Scientific American Supplement, No. 443, June 28, 1884 | Page 3

have to contend with, and that raw linseed oil seasoned by age is the
only source to bind pigments for durable painting; but how to procure it
is another trouble to overcome, as all our American raw linseed oil has
been heated by the manufacturers, to qualify it for quick drying and an
early market, thereby impairing its quality. After linseed oil has been
boiled, it becomes a poor varnish; it remains soft and pliable when used
in paint, giving way to air pressure from the wood in hot weather,
forming blisters. Turpentine causes no blistering; it evaporates upon
being exposed, and leaves the paint in a porous condition for the gas in
the wood to escape; but all painters agree that blistering is caused by
gas, and on investigation we find two main sources from which gas is
generated to blister paint--one from the wood, the other from the
ingredients of the paint. The first named source of gas is started in hot
weather by expansion of air confined in painted wood, which presses
against the paint and raises blisters when the paint is too soft to resist.
Tough, well-cemented paint resists the pressure and keeps the air back.

These blisters mostly subside as soon as the air cools and returns to the
pores, but subsequently peel off.
W.S. and others assert that damp in painted wood turns into steam
when exposed to sun heat, forming blisters, which cannot be possible
when we know that water does not take a gaseous form (steam) at less
than 212° F. They have very likely been deluded by the known way of
distilling water with the aid of sunshine without concentrating the rays
of the sun, based upon the solubility of water in air, viz.: Air holds
more water in solution (or suspension) in a warmer than in a cooler
degree of temperature; by means of a simple apparatus sun-heated air is
guided over sun-heated water, when the air saturated with water is
conducted into a cooler, to give up its water again. But water has an
influence toward hastening to blister paint; it holds the unhardened
woodsap in solution, forming a slight solvent of the oil, thereby
loosening the paint from the wood, favoring blistering and peeling.
There is a certain kind of blister which appears in certain spots or
places only, and nowhere else, puzzling many painters. The explanation
of this is the same as before--soft paint at these spots, caused by
accident or sluggish workmen having saturated the wood with coal oil,
wax, tar, grease, or any other paint-softening material before the wood
was painted, which reacts on the paint to give way to air pressure,
forming blisters.
The second cause of paint blistering from the ingredients of the paint
happens between any layer of paint or varnish on wood, iron, stone, or
any other substance. Its origin is the gaseous formation of volatile oils
during the heated season, of which the lighter coal oils play the most
conspicuous part; they being less valuable than all other volatile oils,
are used in low priced japan driers and varnishes. These volatile oils
take a gaseous form at different temperatures, lie partly dormant until
the thermometer hovers at 90° F. in the shade, when they develop into
gas, forming blisters in airtight paint, or escape unnoticed in porous
paint. This is the reason why coal-tar paint is so liable to blister in hot
weather; an elastic, soft coal-tar covering holds part of its volatile oil
confined until heated to generate into gas; a few drops only of such oil
is sufficient to spoil the best painted work, and worse, when it has been

applied in priming, it settles into the pores of the wood, needing often
from two to three repetitions of scraping and repainting before the evil
is overcome. Now, inasmuch as soft drying paint is unfit to answer the
purpose, it is equally as bad when paint too hard or brittle has been
used, that does not expand and contract in harmony with the painted
article, causing the paint to crack and peel off, which is always the case
when either oil or varnish has been too sparingly and turpentine too
freely used. Intense cold favors the action, when all paints become very
brittle, a fact much to be seen on low-priced vehicles in winter time.
Damp in wood will also hasten it, as stated in blistering, the woodsap
undermining the paint.
To avoid peeling and blistering, the paint should be mixed with raw
linseed oil in such proportions that it neither becomes too brittle nor too
soft when dry. Priming paint with nearly all oil and hardly any pigment
is the foundation of many evils in painting; it leaves too much free oil
in the paint, forming a soft undercoat.