Merrifield, Mary P.: The art of fresco painting, as practised by the old Italian and Spanish masters, with a preliminary inquiry into the nature of the colours used in fresco painting: with observations and notes (London: Charles Gilpin, 1846)

LEON BATISTA ALBERTI. 23
seen in the works of the ancients, that factitious gems,a fastened to
the wall, with wax, or perhaps with white stucco, become so hard, in
course of time, that they separated neither with fire nor water.
You would say that it was burnt glass; and I have seen some who,
with the white flower of lime, have fastened colours, particularly
vitrified colours, to the walls while still fresh.”
a The words in the Italian translation of Cosimo Bartoli, which I have used,
are, “ colore di gemme,” by which I think he means the coloured pastes, of
which artificial gems are made, like those described by Cennino (translation),
p. 74, and by Cicognara. See note to Cennino, p. 149.