"Chap. 116.--_How to prepare gesso sottile (slaked plaster of Paris) for grounding panels._
"You must now prepare a plaster for fine grounds, called gesso sottile. This is made from the same plaster [plaster of Paris] as the last, but it must be well purified (purgata), and kept moist in a large tub for at least a month; renew the water every day until it almost rots, and is completely slaked, and all fiery heat goes out of it, and it becomes as soft as silk. Throw away the water, make it into cakes, and let it dry; and this gesso is sold by the druggists to our painters. It is used for grounding, for gilding, for working in relief, and other fine works." [p167]
APPENDIX: ON GILDING (_By Graily Hewitt_)
Success with raised gilding can only be expected when practice has rendered attention to the details of the process automatic and there is no need to pause and think. Even then the results must be somewhat uncertain and experimental. For our own preparations of size are usually unsatisfactory, and the ingredients of the best we can buy are unknown to us. And our vellum is certainly not of the quality we find in the old books. Some one is badly wanted to investigate the chemistry of the one and an appropriate preparation of the other. But we can take as much care as our time allows, passing nothing as "good enough" which we have not well examined, and bringing to the business all the patience and deftness available.
Vellum is too stiff, or too dry, or too greasy. When stiff, it is too thick for books; when dry, too apt to crack or cockle; when too greasy, exasperating. And yet the soft and rather greasy sort can be rendered more agreeable than the rest with labour. It should be rubbed by the flat of the hand with powdered pumice (or even fine sandpaper on the rough side) and French chalk, especially on its split (or rougher) side, until it is serviceable. A few trials will teach how long to give to this. Five minutes for one side of a lamb's skin would not be too much. It can then be beaten with a silk handkerchief, but not rubbed with this until the size has been laid. It may be rubbed cleaner between the laying of the size and the gilding. Especially must those parts of pages be thoroughly rubbed clean which in the book, when made up, will lie upon and be pressed against gold letters on the page opposite; or the pumice left behind will scratch them. On the other hand, if the vellum has not been thoroughly pumiced on both pages, the greasiness in [p168] it will dim the gold in time, both from above and below; or even make the size flake off altogether. The size is often blamed for faults of the vellum and its want of preparation.