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 deg.
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