7g PAINTING MATERIALS
A useful synthetic resin can be made by replacing part of the acetate groups
of polyvinyl acetate by acetaldehyde. It is known as polyvinyl acetal. Elliot
(loc. cit.) describes it and says that protective films made from it have greater
toughness, hardness, and adherence than polyvinyl acetate, but that resistance
to weathering is not so good. Woodbury has described the use of this resin in an
anthropological museum for impregnating decayed and brittle bone specimens.
The polyvinyl acetals so far produced have a pale straw color in solution.
Polyvinyl alcohol is of some interest because it is soluble in water to the
extent of forming a ao per cent solution, but it is insoluble in most organic sol-
vents. In water the polyvinyl alcohol behaves as a reversible colloid. Solutions
are similar to those of dextrin, albumen, and gum arabic. Its film-forming proper-
ties seem to be as good as those of the polyvinyl esters. It can be used as a water-
color medium, and, in aqueous solutions, as a sizing agent for fabrics (see Ellis,
Chemistry of Synthetic Resins, p. 1058).
Because of their recent development, the vinyl resins have had a small place
in painting. Herbert E. Ives and W. J. Clarke, however (' The Use of Polymerized
Vinyl Acetate as an Artist's Medium/ Technical Studies^ IV [1935], pp. 36-41),
have suggested in detail the way to use polyvinyl acetate as a medium; Stout
and Gettens have described its use, also, for the treatment and transfer of Oriental
wall paintings.
Viscose (see also Cellulose Coatings). Viscose is a regenerated cellulose now
widely used in the making of rayon and transparent wrapping films (cellophane).
It is made by treating sulphite wood-pulp, first with sodium hydroxide and then
with carbon disulphide. The product, cellulose xanthate (a yellow solid), is a
definite though unstable compound, is soluble*in caustic soda (viscose proper),
and from this caustic solution cellulose can be precipitated rapidly by an acid
in a setting bath. By forcing the viscose through spinnerets directly into the acid
bath, viscose rayon threads are formed. Transparent films (cellophane) are made
by extruding a viscose solution into a long, narrow slit in a setting bath. After
passing through baths to neutralize the acid, it passes through a glycerine solu-
tion from which it takes up 17 per cent. The process is continuous and a sheet
of any length can be produced. Riegel says (p. 347) that cellophane is made
moisture-proof and waterproof by passing the sheet, after the glycerine bath,
through a dilute lacquer solution with rather volatile solvents such as ethyl
acetate; after passing through driers, the lacquer, with the proper amount of
plasticizer, remains as a coating.
This regenerated cellulose is not soluble in organic solvents. Wire netting
and fabrics, however, can be impregnated by precipitating the viscose directly
upon them. Heaton (p. 426) says that it has been suggested that the glue sizing
of artists* canvas be replaced by a viscose sizing since a canvas treated this
way would be less likely to mold. For application, the canvas is first impregnated