According to Mr. Borlinetto a sepia tone is obtained by using the following cold developer:
| Saturated solution of potassium oxalate | 120 parts |
| Saturated solution of copper chloride | 13 parts |
| Oxalic acid | 1.5 part |
After developing the proofs are immediately immersed for fixing in a solution of hydrochloric acid, 1 to 80 of water, renewed so long as the paper is tinged yellow (about three times), leaving the proofs ten minutes in each solution. Lastly, they are washed to remove the acid.
The platinotype has been still improved by Captain Pizzighelli, who devised the following methods of operating by which the impressions are obtained by the continuous action of light, that is, without development, thus rendering the platinotype just as simple as the ordinary printing-out silver process.
In these new processes to the sensitizing solution is added the alkaline oxalate, which effects the reduction of the platinous salt during the exposure to light. Consequently the prepared paper is insolated until the image appears as it should be, or—which is exceedingly useful in cloudy weather—until it is entirely visible but still deficient in delicate half tones, for in the dark the action proceeds and the image developing itself will be found finished in a period which may extend to a few hours. But it can be, however, developed in a few seconds by immersion in a cold or slightly warm solution of sodium carbonate, 1:25 of water. The image is fixed as directed in the foregoing process.
The paper, prepared exactly as in the former process and kept in the calcium box until wanted for use, should not be employed quite dry, but allowed to absorb a little moisture by hanging it in the dark room. Hence, the India rubber and other protecting pads can be dispensed with. They are even objectionable, for dampness is absolutely necessary to promote the chemical changes by which the image is developed.
| A. AMMONIO-FERRIC OXALATE SOLUTION | |
|---|---|
| Ferric oxalate solution | 100 parts |
| Neutral ammonium oxalate | 18 to 20 parts |
| B. SODIO-FERRIC OXALATE SOLUTION | |
|---|---|
| Ferric oxalate solution | 100 parts |
| Neutral sodium oxalate | 15 to 18 parts |
To prepare these two solutions the ammonium or sodium oxalate is dissolved by small quantities at a time, and when the emerald color due to the formation of the double oxalate commences to darken, the saturation being then complete, no more of either salt should be added. The solution is now well shaken with 3 parts of glycerine, allowed to settle and filtered.
Any one of the double oxalates can be used. The ammonium tends to produce softer pictures and bluish tones. To obtain more contrasts a little potassium chlorate may be added.