MEDIUMS AND ADHESIVES 29
glues, and proteins, which form similar mucilaginous solutions, in that the latter
are definitely nitrogenous bodies, while the gums contain practically no nitrogen.
They are insoluble in alcohol, do not melt but char on heating, and do not give
off a nitrogenous odor. The one chiefly used for a painting medium is gum arabic;
gum tragacanth and cherry gum are of less importance.
With the exception of gum arabic, the chemistry of plant gums has not been
very thoroughly studied. The plant gums are salts of complex, organic acids,
usually with calcium, magnesium, and potassium. The complex acids are built
up of sugar units in combination with the acidic part of the molecule. C. L. Butler
and L. H. Cretcher (' The Composition of Gum Arabic/ 'Journal of the American
Chemical Society^ LI [1929], pp. 1519-1525) have identified rhamnose, J-galactose,
and /-arabinose in the sugar fraction of the hydrolysis product of gum arabic.
Calcium, which is the principal metal of this gum, is present as the salt of aldo-
bionic acid. The solution of gum arabic may be precipitated by basic lead acetate,
and it is thickened or rendered turbid by the addition of solutions of borates,
ferric salts, or alkaline silicates. Mixtures of copper sulphate and sodiun/hydrox-
ide, and of neutral ferric chloride and alcohol are valuable confirmatory tests.
Gum arabic yields about 3 per cent ash, consisting of calcium, magnesium, and
potassium carbonates. The percentage of moisture varies with the different
varieties of the gum. Senegal gum contains more moisture (16*1 per cent) than
the Sudan gums, which may make it a superior product, although the Sudan gums
are lighter in color.
As sizing materials and as tempera mediums, gums have probably been used
for a long time. In chapter XXVII of the Schedula diversarum artium of The-
ophilus, early XII century, there is an account of the use of gum in place of sun-
dried oil (see Laurie, Materials of the Painter's Craft\ p. 164). Jehan le Begue
(Merrifield, I, 284) describes the preparation of a rose color in which powdered
brixillium is ground with a gum water made of two thirds gum arabic and one
third clear water. And there is good evidence that long before this it had been a
medium common in the practice of classical painting.
Hempseed Oil. Hemp (Cannabis sativa) is cultivated in Western Europe, in
North America, and in Japan. The oil from its seed is greenish in color and is
slow-drying; it also wrinkles badly and has a somewhat greasy texture.
Honey (see also Tempera) has been used from early times as an addition to
water-soluble mediums, such as gum arabic or size or glair, and as late as the XIX
century was a common ingredient in moist water colors. By retaining a certain
amount of water, it had the effect of keeping these materials from becoming
brittle through extreme dryness. It is made up of two sugars, dextrose and levulose,
with other compounds, and about 20 per cent of water. Church (pp. 82-83)
recommends that a solution of levulose be prepared from pure honey and used
instead of the more complex substance. In modern practice, small quantities of
glycerine take the place of honey in aqueous mediums.