Historical Painting Techniques, Materials, and Studio Practice

Why have some green paint layers of verdigris and copper resinate turned
brown while others remained intact? After a search through the treatises for
instructions on how to use this dangerous green pigment and a comparison
of the written instructions with the actual methods of applying the pigment
on the paintings, results seemed to suggest that three interesting factors determine
appearance: (1) verdigris has to be thoroughly embedded in oil or
an oil-resin varnish to be protected from air and humidity; (2) colored underlayers
contribute to the beauty of the green glaze; and (3) admixtures of
yellow lakes or yellow glazes on top of the transparent green layer were
applied to soften the sharpness of the bluish-green tone of verdigris and
copper resinate.
The instructions for the use of verdigris emphasize that it should be thoroughly
incorporated into the medium and covered with varnish as soon as
possible. The early use of verdigris for pictura transludda on gold leaf and tinfoil
described by Theophilus, Cennini, and in the Tegernsee Manuscript (ca.
1500) points to this fact (9, 10, 11). The Strasburg Manuscript simply states
that all colors should be ground in oil. It proceeds to explain how to mix
colors and how to achieve good results by painting in several layers, the
painting technique generally found in fifteenth-century paintings (12).
In On the true precepts if the art of painting (1587), Armenini gives very detailed
instructions on painting technique, particularly on how to paint green drapery
(13):
if the drape is to be green, one does as follows: After the sketch is made
using somewhat coarse green, black and white, it is lightly painted with a
mixture if verdigris, a little common varnish, and some giallo santo. With
a coarse brush of miniver, one veils the sketch uniformly; next one pats it
either with the palm if the hand or with a little wad if cotton wool covered
with linen, unta the given colour is uniform and no brush strokes can be
detected. And if the result is not to one's satisfaction, after the veiling is
dry one repaints with the same mixture and then pats in the prescribed
way.
He also explains that before veiling, the thoroughly dried sketch has to be
oiled out very thinly to stop the glaze from being repelled by the underlayer.
A painting by Palmezzano depicting the Mystic Marriage if Saint Catherine,
signed and dated 1537, shows exactly this technique in the green drapery of
the throne and St. Catherine's green garment (Plate 9). The pattern of the
textile used for dabbing on the glaze is clearly visible (Plate 10). The buildup
shown in the cross section also corresponds to Armenini's instructions
(Plate 11); even the oiling-out layer is visible in ultraviolet light (Plate 12).
The glaze was apparently too viscous to be spread out evenly with a brush,
therefore the glaze was spread by dabbing it with a rag. Traces of textile
pattern are also visible in a Flemish altarpiece, painted in Antwerp around
1520, now at Oxburgh Hall in Norfolk. In the process of dabbing on the
green glaze the artist could not always keep within the outline of his green
drapery, so he had to retouch the background in some places. Minor overlaps
were simply left; one can see the weave pattern of the rag, the glaze is partly
discolored and some green particles are still visible.
Experimentation
As an experiment, some neutral recrystallized verdigris was ground in linseed
oil and the mixture was heated very gently to approximately 50 °C, until the
copper acetate had dissolved and the pigment grain had disappeared. The
mixture was intensely green and quite viscous. It could be spread with a
brush while warm, but congealed very quickly, making the brush strokes very
coarse and imprecise. However, it was quite easy to spread the glaze by dabbing
it on with a piece of canvas. After a few hours, the glaze dried. Within
a few days the thick brush strokes of green glaze showed drying wrinkles,
whereas the dabbed area looked the same as when it was applied, demon-
66
Historical Painting Techniques, Materials, and Studio Practice