I32	MODERN PAPER-MAKING
strained through a fine-meshed strainer cloth or bag. Warm, but not hot or
boiling, water can be used, and the colour well diluted, not more than i Ib. to
3 gallons of water, and this only when a large quantity is being used. The stock
must be definitely acid to litmus or the colour will not hold on the machine.
For this reason it is not suited to the colouring of blottings, which require
a huge quantity to make any appreciable effect. Also, a blotting coloured
with ultramarine in any quantity will be found to emit a disagreeable odour
when used to absorb ink.
Smalts Blue is finely powdered cobalt-blue glass. It is quite unaffected by
acids or alkalis, exposure to light or by atmospheric conditions. It is expensive
to use, owing to the fact that about twenty times the quantity is required
compared to ultramarine, and its first cost is high. It is found in hand-made
writings and ledgers, and sometimes in high-class machine-made paper. In
the latter case a great deal sinks in the sand traps and is lost, so that the modern
paper-maker does not look on it with much favour and uses it only when it
is specially asked for.
Prussian jB/tte.—This is a chemical pigment of a green shade. It is too green
to use in white papers, but is very useful in small proportions in azures and
blues that must have a greener tint than can be obtained with ultramarine.
It is affected by alkalis, but not. by acids. Formerly it was produced by a com-
bination of chemicals in the beater, but it can now be obtained in the form
of paste or powder, which give more regular results.
Yellow Chrome.—This also is a chemical pigment which may be made in
the beater, but it is less trouble to buy the colour in paste form. It is a lead
chromate and is very poisonous. The shade varies from a canary yellow to a
brown or orange tint.
Carbon Black.—This is a very useful pigment to have in the mill. It produces
grey tints and can be employed to sadden too brilliant tints or give darker tints
of blues, greens, etc., than can be got by these dyes alone. The paste form of
carbon black is much to be preferred to the dry condition. The latter has an
oily and fluffy character, which prevents it from mixing well with paper stock,
resulting in specks and streaks appearing in the paper after it has passed through
the presses and calenders of the machine.
Ochres and Earth Colours.—This'is a very extensive class of mineral colours
ranging from a light yellow to a dull brown or red. They owe their tints
mainly to the presence of iron oxides. Many are of natural origin, others are
produced by burning or calcining various earths, ochres and clays. They are
all heavy colours, practically permanent, and act as fillers more than dyes.
Nitrate of Iron or Iron Liquor is a chemical combination of iron and nitric
acid. It acts on fibrous materials to produce a brownish-yellow tint, Used