Chemical removal of the upper script
Three major recipes were used to recuperate the faded lower texts. One of them was based on gallnut, which is a gall produced by insects on an oak tree, which resembles a nut. The oak-gall solution is an alcohol-based essence of oak galls which enhances the ancient metallic inks of faded scripts and turns the parchment into a dark brownish colour and sometimes, if applied in large quantities, black. The iron inks of the upper scripts were the major reason of such corrosion. This was the method preferred by Angelo Mai for its efficiency.
The other recipe was the liver of sulfur tinctures. The shared principle of these recipes is “that the metallic traces of the ink” in the removed lower texts “precipitate through contact with the various sulphide solutions and thus help to freshen up the optical effect of the old ink traces” (cited by Albrecht, Between Boon and Bane, p. 154). The third widespread recipe is the ‘Gioberti tincture’ invented by the Italian chemist, Giovanni Antonio Giobert (1761-1834), which involved the application of hydrochloric acid and then a mixture of six portions of water and 1/8 of potassium ferrocyanide. This is the recipe described by Amedeo Peyron (1785-1870) who used it to decipher a Turin manuscript that included unknown speeches of Cicero (cf. English citation by Albrecht, Between Boon and Bane, pp. 156-157). By mentioning “Idrosulfuro di potassa” and “Prussiato di potassa” on Vat. gr. 19, IIv + IIIr, Angelo Mai seems to be referring to this recipe. We can see the results: the lower scripts turned bluish grey or green and can be more easily distinguished from the upper script; the gallnut brought out the best results. All these three recipes, however, are intrusive and harmful to the parchment. Nevertheless, they remained in use throughout Europe, wherever palimpsests were kept and studied, until the end of the nineteenth-century.
Vat. gr. 19, f. IIIr with Mai's annotation