it is well to keep the paper
in a damp place, in the cellar for example. Before sensitizing, which
should be done by a very diffused light, a quantity of the solution
proportionate to the surface to be sensitized (about 15 c.c., for a whole
sheet of Rives' or Saxe paper) must be measured, and spread with a large
brush(22) on the paper fixed with drawing pins on a board covered with a
sheet of blotting paper. When well impregnated, the paper is hung up to
dry in the dark room, and as soon as the apparent dampness of the surface
has disappeared, it should be dried immediately at a temperature of 30--40
deg. C. (86--101 deg. Fahr). If the paper be dried too rapidly the
sensitive compound remains on its surface, and in developing the image
does not come out well. If, on the other hand, the drying is too slow,
the solution penetrates too much in the paper and the image is wanting of
vigor and does not appear very sharp. One cannot depart from this rule
that the desiccation from the moment the solution has been applied until
the paper is dry should last no more than from twelve to fifteen minutes.

The sensitized paper is hygroscopic and must be preserved in a calcium
box. _It is a conditio sine qua non that the paper must be quite dry
before, during, and after printing, to obtain good results._ Dampness is
the greatest enemy in this process.

For printing a pad of India rubber should be placed over the platinum
paper to prevent it from attracting the atmospheric moisture, and in damp
weather it is even advisable to cover it with several sheets of blotting
paper previously heated before the fire.

The platinum paper is at least three times more sensitive than the silver
paper used in the printing-out process, under the reductive action of
light the yellow color of the prepared paper turns brown and then becomes
of a lighter color, nearly orange, so that the darker parts of the image
often appears more luminous than the dark half tints. No rule can be
given to regulate the insolation, but after a few trials it is easy to
judge when it is right by observing the progress of the reduction and the
color of the image. The orange color indicates the complete reduction of
the ferric oxalate. When the details in the lights are _faintly_ visible,
the exposure is generally right.

The developer consists of an almost saturated solution of potassium
oxalate _acidified by oxalic acid,_ and for use heated to 80--85 deg. 0.
(176