MEDIUMS AND ADHESIVES 23
each present. There will be some ratio of calcium to sodium at which the two
oleates will practically balance each other and the slightest relative change will
change the type of emulsion. Although most emulsions are made with gelatinous
colloids as emulsifying agents, theoretically, this is not necessary. Anything that
will go into the interface and make it sufficiently viscous will give the same result.
If enough of a fine powder is put into the interface, a plastic mass is formed there
which will stabilize the emulsion. It is not always easy to tell by inspection whether
water is the external phase or the internal phase in a given emulsion. One way is
to examine the emulsion under a microscope while a little water or a little oil is
being added. The one that is the external phase will mix readily with the emulsion
and the other will not. If the emulsion is not deeply colored, its type may be
recognized by means of a few minute crystals of a fat-soluble dye, such as Sudan
III or Scarlet R, which are dropped on the surface and give a spreading color to a
water-in-oil emulsion but not to an oil-in-water type.
It is not possible to say to what extent emulsions have been used as mediums
in painting. Berger interpreted the description of Punic wax given by Pliny (XXI,
49) and by Dioscorides (II, 105) as an emulsion. His theory, however, has been
overwhelmingly refuted by the studies of Eibner, Laurie, Schmid, and others.
Modern attempts to explain the Flemish method of the XV century, particularly
that of the Van Eycks, have brought about, among others, the view that both oil
and tempera were used. There is a further difference of opinion, however, as to
whether the medium was an emulsion of the two or whether the two were used
either alternately or in juxtaposition to produce a final result. Maroger has
strongly argued for emulsion as the explanation. It is, of course, possible to emul-
sify either wax or oil, and many experiments have been made in recent times with
both emulsions as painting mediums.
Encaustic (see also Waxes) was a method of painting with wax common in
ancient times. The word refers, literally, to the process of melting or burning the
color into the surface on which it was applied.
Fatty Acids are the organic, aliphatic acids which are combined with glycerine
to form fats and oils (see Oils and Fats). In most fats and oils there is a small
per cent of uncombined, free fatty acid.
Fish Glue is impure gelatin prepared from fish heads, bones, and skins. The
pure gelatin from fish bladders is known as isinglass (see also Gelatin, Glue, and
Isinglass). Glue made from the skins is clearest and best. Usually, fish glue is
marketed in liquid form but it can sometimes be obtained in the form of cakes or
broken sheets which are hygroscopic and readily soluble in water. As a liquid it
contains a preservative and sometimes an essential oil like wintergreen or cinna-
mon to mask the odor. This glue is inferior to animal glues as an adhesive and is
more easily spoiled by bacterial decomposition. Alexander says (p. ai8) that the
joint strength of a common commercial fish glue was only 260 pounds per square
inch, but, according to the Forest Products Laboratory (Technical Note, F-a),
high-grade skin glue should average 1,700 to 1,800 pounds.