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,