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)

ARMENINO.

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they retouch the naked figures, and produce a most beautiful effect,
because they make hatchings upon the painting, as it is usual to do,
while drawing upon paper with black lead. And for this purpose
they use a rather large hair pencil of minever, with which they work
carefully and gradually, in the way in which granite is painted.
Some persons temper these dark tints with gum, some with thin glue,
and some with yolk of egg (tempera), with which latter vehicle they
are darker and more permanent than with the others. This I affirm
from what I have both seen and done, and also w’hat I have been
told by the best painters. And, therefore, pictures painted in secco
upon canvass, should be retouched in this same manner. Armenino,
De Veri Precetti. p. 147. Ed. Pisa, 1823.