in the white powder which seems to be nothing but pure calcium carbonate free from any calcium sulphate and mixed with glue or gelatine, not albumen. The authorities in the Egyptian Department have never heard of leather or skin being used in this way and all the pieces of gilt gesso which we have so far got from them are much thinner than that from the shrine. If this is really a new discovery could you let me have some small pieces of the gilt gesso from each of the shrines to test in the same way. When one thinks of it, it is easy to see why the damp skin was so used, especially if the gesso was to be thick and receive deeper markings. We have made microscopic sections of the leather and might, even with the small pieces which we have be able to identify the kind of animal from which the skin was obtained.
I enclose a tiny specimen for your inspection which has been treated with a dye dissolved in spirit. This colours the gesso but not the leather.
Yours very truly,
Alexander Scott
See note by Plenderleith under Chemistry
(September 12, 2008)
