you actually need it in bad
drying colors, and then very little of it.

The essences of turpentine and of petroleum may be used to thin the
paint, and are preferable to oil, because they have less darkening
tendency. They do not, however, bind the color so well, and the paint
should not be put on too thinly with them. Usually there is enough oil
ground with the pigment as it comes in the tubes to overcome any
probability of the paint scaling or rubbing when thinned with
turpentine, but in the slow-drying, transparent colors there will be a
liability to crack. Moderation in the use of any and all vehicles is
the best means of avoiding difficulty. Use vehicles only when you need
them, not habitually, and then only as much as there is real need of.
If you use oil, use the lighter oils, and expect some darkening in
time. Prefer turpentine to oil, and expect your color to dry rather
"dead," or without gloss, by its use. If you intend to varnish, this
is all right. If you do not intend to varnish the picture, keep the
color as near the pure tones as you can. The grayer the color, the
more the "dead" or "flat" drying will make it look colorless.

=Varnishes.=--When the picture is done, after it is dry, varnishes are
used to bring out the freshness of color, and to preserve the surface
from outside influences of all sorts. A picture must be well dried
before it is varnished, or it is likely to crack; six months is not
too long to be safe. If you are in a hurry to varnish, use a temporary
or retouching varnish.

The best varnish is necessary for use on pictures. Never use any
except a varnish especially made for the purpose by a reliable
colorman. Those made by Winsor and Newton may all be depended upon.
Pay a good price for it, and don't use too much.

Mastic varnish is that which is most favorably known. Be sure you get
a good and pure quality.

Varnishes are made from various gums or resins dissolved in a solvent
such as alcohol, turpentine, or oil, as the case may be. The lighter
gums are the best for pictures, because they do not affect the color
of the picture. Much care should be used in putting on the
varnish--that it is even and as thinly distributed as will serve the
purpose. It should not be flowed on, but carefully worked out with a
clean brush, and then kept from dirt and dust until dry.

The finer varnishes in oil or turpentine are best for ordinary use.
Those in alcohol do not hold their freshness so well.