[Footnote 1: A communication to the North London Photographic
Society.]
Platinotype, which may be considered to be the most artistic of
photographic printing processes, may be separated into its three
modifications--the hot bath and cold bath, in which a faintly visible
image is developed, and the Pizzighelli printing-out paper. The hot
bath process, again, may be divided into the black and white and sepia
papers. I intend to give you a rough outline of the preparation of the
paper and working of these modifications, concluding by demonstrating
the hot bath method, and handing around prints by it.
Platinotype may almost be styled an iron printing process, for, while
no trace of iron or its salts is found in the finished print, certain
salts of iron are mixed with the platinum salt, which is platinum
combined with two atoms of chlorine (PtCl2), as a means for readily
reducing it; this, however, cannot be effected without the presence of
neutral oxalate of potash, hence the use of the oxalate bath. There is
no platinum in the paper for the cold bath process, it being coated
with ferric oxalate mixed with a very small quantity of chloride of
mercury--somewhere about one grain to an ounce of ferric oxalate
solution. When dry it is ready for exposure, which is about three
times less than with silver printing.
It is absolutely necessary to store all papers for platinum printing
in an air-tight tin containing chloride of calcium, which must be
dried by heating from time to time. For the cold bath, however, it is
important to have moisture present during printing, or it may be after
printing and before development. If the paper is left in a dampish
room for fifteen minutes, it should be sufficient. Prints made by
exposing damp paper, or damping dry paper just before development,
must be developed within one hour if the maximum of vigor is desired;
by delaying the development some hours, the prints in the meantime
being stored in a drawer so that they may retain their moisture, an
increase of half tone and warmth of color will be obtained. If it
should be necessary to delay development for a day or two, the prints
must be dried before a fire soon after being removed from the frames,
and then stored in a calcium tube until wanted for development.
While printing, the lemon color of the paper receives a grayish
colored image, which, although faint, can, with practice, be judged as
easily as silver printing.
The developer consists of oxalate of potash and potassic
chloro-platinite--about thirty grains of the platinum salt to half an
ounce of oxalate forming about six ounces of solution; a great many
variations, however, may be made in the proportions of platinum salt
and oxalate, and different effects secured. Development is effected by
sliding the print face downward on to the developer, which must be
rocked after the development of each print to avoid scum marks. To
clear the prints they are washed in three or four baths of a weak
solution of hydrochloric acid after leaving the developer, to remove
all traces of the iron salts, and finally washed for a quarter of an
hour in three changes of water; they are then finished, and may be
dried between clean blotting paper.
Pizzighelli's process differs from the above in being one that prints
fully out in the frame without development; the paper contains the
platinum and iron salts as well as the developer, and so prints and
develops at the same time. Although excellent prints can be produced
with it, for general work the results of the paper, as at present
made, will not compare with the hot and cold bath processes. It is,
however, excellent for printing from very dense negatives, and
occasional negatives that seem extremely suitable for it. The paper
should be breathed on before printing, as if it is quite dry the
printing will be very slow and irregular. The best conditions for the
preparation of the paper have scarcely been decided upon yet, and it
is not quite fair to judge the process. The prints are cleared in the
acid baths and washed for about a quarter of an hour.
The sepia and black hot bath processes are much alike in the general
treatment. There are, however, some special precautions to be observed
with the sepia paper, the chief being to protect it from any but the
faintest rays of light; the prints, unlike the black ones, may be
affected by light when in the acid bath. A special solution must be
added to the developer to keep the lights pure. Over-exposure cannot
be corrected by using a cooler bath, as is the case with the black
prints, and the paper does not remain good so long.
The paper for the black prints by the hot bath process is washed with
a mixture of potassic platinous chloride and ferric oxalate, the
proportion being about sixty grains of the platinum salt to one ounce
of the iron solution. It will not keep good longer than twenty minutes
or so, and must be applied to the paper directly after mixing. The
ferric oxalate in the paper is reduced by the action of light to
ferrous oxalate, which forms the faint visible image; this, when the
paper is floated on the oxalate of potash bath, is capable of reducing
the platinum salt in contact with it into metallic platinum; but the
ferric salt, which remains unaltered, has no action on the platinum
salt, leaving these parts, which represent the high lights of the
print, untouched. The ferric oxalate is removed by the acid baths
which follow the development. A good temperature for development is
150° Fahr., and when using this so much detail should not be apparent
as when printing for the cold bath process, in which all the detail
desired should be very faintly visible. There are, however, many
methods of exposing the paper and developing it, and no fixed rule can
be made, but the development must in every case be suited to the
exposure or the result will be a failure. For instance, the paper may
be printed until all detail is visible, but a very much cooler
development must be used, say 80° or 90°; on the other hand, a
slightly short exposure may be given, and a temperature of 180° to
200° used. 150° should be taken as the normal temperature, and kept to
until some experience has been gained, as employing all temperatures
will lead to confusion, and nothing will be learned. Some negatives
require a special treatment, and both printing and development must be
altered, while for a very dense negative the paper may be left out in
a dampish room for some time. It will then print with less contrast
and more half tone. A thin negative is better printed by the cold bath
process, but negatives should be good and brilliant for platinotype
printing. Any one taking up platinotype and getting only weak prints
would do well to look to his negatives instead of blaming the paper,
as the high lights should be fairly dense, and the deep shadows nearly
clear glass.
Time for complete development should always be allowed; with a hot
bath fifteen seconds will be sufficient, but if a cooler development
is used, or the prints are solarized in the shadows, more time should
be allowed. When the deep shadows are solarized, or appear lighter
than surrounding parts, a hot and prolonged development is required to
obtain sufficient blackness, as they have a tendency to look like
brown paper. I have found breathing on solarized shadows useful, as in
the presence of slight moisture they begin to print out and become
dark before development, getting black almost directly the print is
floated on the oxalate. Three or four acid baths of about ten minutes
each are used, and the prints are washed as before. The process
throughout takes much less time than silver printing, and can be kept
on all the winter, when it is nearly impossible to print in silver.
Prints can be developed in weak daylight or gaslight, and prolonged
washing is dispensed with.--_N.P. Fox, reported in Br. Jour. of
Photo._
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[Continued from Supplement, No. 706, page 11283.]