CRAYONS.
Crayons are coloured pencils by which pictures may be, so to speak,
”dry painted”. At present this method of painting is little used,
though it was in vogue in the last century, but coloured pencils,
especially blue and red, are much used for writing. Crayons are now
used in a very convenient form, being generally made in the same manner
as lead pencils. The coloured mixture from which the crayon is made
is produced in the form of a paste containing a soft, finely ground
mineral, well mixed with the colouring matter and an amount of binding
medium just sufficient to hold the powders together.
Gypsum is generally used as the soft white mineral forming the base
of the coloured mass. It is much better to use steatite (soap-stone),
which is not much dearer, and has many advantages over gypsum. The
preference given to steatite is based upon a comparison of the
properties of the two minerals; gypsum is crystalline, steatite is
non-crystalline. Powdered gypsum has essentially a dry character,
whilst powdered soap-stone is of a peculiar greasy nature, and
consequently can be readily smeared upon a flat surface; it also
imparts a pleasing lustre to the colours with which it is mixed.
The manufacture of crayons consists in preparing and moulding the
coloured mass. The process is commenced by mixing the powdered colour
and soap-stone in a closed rotating cylinder. The rotation is continued
until the mixture is uniform in colour. The quantity of colour to
be mixed with the soap-stone depends upon the shade to be produced.
Manufacturers who make a speciality of crayons prepare a number of
shades of each colour. The artists who use crayons, however, usually
require only those of a pure shade, since they themselves can produce
the intermediate shades from the essential colours. On this account it
is advisable to make crayons of distinct and deep colours. The mineral
pigments are the most suitable: for yellow, deep chrome yellow; for
red, vermilion or deep madder lake; for green, one of the pure green
pigments, such as chrome green; for blue, Chinese blue or ultramarine;
for brown, burnt sienna or manganese bistre. For white and black
crayons levigated chalk is used, alone or mixed with a sufficient
quantity of fine vine black or some other good black. Either gum Arabic
or size may be used as binding material. When the former is employed
the crayon becomes so brittle when dry that it breaks when pointed with
the knife, in spite of the greatest care. Size produces a less brittle
crayon, and is also much cheaper, so that it is preferred.
The paste from which the crayons are to be formed is made by mixing
thin size with the colouring matter and steatite to a soft pulp, which
is then kneaded to secure uniformity. The process then proceeds in a
different manner according as the crayon is to be pressed or sawn out.
For producing the crayons by pressing, a simple apparatus is required.
It consists of a horizontal metal cylinder with a well-fitting piston.
The front of the cylinder is closed by a metal plate in which is an
orifice of rather greater diameter than the crayons to be made. In
front of the cylinder is an endless band which moves away from it. In
using this apparatus to shape the crayons, the paste is made of such
consistency that a slight pressure forces it out of the orifice in the
front of the cylinder in a coherent rod. When the cylinder is filled
with the paste, care must be taken that it includes no air bubbles,
for these would cause the rod to break. The piston is then put in
position and the mass forced by uniform gentle pressure out of the
narrow opening. The endless band must move away at the same rate as
the rod proceeds from the cylinder, so that a long stick of the crayon
mass rests upon it. This stick is cut up by a blunt knife into uniform
lengths, which are dried upon boards covered with blotting paper, and
are then enclosed in casings similar to those of the ordinary lead
pencil. The rods must be given a rather greater diameter than they are
to possess when dry, since they shrink somewhat in drying.
The second method consists in producing a thick paste which is moulded
into blocks of the length of the crayons. These blocks are very slowly
dried at a uniform temperature, and when completely dry, are cut by a
fine saw into thin rods, which are then enclosed in a wooden case. The
powder produced in sawing is used in the next operation. This simple
process has many drawbacks. In the first place it is difficult in
drying large blocks of the crayon mass to avoid cracks, which must be
carefully filled with thin paste. Also in sawing up the blocks a large
number of rods will be broken, even with the most careful treatment
(these are again worked up with the powder produced in sawing). Thus,
generally, the formation of crayons by pressure is preferred.
It is important to dry the crayon rods so far that they do not shrink
further after they are placed in the case; otherwise they would break
up when sharpened. At times it is quite impossible to make a usable
point on such a faulty crayon.
=Crayons for Earthenware.=—Crayons for this purpose are made, according
to M. Rösler, by finely powdering colours suitable for glass and
earthenware, and mixing them to a paste with a solution of 2 parts of
gum Arabic and 1 part of Marseilles soap. Pencils are then moulded from
the paste and wrapped in a strip of paper just previously dipped in a
paste of gypsum. Upon unglazed articles or ground glass these crayons
can be used like a lead pencil. The colour is fixed by burning in the
usual manner.