20 PAINTING MATERIALS
beaten glair is better. The white is separated from the yolk and is thoroughly
beaten in a platter with a wooden whisk until it sticks to the platter even when
that is turned bottom-side up. Then the platter with the froth is left in a cool
place, tilted slightly, until the glair liquid has settled out. With this the colors are
tempered. Of this medium Thompson says (The Materials of Medieval Painting,
PP- 55-56):
It is a delicate binder, very modest and retiring and inconspicuous; and it preserves
the individual quality of a pigment beautifully. . . . Glair is rather weak and
brittle, especially when newly made, and partly for this reason (which militated
against its use in strong concentrations), partly because it was not dense enough to
bring out the full quality of some pigments, it was often supplemented in book
painting by gum arabic.
Egg White (see also Egg Tempera and Egg Yolk) contains in quite different
amounts the same substances found in the yolk. Church (p. 72) gives the per-
centage proportions of these as follows:
Water.................................. 84.8
Albumen, vitellin, etc..................... 12.0
Fat or oil............................... 0.2
Lecithin................................ trace
Mineral matter.......................... 0.7
Other substances........................ 2.3
The albumen is the adhesive substance of egg white and is complex, containing,
besides carbon, hydrogen, nitrogen, and oxygen, about 1.6 per cent of sulphur.
As a pure film it is clear and brittle and is readily dissolved by water. Church has
suggested (p. 73) that if paintings in tempera, before they are quite dry, were
heated to 70° or 75° C. the albumen would be changed to an insoluble form, or
that treatment with tannin would serve the same purpose. Egg white as a medium
is called also by its other name, * glair.'
Egg Yolk (see also Egg Tempera and Egg White) is an oily emulsion in which
the oil particles are suspended in a solution of albumen. Its average composition
is given by Church (p. 72) in percentage proportions:
Water.................................. 51.5
Albumen, vitellin, etc..................... 15.0
Fat or oil............................... 22.0
Lecithin................................ 9.0
Mineral matter.......................... i .o
Other substances........................ 1.5
The lecithin is a fatty substance to which has been given the empirical formula,
C42H84NPO0, but it differs from most fats (see Oils and Fats) in containing nitro-
gen and phosphorus and in being very hygroscopic. It evidently acts as an emulsi-