Gelatine 35 parts
Acetic acid, No. 8 250 parts
Alcohol, 95 deg 50 parts
Water 700 parts
Chrome alum, 4:100 60 parts
Dissolve the gelatine in the acid at a moderate heat, add afterwards the
alcohol and water, and lastly mix the chrome alum by small quantities at a
time.
These substrata are employed to avoid the peeling off of the image. To
prevent the entire desiccation of the gelatine, which is the cause of the
defect above alluded to, it is advisable to add glycerine to the washing
water after the image is cleared. Some operators recommend a coating of
flexible collodion, that is, prepared with castor oil, for the purpose in
question. We do not think that necessary when the transparencies are not
exposed to sunshine. If anything should be applied we would prefer the
encaustic.
Carbon transparencies are invaluable for reproducing negatives in the
original size by the same (carbon) process, or for enlarging by the
collodion or gelatine process. For these purposes they should be made on
the special red tissue manufactured by the Autotype Company, of London,
Eng. They can, however, be made on the ordinary tissues.
Whatever be the tissue employed, the transparencies for the reproduction
of negatives are seldom opaque enough, and should be intensified. This is
done by treating them with a very dilute solution of sodium permanganate,
which colors them olive green.
Transparencies for lantern slides, etc., are best colored with the
couleurs à l’albumine of L. Encausse, sold by J. Reygondaud, Paris
(France). They are transparent.(33)
_Toning and Intensifying.—_The carbon proofs can be toned and at the same
time intensified by reagents acting with chromic oxide.
The dyes or coloring matters precipitated are not opaque, and, as a
consequence, not objectionable for transparencies. The following
processes are the most employed:
Prepare three solutions as follows:
A. Ferric sulphate 5 parts
Water 100 parts
B. Sodium carbonate 2 parts
Water 100 parts
C. Gallic acid 5 parts
Water 100 parts
Dissolve the gallic acid in warm water. Filter each solution. They keep
well.
To tone, the plate is immersed for, say, ten minutes in A, then, after
rinsing slightly, it is placed in B for the same period, rinsed again and
flowed with C until the desired color is obtained. The tone is a splendid
purple black color. If a solution of pyrogallol be substituted to that of
gallic acid, the tone is green, and to a green bordering to black when a
solution of catechu is used, the catechu exerting at the same time a
tanning action on the gelatine. After toning, the plate should be
thoroughly washed.
A similar process consists to wet the plate under the tap, then to flow
over a mixture by about equal volumes of
A. Ferrous sulphate 5 parts
Acetic acid, No. 8 5 parts
Water 100 parts, filter
B. Gallic acid 5 parts
Water 100 parts
When toned, the plate is well washed, then flowed once with the alum
solution and again washed. The tone by this process easily turns to an
inky blue not very agreeable. The action should be stopped a little
before the desired color is obtained.
It sometimes happens that the image in drying intensifies more then
necessary. It can be cleared with a solution of oxalic or citric acid.
A brown sepia is obtained by toning first with potassium permanganate, 1
per cent. of water, then, after washing, with a solution of pyrogallol.
If gallic acid be used instead of pyrogallol, the tone is black. By this
process a great intensity is obtained. A dilute solution of ammonium
sulphide can be employed as a clearing agent.
Pyrogallol and silver nitrate give a warm black tone.
Potassium bichromate followed by silver nitrate form a brick-red
precipitate of some opacity.(34)
Chloride of nickel and potassium ferrocyanate produce a fine brown.
Lime water and alizarine dissolved in alcohol dye violet.
Alizarine and the caustic alkalies produce a variety of tints, from violet
to purple, according to the concentration of the solutions.
Lead acetate and alizarine in ammoniacal solution dye purple.
Potassium ferrocyanide and uranium nitrate produce a warm sepia tone. With
chloride of nickel the tone is brown.
Ammoniacal solution of coralline diluted with water gives carmine red.
Potassium bichromate and extract of indigo produce a fine greenish tone
suitable for landscapes.