There are three kinds of photographic enlargements used as a basis for
crayon portraits, and, with a little experience, the student can
determine for himself which kind will prove the most satisfactory.
Free-hand crayons are made on Steinbach and other crayon papers,
without any photograph as a basis. Silver enlargements are made on
paper coated with a solution of chloride of silver, which the action of
e light reduces to salts of silver. This is the oldest form of
photography, and has been used since its introduction by Scheele in
1778. Silver enlargements are made by the aid of the sun (and are then
called solar enlargements) or they can be made with the electric light.
Platinum enlargements are a recent advance in photographic printing
with iron salts, the process which has been worked out and patented by
W. Willis, Jr., being a development of such printing. Its principle is
that a solution of ferrous oxalate in neutral potassium oxalate is
effective as a developer. A paper is coated with a solution of ferric
oxalate and platinum salts and then exposed behind a negative. It is
then floated in a hot solution of neutral potassium oxalate, when the
image is formed.
This process was first introduced by Mr. Willis in 1874, and he has
since made improvements. He claims that the platinotype paper does not
contain any animal sizing. The early experiments convinced him that the
paper upon which the image was to be printed would prove an important
factor, as all photographic paper contained animal sizing, which was
found to be antagonistic to platinum salts. The action of platinum
salts upon a paper containing animal sizing gave it a tint which no
amount of acid washing could remove. For the past nine years Mr. Willis
has had manufactured for his special use a Steinbach paper, free from
the animal sizing, and he also uses a cold developer, thereby causing
the paper to retain its original elasticity.
The chief points of difference between bromide enlargements and silver
or platinum enlargements are that, in the former, we have the sensitive
compound of silver suspended in a vehicle of gelatin, and, in the
latter, a thin coating of an aqueous solution of the sensitive salts.
In the former process, the image is not shown until the paper has been
developed in the bath, while in the latter, the image is shown upon the
paper when it is exposed to the light; so that, in the latter, the
image or picture has only to be fixed or made permanent, while in the
former, it is developed, then fixed. The gelatin bromide paper is
coated with a solution of gelatin, bromide of potassium and nitrate of
silver, developed with a solution of oxalate of potash, protosulphate
of iron, sulphuric acid and bromide of potassium and water, and fixed
with hyposulphate of soda. It is manufactured in America by E. and H.
T. Anthony & Co. and by the Eastman Dry Plate Company.