This process, discovered by William Willis,(16) yields very fine
impressions which wholly consists of platinum and are, therefore,
chemically permanent. It has been described theoretically and practically
by Pizzighelli and Kübl in a paper for which the Vienna Photographic
Society has awarded the Voightlander prize.(17) The following is an
abridgment of this important process, as described by the authors:
The paper, calendered or not,(18) is sized with gelatine or arrowroot.
The color of the proof with the latter size is brownish black, and bluish
black with the former.
To prepare the gelatine solution 10 parts of gelatine are soaked in 800
parts of water and then dissolved at a temperature of 60 deg. C. (140 deg.
Fahr.), when 200 parts of alcohol and 3 parts of alum are added and the
solution filtered.
To prepare the arrowroot solution 10 parts of the substance are powdered
in a mortar with a little water and mixed to 800 parts of boiling water,
added gradually in stirring. After boiling for a few minutes 200 parts of
alcohol are added and the mixture filtered.
These solutions are employed warm. The paper is immersed for two or three
minutes and hung up to dry in a heated room, then immersed a second time
and dried by hanging it up in the opposite direction, in order to obtain
an even coating.
The potassic platinic chloride is an article of commerce. It should be
soluble without residue in 6 parts of water and without acid reaction. In
this proportion it constitutes the normal stock solution employed in the
various formulas.
The standard ferric oxalate solution is also found in commerce. Treated by
potassium ferricyanate it should not be colored blue, nor become turpid
when diluted with one-tenth part of water and boiled. The former reaction
indicates that it contains no ferrous salt, and the latter no basic
oxalate.
The authors give the following instructions for preparing the ferric
oxalate solution, to which they attach much importance:
Five hundred parts of ferric chloride are dissolved in 5,000 parts of
water and heated to boiling, when a solution of soda is added until the
liquid becomes alkaline.(19) About 250 parts of caustic soda are generally
employed for this purpose. The precipitate—ferric oxide—is now washed in
warm water until the last washing water is quite neutral to test paper,
then drained and mixed with 200 parts of pure crystallized oxalic acid.
The mixture is then allowed to stand in the dark for several days at a
temperature not exceeding 30 deg. C. (86 deg. Fahr.) At first the
solution from green turns to a yellow green, and finally becomes almost
brown. At this moment the excess of ferric oxide is filtered out and the
liquor submitted to a quantitative analysis, the result of which leads to
ascertain the quantity of ferric oxalate in 100 parts of the solution and
the excess of oxalic acid. The solution should then be diluted with
distilled water, such as it contains 20 parts of ferric oxalate per 100
parts of water, and oxalic acid must be added in the proportion of from 6
to 8 per 100 of the ferric oxalate, taking into account the quantity of
acid the solution already contains. The solution should be kept in the
dark. It is altered by light.(20)
IRON CHLORATE SOLUTION
Ferric oxalate solution 100 parts
Potassium chlorate 0.4 parts
This solution is employed to obtain more contrasts.
PREPARATION OF THE SENSITIZING SOLUTION
Platinum solution 12 parts
Ferric oxalate solution 11 parts
Distilled water 2 parts
This solution gives very soft tones with intense black. To obtain more
brilliancy we use the following proportions:
Platinum solution 12 parts
Ferric oxalate solution 9 parts
Chlorate of iron solution 3 parts
Distilled water 2 parts
To obtain results comparable to those which the silver printing out
process gives, the following mixture is employed:
Platinum solution 12 parts
Ferric oxalate solution 8 parts
Chlorate of iron solution 4 parts
Distilled water 8 parts