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.
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