EXAMINATION OF PIGMENTS.
Besides the chemical tests for purity and adulteration, which
necessarily must vary with each pigment, there are certain other
examinations which partake rather of a mechanical nature, and which
are applicable to practically all pigments without any modification.
They are directed chiefly to ascertaining fineness, body, colour, and
durability.
FINENESS.--Fineness may be tested for as follows:--A tall
glass cylinder is filled with clean water, and about ½ oz. of the
pigment under examination is well shaken in the water; the glass is
placed on one side to settle out, and the length of time taken to
settle may be noted for future reference. The finer the sample, the
longer the time it takes to settle out; and the time in seconds may be
taken as an approximate estimate of the fineness of the sample.
BODY OR COVERING POWER.--An equal and exact quantity, say 50
gr., of the sample under examination and of a standard sample of the
same degree of fineness is weighed out, and placed on two separate
sheets of paper. To each sample is added an equal quantity, say 15
gr., of vegetable black or of very finely ground barytes, according
as the pigment is a light or a dark-tinted kind. The ingredients of
each sample are most intimately and completely mixed, and the tints
of the two mixtures are compared by observation with the naked eye.
The pigment which most nearly retains its own colour, possesses the
greatest body or covering power.
COLOUR.--The colour or tint of a pigment can only be
estimated by comparing it with a standard sample; this is done as
follows:--A sheet of black paper, with, a dead surface, is spread
out on a table in front of a window, a small heap of the standard is
placed on the paper, and next to it a similar heap of the sample to
be compared; by means of a palette knife the surface of the two heaps
is flattened out; on now carefully looking at the two heaps, the one
which has the purest colour can readily be picked out. The heaps should
be looked at from several points of view before a final judgment is
arrived at. If the pigment is dark-coloured, it should be spread on
white paper, taking care that the same kind of paper is used for the
two samples.
_Durability._--Durability is not a difficult point to test, but it
takes some time to make a complete test. The best method is to mix a
small quantity of the pigment in question with raw linseed oil, cover
a piece of glass with the mixture, and expose it outside to the action
of the sun and air for some time, noting at intervals how it behaves.
It is well for the sake of comparison to coat a second piece of glass
with the mixture, and keep this in a dark place. The difference between
the two from time to time will show how the pigment behaves under the
influence of light and air. As the durability of pigments is decidedly
different according as they are used in oil or water-colour painting,
the oil in the former case acting as a protective agent, it is well
to use a similar test, using a little gum water as a vehicle to mix
the pigment with. It will take from two to three months at least to
properly test the durability of a pigment in summer, while in winter
the time will be increased considerably. Glass is the best substance to
use, as it is quite neutral, and does not of itself introduce into the
test any injurious element, as wood or paper might do, although these
bodies may be used if thought desirable.