This subject has been treated by many, but out of the numerous ideas
that have been brought to bear upon it, the writers have failed to
elucidate the question fully, probably owing to the fact that in most
parts they were themselves dubious as to the real cause. Last year
W.S. gave a lengthy description in the _Building News_, in which he
classified blistering and peeling of paint into one of blistering
only. He stated in the beginning of his treatise the following:
"The subject of blistering of paint has from time to time engrossed
the attention of practical men; but so far as we can follow it in the
literature pertaining to the building trade, its cause has never been
clearly laid down, and hence it is a detail enshrouded in mystery."
W.S. dwells mostly, in his following explanations on blistering
paints, on steam raised in damp wood. Also an English painter,
according to the _Painters' Journal_, lately reiterates the same
theory, and gives sundry reasons how water will get into wood through
paint, but is oblivious that the channels which lead water into wood
are open to let it out again. He lays great stress on boiled oil
holding water in suspense to cause blistering, which is merely a
conjecture. Water boils at 212° F. and linseed oil at 600° F.,
consequently no water can possibly remain after boiling, and a drop of
water put into boiling oil would cause an explosion too dangerous to
be encountered.
It will be shown herewith that boiled oil, though in general use, is
unfit for durable painting, that it is the cause of most of the
troubles painters 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. For durable
painting, paint should be mixed with as much of a base pigment as it
can possibly be spread with a brush, giving a thin coat and forming a
chemical combination called soap. To avoid an excess of oil, the
following coats need turpentine to insure the same proportion of oil
and pigment. As proof of this, prime a piece of wood and a piece of
iron with the same paint; when the wood takes up part of the oil from
the paint and leaves the rest in proportion to harden well, where at
the same time the paint on iron remains soft. To be more lucid, it
need be explained, linseed oil boiled has lost its oleic acid and
glycerine ether, which form with the bases of pigments the insoluble
soap, as well as its albumen, which in boiling is thrown out. It
coagulates at 160° F. heat; each is needed to better withstand the
action of wind and weather, preventing the dust from attaching itself
to a painted surface, a channel for ammonia in damp weather to
dissolve and wash off the paint. In later years linseed oil has been
extracted from linseed meal by the aid of naphtha and percolation, the
product of a very clear, quick drying oil, but lacking in its binding
quality, no doubt caused by the naphtha dissolving the fatty matter
only, leaving the glycerine and albumen in the meal.
All pigments of paint group according to their affinity to raw linseed
oil into three classes. First, those that form chemical combinations,
called soap. This kind is the most durable, is used for priming
purposes, and consists of lead, zinc, and iron bases, of which red
lead takes up the most oil; next, white lead, the pure carbonate Dutch
process made, following with zinc white and iron carbonates, as iron
ore paint, Turkey umber, yellow ocher; also faintly the chromates of
lead--chrome-green and chrome-yellow, finishing with the poorest of
all, modern white lead, made by the wet or vinegar process. The second
class being neutrals have no chemical affinity to linseed oil; they
need a large quantity of drier to harden the paint, and include all
blacks, vermilion, Prussian, Paris, and Chinese blue, also terra di
Sienna, Vandyke brown, Paris green, verdigris, ultramarine, genuine
carmine, and madderlake. The last seven are, on account of their
transparency, better adapted for varnish mixtures--glazing. The third
class of pigments act destructively to linseed oil; they having an
acid base (mostly tin salt, hydrochloride of tin, and redwood dye),
form with the gelatinous matter of the oil a jelly that will neither
work well under the brush nor harden sufficiently, and can be used in
varnish for glazing only; they are not permanent in color, and among
the most troublesome are the lower grades of so-called carmines,
madderlakes, rose pinks, etc., which contain more or less acidous
dyes, forming a soft paint with linseed oil that once dry on a job can
be twisted or peeled off like the skin of a ripe peach. All these
combinations of paint have to be closely observed by the painter to
insure his success.
Twenty-five years ago a house needed to be painted outside but once in
from five to seven years; it looked well all the time, as no dust
settled in the paint to make it unsightly. Painters then used the
Dutch-process-made white-lead, a base and raw linseed oil, a fat acid,
which formed the insoluble soap. They also put turpentine in the
following coats, to keep up the proportions of oil and pigment. All
held out well against wind and weather. Now they use the
wet-process-made white lead, neutralized by vinegar, with oil
neutralized by boiling, from the first to the last coat, and--fail in
making their work permanent.
W.S., in the _Building News_, relates an unaccountable mysterious
blistering in a leaky house, where the rainwater came from above on a
painted wood wall, blistering the paint in streaks and filled at the
lower ends with water, which no doubt was caused by the water soaking
the wood at the upper ends where there was no paint, and following it
down through the fibers, pushed and peeled off the soft, inadhesive
paint. Green, sappy, and resinous wood is unfit for durable painting,
and to avoid blistering and peeling wood should be well seasoned and
primed with all raw linseed oil, some drier, to insure a moderately
slow drying, and as much of a base pigment as the painter can possibly
spread (much drier takes up too much oil acid, needed for the pigment
base to combine with), which insures a tough paint that never fails to
stand against blistering or peeling, as well as wind, weather, and
ammonia.
The coach, car, and house painter can materially improve his painting
where his needs lie by first oiling the wood with raw oil, then
smoothing the surface down with lump pumicestone, washing it with a
mixture of japan drier or, better yet, gold sizing and turpentine,
wiping dry, and following it up with a coat of white lead, oil, and
turpentine. The explanation is: the raw oil penetrates the wood and
raises the wood fibers on the surface to be rubbed down with
pumicestone, insuring the best surface for the following painting: to
harden the oil in the wood it receives a coat of japan drier, which
follows into the pores and there forms a tough, resinous matter,
resisting any air pressure that might arise from within, and at the
same time reacts on the first coat of lead as a drier. This mode
insures the smoothest and toughest foundation for the following
painting, and may be exposed to the hottest July sun without fear of
either blistering or peeling.