Imitation chalcedony: sifted 'snakes' and a non-yellowing varnish
Sifting transparent chalcedony over a fixed ground; a thin sun-cured oil varnish laid 6-8 times
This page continues the recipe for imitation chalcedony. It explains that if you sift transparent chalcedony through tiny holes and clothe the sifted 'snakes' with fixed chalcedony over a transparent ground, the ground will look full of holes where the snaking is broken at its faces. A second passage gives a varnish that keeps the chalcedony from yellowing: the finest oil, cured in sun and open air, brushed on as thinly as possible, wiped smooth with clean hands so no smear remains, then dried in sun or wind, and repeated six or eight times until well dried and lustrous. Leonardo adds that old white thickened oil, thinly applied several times, would also serve.
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Sifted 'snakes' of chalcedony over a transparent ground
Transparent chalcedony sifted through tiny holes yields 'snakes' that are then clothed in fixed chalcedony over a transparent ground. The ground appears full of holes wherever the snaking is broken at its faces.
A non-yellowing oil varnish applied six to eight times
To keep the chalcedony from yellowing, the finest oil, cured in sun and open air, is brushed on as thinly as possible, wiped smooth with clean hands, and dried in sun or wind. The process is repeated six or eight times until dry and lustrous; old white thickened oil, thinly applied, is said to serve as well.