[...] Prof. Stefano Zamponi [...] feels « che la vernice non sia una polvere (il termine non sarebbe sensato in italiano) ma un liquido, una vernice cioè, ed esattamente quella vernice che si dava col pennello sulla carta, per evitare un assorbimento eccessivo dell'inchiostro (talora la vernice è data solo all'interno dello specchio di scrittura... e quindi è oggi perfettamente visibile) ». Indeed the shiny aspect of many Italian paper manuscripts, often only (approximately) on the written area as Zamponi describes it, is a notable feature. It is often apposed in a haphazard way, with coarse strokes which do not cover all of the relevant terrain and often overshoot into the margins; it is made with some instrument of say half a centimetre in width. I tend to think that it is polishing, with a hard stone (an agate), but am far from certain.
As to the vernice, however, I wish to refer to the recipe given by Palatino himself (p. 240 of the Ogg edition), which runs (according to the Osley translation, p. 93) :
I think this already makes clear that the mixture of crushed egg-shell and crushed incense, which you spread with a hare's foot, is a powder, not a liquid. The recipe is strikingly similar to a fifteenth-century Dutch recipe (unpublished, found in Utrecht UB 387 f. IIv, together with a number of other interesting recipes):
This also is evidently a powder. (In Dutch too the word normally means a varnish.)
Further I wish to quote from L. Rockinger, « Zum baierischen Schriftwesen im Mittelalter », Abh. d. Hist. Classe der kön. Bayerischen Akademie der Wiss. 12.1 (1872) 3-72, 12.2 (1873) 169-230 - an insufficiently known goldmine. On p. 21 he quotes a recipe for creta, which is to be « strewn over » a fresh palimpsest : it consists of two parts of ground eggshells and one part of chalk (creta) ground with white of eggs, « which others use by way of vernisium ». The pulvis vernisy quoted from a Tegernsee source on p. 26, however, which is « a powder one puts on paper or parchment so it does not flow and writing stands cleanly on it », is a different composition.
The relationship of these powders (for which the English name is pounce, and a French word seems to be groi(n)son) with the group of preparations called creta should be investigated. (On pounces, one should read M. L. Hodgson in C. M. Lamb, ed., The Calligrapher's Handbook, 1956, p. 83-86 !) Both vernice and creta normally combine an amount of calcium carbonate (ground egg-shell, marble, chalk...) with organic matter, sometimes a protein but more often a resin - incense in the Palatino and the Dutch text, but often sandarac. And in fact the word vernice is also found as a synonym of sandarac. Both the powder and the fluid are called vernice, not because of their physical appearance (which is different) but because they both contain sandarac (or something similar).
Finally I quote: « Vernice è anche una specie di gomma pulverizzata, con cui si strofina la carta primo di scrivere; detta comunem. Sandracca », from N. Tommaseo, Diz. della lingua italiana 7, 1916, 1803, who quotes sources.