MEDIUMS AND ADHESIVES 19
complicated emulsions, possibly with oil as the continuous phase (see Emulsions).
A more simple medium which makes use of the whole egg is that described by
Cennino Cennini, c. LXXII (Thompson, The Craftsman's Handbook, p. 51). He
speaks of a tempera for wall painting, made of the white and yolk of an egg into
which are put some cuttings of young shoots of a fig tree. These are beaten well
together. A very rare form of egg tempera was developed by the Indians of Canada
(see Douglas Leechman, * Native Paints of the Canadian West Coast/ Technical
Studies, V [1937], pp. 206-207). They used, among other mediums, eggs from
various species of salmon, sometimes taken fresh, sometimes dried, and some-
times worked up by being chewed in the mouth together with a piece of red cedar
bark.
The egg tempera which is traditional and reflects the practice of many cen-
turies is that made simply with yolk of egg. It is described by Thompson in The
Practice of Tempera Painting (p. 96):
Take a raw fresh hen's egg, and crack it on the side of a bowl. Lift off half of the
shell, keeping the. yolk in the lower half, and letting the white run into the bowl.
Pass the yolk back and forth from one half shell to the other several times without
breaking it, so as to get rid of as much of the white as possible; and pinch off between
the shells the little white knots which adhere to the yolk. Put the yolk into a cup,
and break it, stirring up with it one or two tablespoonfuls of cold water. It does
not much matter how much water you add; a little more or less makes no difference.
You will probably develop a preference for a thick egg mixture or a thin one as you
get used to it, and either is all right. The main point of adding the water is to cut
the greasiness of the yolk a little, and make it fairly liquid. Pour it into a four-ounce,
glass-stoppered, wide-mouthed bottle.
He recommends adding to this two or three drops of vinegar or 3 per cent acetic
acid as a preservative and to make the medium less greasy. Into the egg yolk as
prepared, the colors are mixed. They have already been ground in water and about
equal parts of pigment paste and prepared yolk are put together, proportions being
adjusted to the needs of each pigment, and the whole thinned out with water.
White of egg or glair has probably been most used as a medium for illuminating
books, and for powdered or * shell* gold, and for bole. The traditional use of it is
described particularly in two MSS. One of these is in Naples, Biblioteca Nazionale
MS. XILE 27; it is translated with notes by Thompson and Hamilton, De Arte
Illuminandi (New Haven: Yale University Press, 1933). The other is published
also by Thompson, * The De Clarea of the So-Called " Anonymus Bernensis," *
Technical Studies, I (1932), pp. 8-19, and 69-81. The former is of the XIV century
and the De Clarea is described by this translator (p. 11) as * a fragmentary extract
from a lost work of the second half of the eleventh century.' There is little to be
added to that treatise so far as preparation of the glair is concerned. The author
distinguishes two kinds—one made by beating and the other by pressing. The
latter sort is squeezed through cloth and is contaminated in the process* The