THE PLATINOTYPE.
This process, discovered by William Willis,[16] yields very fine impressions which wholly consists of platinum and are, therefore, chemically permanent. It has been described theoretically and practically by Pizzighelli and Kübl in a paper for which the Vienna Photographic Society has awarded the Voightlander prize.[17] The following is an abridgment of this important process, as described by the authors:
The paper, calendered or not,[18] is sized with gelatine or arrowroot. The color of the proof with the latter size is brownish black, and bluish black with the former.
To prepare the gelatine solution 10 parts of gelatine are soaked in 800 parts of water and then dissolved at a temperature of 60 deg. C. (140 deg. Fahr.), when 200 parts of alcohol and 3 parts of alum are added and the solution filtered.
To prepare the arrowroot solution 10 parts of the substance are powdered in a mortar with a little water and mixed to 800 parts of boiling water, added gradually in stirring. After boiling for a few minutes 200 parts of alcohol are added and the mixture filtered.
These solutions are employed warm. The paper is immersed for two or three minutes and hung up to dry in a heated room, then immersed a second time and dried by hanging it up in the opposite direction, in order to obtain an even coating.
The potassic platinic chloride is an article of commerce. It should be soluble without residue in 6 parts of water and without acid reaction. In this proportion it constitutes the normal stock solution employed in the various formulas.
The standard ferric oxalate solution is also found in commerce. [pg 73] Treated by potassium ferricyanate it should not be colored blue, nor become turpid when diluted with one-tenth part of water and boiled. The former reaction indicates that it contains no ferrous salt, and the latter no basic oxalate.
The authors give the following instructions for preparing the ferric oxalate solution, to which they attach much importance:
Five hundred parts of ferric chloride are dissolved in 5,000 parts of water and heated to boiling, when a solution of soda is added until the liquid becomes alkaline.[19] About 250 parts of caustic soda are generally employed for this purpose. The precipitate—ferric oxide—is now washed in warm water until the last washing water is quite neutral to test paper, then drained and mixed with 200 parts of pure crystallized oxalic acid. The mixture is then allowed to stand in the dark for several days at a temperature not exceeding 30 deg. C. (86 deg. Fahr.) At first the solution from green turns to a yellow green, and finally becomes almost brown. At this moment the excess of ferric oxide is filtered out and the liquor submitted to a quantitative analysis, the result of which leads to ascertain the quantity of ferric oxalate in 100 parts of the solution and the excess of oxalic acid. The solution should then be diluted with distilled water, such as it contains 20 parts of ferric oxalate per 100 parts of water, and oxalic acid must be added in the proportion of from 6 to 8 per 100 of the ferric oxalate, taking into account the quantity of acid the solution already contains. The solution should be kept in the dark. It is altered by light.[20]
| IRON CHLORATE SOLUTION | |
|---|---|
| Ferric oxalate solution | 100 parts |
| Potassium chlorate | 0.4 parts |
This solution is employed to obtain more contrasts.
| PREPARATION OF THE SENSITIZING SOLUTION | |
|---|---|
| Platinum solution | 12 parts |
| Ferric oxalate solution | 11 parts |
| Distilled water | 2 parts |
This solution gives very soft tones with intense black. To obtain more brilliancy we use the following proportions:
| Platinum solution | 12 parts |
| Ferric oxalate solution | 9 parts |
| Chlorate of iron solution | 3 parts |
| Distilled water | 2 parts |
To obtain results comparable to those which the silver printing out process gives, the following mixture is employed:
| Platinum solution | 12 parts |
| Ferric oxalate solution | 8 parts |
| Chlorate of iron solution | 4 parts |
| Distilled water | 8 parts |
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.
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.
| C. IRON CHLORATE SOLUTION | |
|---|---|
| Solution B | 100 parts |
| Potassium chlorate | 0.4 part |
| D. MERCURIC SOLUTION. | |
|---|---|
| Mercuric chloride solution at 5:100 | 20 parts |
| Sodium oxalate solution at 3:100 | 40 parts |
| Glycerine | 2 parts |
| SENSITIZING SOLUTIONS. | |
|---|---|
| FOR BLACK TONES. | |
| Platinite solution, 1:6 | 5 parts |
| Solution B | 6 parts |
| Solution C | 2 parts |
| FOR SEPIA TONES. | |
|---|---|
| Platinite solution, 1:6 | 5 parts |
| Solution C | 4 parts |
| Solution D | 4 parts |
Intermediate tones are obtained by diminishing the dose of C and replacing it by an equal volume of B. For this process the paper should be sized with
| Arrowroot | 2 parts |
| Sodium oxalate at 3:100 | 100 parts |
To dispense with this preliminary sizing Captain Pizzighelli adds gum arabic to the platinite solution, whereby the sizing and sensitizing are done in one operation.
The gum arabic solutions are prepared as follows:
| E. | Gum arabic in powder | 40 parts |
| Sodium ferric oxalate solution, B | 40 parts | |
| Sodium oxalate solution at 3:100 | 100 parts | |
| Glycerine | 3 parts |
Place the glycerine and the gum arabic in a mortar, then, stirring with the pestle, dissolve by adding, little by little, the mixture, heated to 40—45 deg. C. (104—113 deg. Fahr.), of the solution of sodium ferric oxalate and sodium oxalate. Let stand for about two hours and grind again to dissolve entirely the gum arabic. Filter through muslin.
| F. Mercuric chloride solution, 5:100 | 20 parts |
| Sodium oxalate solution, 3:100 | 40 parts |
| Gum arabic in powder | 24 parts |
| Glycerine | 2 parts |
Dissolve as said above.
| SENSITIZING SOLUTIONS. | |
|---|---|
| FOR BLACK TONES. | |
| Platinite solution, 1:6 | 5 parts |
| Solution E | 6 parts |
| Solution C | 2 parts |
| FOR SEPIA TONES. | |
|---|---|
| Platinite solution, 1:6 | 5 parts |
| Solution C | 4 parts |
| Solution F | 4 parts |
Mix just before use. The solutions do not keep. The paper prepared by either one of these two processes can be exposed as in the old process, and the image developed bythe hot oxalate solution.
The preparation of wood, canvas, etc., for the platinotype printing need not to be described; it suggests itself.