MEDIUMS AND ADHESIVES 21
fying agent. When egg yolk is used as a painting medium, it dries to a strong film,
first by evaporation of the water and then by a slow hardening of the oil which
remains suspended in the albuminous matrix. This oil content is greater than that
of the albumen and, in consequence, the ultimate film is very little affected by
water.
Elemi. This is a generic term applied to a large number of resins obtained from
trees of the Burseraceae family. Manilla elemi, or * soft elemi,' the only kind that
has been closely examined, comes from a species of Canarium, C. commune grow-
ing in the Philippines. Other varieties come from South America, Africa, and the
East and West Indies. American or West Indian, Yucatan elemi, is generally
found in commerce. Manilla elemi is a soft, semi-crystalline, yellow resin, with a
fennel-like odor. It is usually viscous like a balsam, but may be quite hard. The
true elemis have comparatively low acid and saponification values, and one per
cent of ash is the highest limit for a good sample. All varieties are easily soluble;
ether, alcohol, chloroform, carbon disulphide, and benzol are effective solvents,
benzine and petroleum ether being less so. Elemi is used chiefly to modify the
consistency of varnish. It is not employed as a paint medium, and, when added to
recently ground colors, gives them the appearance of being covered with frost.
Since the so-called * Dutch Process * of relining pictures on canvas came into
practice early in the XX century, gum elemi has been used in this process as an
addition to waxes, its effect being to increase the tackiness of the wax. It is in-
cluded in a number of formulas given by Plenderleith and Cursiter (pp. 92 and 93).
Emulsions (see also Egg Yolk, Oils and Fats, Oils, history in painting, and
Waxes, history in painting). An emulsion consists of drops of one liquid suspended
in another liquid. In most cases there is an actual film around the globules which
keeps them from coalescing. With any pair of non-miscible liquids, such as oil
and water, there may be two kinds of emulsions, one with drops of oil suspended
in water and one with drops of water suspended in oil. The necessary conditions
for forming a stable emulsion are that the drops shall be so small that they will
stay suspended and that there shall be a sufficiently viscous or plastic film around
each to keep the drops from coalescing. An emulsifying agent is a substance which
goes into the interface and produces a film having satisfactory physical properties.
According to Bancroft's theory, an oil-m-water emulsion is formed if the emulsi-
fying agent at the interface is chiefly in the water phase, and a water-in-oil
emulsion is formed if the emulsifying agent at the interface is chiefly in the oil
phase. For example, sodium and potassium oleates are water-soluble colloids, and
they are excellent for emulsifying oils in water. The gums are also water-soluble
colloids and certain ones are much used in pharmaceutical work for emulsifying
oils in water. Calcium and magnesium oleates form colloidal solutions in oil and
can, therefore, be used to emulsify water in oil; rosin and the resinates behave in
the same way. Since sodium oleate emulsifies oil in water and calcium oleate
emulsifies water in oil, a mixture will behave according to the relative amounts of