Under the general term varnishes are comprised those
solutions of gums which, when allowed to dry in thin layers,
leave the gum as an adherent covering, either in combination
with the solvent or otherwise. They are employed in all
instances where it is desired to give to solid surfaces a
brilliancy which they would not otherwise possess.
When of good quality, varnish also preserves from time
effects of air and moisture the articles to which it is
applied. A good varnish should possess great hardness,
combined with a certain degree of tenacity, so as to afford
polish, and it should be unalterable by atmospheric
influences. It may be colored or not.
The resins and gums usually employed are: mastic, amber,
copal, shellac, sandarac, elemi, colophony, and asphalt. The
menstrua are: ordinary and methylic alcohol, spirits of
wood, spirits of turpentine, linseed and poppy oil.
alcohol-like solvents are employed, time products are termed
“spirit varnishes;” and when oils are the vehicles used,
“fat or oil varnishes” are produced.
Those for which
alcohol, ether, or benzol have been used are least durable.
Those made by spirits are more durable, as the turpentine
becomes partly oxidized and forms with the gum a tough and
very durable compound of turpentine.
Such varnishes are seldom used alone, but generally employed
in combination with oil varnishes. With respect to
durability, especially in relation to heat and moisture, the
latter are excelled by none.
The manufacture of spirit varnishes is exceedingly simple,
it being only necessary to pulverize the gums finely and to
heat them slightly with the solvent, while, in order to
prevent balling together of the resins they are mixed with a
uniformly coarse glass powder.
The solution itself is mostly
effected in a distilling apparatus of copper, which is so
arranged that the volatilized liquid may uninterruptedly
flow back in a condensed state.
Great care ought to be paid to the assortment and selection
of the resins. The lightest colored pieces should be
reserved for fine and colorless varnishes, while the colored
pieces should be put aside for the ordinary kinds.
using them, they should be washed in hot water to remove
impurities, and then well dried. In order to obtain clear
varnishes, it is also necessary to allow them to remain
quiet for some time, after which they are filtered. If
previously digested within freshly burned and cooled
bone-black, they gain greatly in brilliancy.
The use of paper in filtering should be avoided, as the
varnishes run too slowly through it, and moreover, they do
not become perfectly clear. A far better plan consists in
stopping the funnel within a bunch of cotton, and allowing
the liquid to flow through the same.
the preparation of fat or oil varnishes, for which copal and
amber are most extensively used, great care ought to be paid
to the selection of the oil, which should be cold-pressed,
clear, and of considerable age. These three requisites being
complied with, good varnishes may always be expected,
provided proper care has in other respects been bestowed
upon their manufacture. To cause the resins to be more
easily taken up, they are to be previously melted.
convenient apparatus for this purpose consists of a copper
cylinder which terminates at its lower end in a funnel, and
may be well closed with a lid. When the cylinder has been
filled with the gum, which is poured on a perforated plate
resting in the funnel, it is set into a small furnace, in
such a manner that the tube of the funnel may pass through
the grates and through an aperture in the ash-box. Under the
latter is placed a vessel containing the oil, which is
heated by a charcoal fire. If the cylinder is now surrounded
with glowing coals, the resin melts and drops into the oil,
by which it is immediately dissolved.
The boiling of the oil as well as the solution of the resin
is effected in copper pans, which should be more shallow
than deep; they must be of a capacity sufficient for double
the quantity of the charge of oil. The varnish should be
made in dry weather; otherwise moisture is absorbed, and its
transparency and drying quality impaired.
With respect to the mode of heating, a charcoal fire serves
this purpose best, as it yields a uniform heat, and is not
likely to overheat the sides of the pan. The pan is set into
an iron ring, which should only be so wide as to leave the
lower part of the kettle free. The oil is heated very
gradually until it commences to simmer, when the scum is
skimmed off. It was at this stage in the old method that the
dryers (litharge, minium, oxide of zinc, etc.) were added.
The heat is then gradually increased until the oil bubbles
and emits a gaseous vapor, which inflames when a lighted
taper is held near by.
During the boiling, it is a good plan to stir the oil
constantly and to bring it as much as possible in contact
with the air. This may be effected to a sufficient extent by
ladling a part out and letting it again flow in a stream
into the kettle. As to the quantity of copal which oil will
bear, it is about one half its own weight - ten pounds of
copal requiring at least twenty pounds of oil.
the oil varnish is to be mixed with spirits of turpentine,
the latter must be added during continual stirring. If the
boiling-pan is not sufficiently spacious, the mixture must
be effected in another vessel, and the spirits are to be
added while the mixture of oil and resin is still hot.
The boiling-pan is finally well wiped out with some spirits
of turpentine, and then scoured with a rag which has been
dipped into powdered pumice-stone. In this manner all other
utensils are cleansed, then rinsed with pure spirits of
turpentine, and at last dried with a clean cloth.