MEDIUMS AND ADHESIVES 35
Mat Varnish (see also Resins and Waxes) is usually prepared by adding white,
purified beeswax, dissolved in turpentine, in the proportions of I : 3, to mastic or
dammar resin.
Medium is the word usually applied to the binding material or vehicle that
holds together pigment particles in paint. The relation of the quantities of these
two principal ingredients of the paint film has been studied, particularly with
regard to oils (see Oils, relation to pigments). Frequently the word is mistakenly
used to name a fluid which has no film-forming properties but is added actually
as a diluent* Examples of this are turpentine, commonly used with oil, or water
used with tempera.
Megilp (see also Mastic) is a paint medium made by dissolving mastic resin in
turpentine, with the addition of linseed oil. This medium, which is gelatinous in
consistency, has been used by artists on account of its excellent working qualities.
It gives an enamel-like effect but becomes brittle and yellow with age.
Methacrylate Resins (see Acrylic Resins).
Mineral Oils are not oils at all in the sense that artists' oils are (see Oils and
Fats). They are hydrocarbons (compounds of carbon and hydrogen) obtained in
the distillation of petroleum. They are not used in painting, except rarely as
plasticizers for some resins.
Mineral Waxes are all unsaponifiable on treatment with caustic alkalis; they
contain no alcohols, but consist entirely of hydrocarbons. They occur in the
earth's crust and may be divided into (i) those obtained by distillation—paraffin
wax from petroleum and shale, and (2) those obtained by direct mining—ozo-
kerite. (See also Waxes.)
Montan Wax (see also Waxes) is obtained from lignite or peat by extraction
with petroleum ether or similar solvents. It is a material of high but variable
melting point and is largely used for the same purposes as carnauba wax. Samples
from different sources show different composition, but all contain esters of nion-
tanic acid (C^H^COOH) and, also, the free acid. The alcoholic components have
not as yet been clearly characterized; there is always a certain amount of hydro-
carbon also present.
Mordant is the adhesive film which, in a tacky state, is used to catch and hold
gold leaf laid over it. Concerning the origin of mordant gilding, Thompson
(The Materials of Medieval Painting, p. 202) says:
Quite early in the Middle Ages, certainly before the twelfth century (when docu-
mentary evidence first begins to be common), someone found that if he made a
mark with gum or glair on parchment, and clapped a bit of gold leaf on it before
the mark was dry, the gold would stick, and when it was dry it could be burnished
bright. From this it followed that he could write a letter with some sticky material
and put leaf on it and burnish it, and get something like the effect produced by
writing with gold ink*