214 • PAINTING MATERIALS
per cent should be distilled over below I7O°C. When spread, it should evaporate
almost completely, and when poured on clean filter paper should leave no appre-
ciable residue after a half hour. The normally moderate and uniform rate of
evaporation makes turpentine a good thinner for paint and varnish, and it seems
to give paint a particularly acceptable handling quality under the brush.
The components are active chemically. Being unsaturated compounds,, they
absorb oxygen and produce non-volatile, resinous substances. Apparently, also,
some polymerization reactions take place. These, of course, occur only in the
presence of air and are particularly noticeable in a light and warm location,
Poorly sealed containers will allow turpentine to thicken into a sticky substance
which can not longer be used as a safe thinner for paints. If the distillate is pure, it
probably has on paint a thinning action and no other. There has been some belief
that through the formation of peroxides it might act as an oxygen carrier and,
hence, promote the drying of oil films, but any siccative action of this kind is
quite uncertain. Heaton (p. 62) says that there is enough oxidizing action and
enough acidity in turpentine to have a destructive effect on fabrics, and he sug-
gests that its use in connection with artists' canvases should be avoided where
the maximum durability is essential.
All vegetable and mineral oils in a fluid state are miscible with turpentine,
and it dissolves most resins, except those of a fossil origin, and most waxes. It is
immiscible with water, but mixes in most proportions with nearly all organic
solvents. It is not toxic or narcotic, but prolonged exposure to its vapors may
cause headache and sickness, usually temporary, to some persons.
A variety of turpentine, produced chiefly in America, is made by steam dis-
tillation of pine wood logs and stumps. After such wood is first reduced to fine
chips or powder, there is an initial distillation to extract turpentine and pine oil.
The wood is then digested with petroleum spirit to extract the resin. It is said that
the yield of turpentine from this source is between four and eight gallons per ton
of the wood. Wood turpentine, as it is called, produced on a large industrial scale,'
is more standardized and uniform than gum spirits. It is similar to the oleoresin
product, the main difference being that it contains a greater proportion of dipen-
tene, limonene, and terpinene. In consequence, it has a greater solvent strength.
The odor is distinct from that of ordinary turpentine. Except for this, for slightly
different physical properties, and for a narrower distillation range, there is not
much distinction between it and the turpentine from gum spirits. That made by
destructive (dry) distillation of pine wood is an inferior product.
The history of turpentine is a long one. In crude form, at least, it appears to
have been known in classical times or earlier. Greek writers tell about a material
obtained from coniferous woods and juniper, and Pliny (XV, 7) describes pissinum,
made by.boiling pitch from trees and catching the vapors in a fleece spread over
the tops of the vessels. A yellow fluid was wrung out. He also mentions (XVI, 22)
a tarry substance from the destructive distillation of coniferous wood, and that