For very weak negatives, reproductions of drawings, etc., we use—
| Platinum solution | 12 parts |
| Chlorate of iron | 11 parts |
| Distilled water | 2 parts[21] |
To obtain proofs not completely black, as, for example, reproductions of lead drawings, the solution may be diluted with half or the whole volume of distilled water. But if the solution be applied on little absorbent surfaces or on paper strongly sized it is not advisable to dilute it.
Preparation, of the Paper.—The paper should be kept slightly moist in order that it does not too completely absorb the sensitizing solution. Therefore, when the atmosphere is very dry, 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 [pg 75] 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—184 deg. Fahr.),[23] in an agate glazed [pg 76] iron tray placed upon a water bath at the above temperature. By simply drawing the proof over it, the image is at once developed.[24]
When the proof is thought to be over-exposed, the oxalate solution can be employed at a lower temperature. If, on the contrary, it is under-exposed, the solution may be heated even to the boiling point.
The developer can be used over and over again. It should always have an acid reaction.