SOLVENTS AND DILUENTS 187
preferable as a lacquer solvent because it is miscible with a wider range of solvents
than is the usual industrial product and because it is not so likely to cause bloom.
Unlike the ordinary alcohol, it is miscible with turpentine and benzene. It is,
however, strongly hygroscopic and does not remain anhydrous unless carefully
protected from the air.
Alcohol probably came first into use as a paint material for the preparation of
spirit varnishes—soft resins in a suitable solvent. The slight literature that exists
in connection with the restoration of pictures mentions it as a solvent to be used
for the removal of old varnish film. The softening and gelation caused by alcohol
vapors on varnish films is the basis of the so-called' Pettenkofer process ' formerly
used for the regeneration of paintings (see Max von Pettenkofer, Uber Qlfarbe und
Konsermerung der Gemdlde-Galerien durch das Regenerationsverfahren [Braunsch-
weig, 1870]; also Eibner, pp. 449-452). In this process the painting was put in a
sealed chamber with alcohol vapors. The solvent vapors coalesced the edges of
the cracks of the varnish film, made that film continuous, and gave it added
transparency and the appearance of a revarnished surface. Sometimes copaiba
balsam was first applied and was driven into the film by the vapors. The assertion
is frequently made that alcohol is a strong solvent for oil paint, but the description
of the oil paint is not given. Possibly action on immature films or films containing
some amount of resin as an admixture have been the basis for this belief, although
it remains to be categorically denied.
Ammonia (NH3) and Ammonium Hydroxide (NH4OH). The colorless gas is
very soluble in water. Concentrated ammonia water is 28 to 29 per cent NHs and
has a specific gravity of 0.90. It is also very soluble in alcohol. It goes out of solu-
tion much more rapidly than the fluid evaporates until, in the case of water, the
concentration of dissolved ammonia falls to 6.1 per cent. There the evaporation
rate is the same for both and the gas can be expelled from the water only by heat-
ing. In water the solution is alkaline, and takes the name, ammonium hydroxide.
Because of alkalinity, it neutralizes acids and forms ammonium salts. It is a weak
alkali, however, and the weakness seems to come from its low ionization. A dilute
solution has certain uses because it provides a weak alkali which leaves no residue
on evaporation of the water. The presence of the dissolved gas lowers the surface
tension of water and allows it more readily to wet surfaces and to penetrate into
fabrics or other rough textures. The mild alkali properties enable it to cut thin
films of oil and grease. The presence of the dissolved gas in the mixture of water
with organic solvents acts rapidly on fatty or greasy materials and can be used
occasionally with safety in removal of them from surfaces of paintings. As a rule,
however, the water content of such a mixture is dangerous, even if the alkalinity
does not threaten the preservation of the linoxyn film.
As ammonium carbonate, a white solid called spiritus salts urinae, it was known
to the early alchemists. Another name for it was c spirit of hartshorn/ because it
was obtained from the dry distillation of hoofs, bones, and horns. This, in effect,
was the only volatile form known up to about the XVIII century. Ammonium