In order to draw with the silverpoint on the paper, it needs a sulphur-containing prime.
The silver abrasion of the pin reacts with the sulphur of the primer to form silver sulphite. Grey-blue lines appear, which only darken over the years. A bad primer can be recognized by the fact that a drawing made on it has very weak light-dark contrasts. Darker shades of grey are hardly achievable. The drawing generally appears to be very pale.
The old masters used gypsum (calcium sulphate = sulphurous) or bone meal (a calcium phosphate) with additions of lead white and egg yolk (egg yolk =
sulphurous) for priming the paper. The bones (mostly from chickens) were burned in a fire and then finely ground to powder with a mortar. The powder was mixed with glue water (rabbit glue) and
coated onto the paper so that it adhered to the paper. The primer was usually slightly grey or, depending on the fashion, also coloured (similar to tone paper). This coloured paper was called
Carta Tinta. With it, light and shadow effects can be worked out better than with pure white paper (White heightening).
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