No photographic operations can be termed easy in the sense that they allow much margin for careless working. Some, however, are simpler than others, and amongst these must be reckoned the platinum process. The developing and fixing solutions are elementary and the directions few in number. If the latter are faithfully obeyed, success follows as a matter of course; and the results more than compensate for the time and trouble involved in their achievement. It is a case of little toil, less anxiety, and much reward; short exposures, quick development, and rapid drying, all ending in a print full of tone and atmosphere, which is absolutely permanent in a metal more invincible than any other known.

The principle of the platinotype process consists in the reduction of a ferric salt to the ferrous condition under the action of light, and its subsequent action on certain salts of platinum, which it leaves in the black metallic state. It is not economical or advisable for the amateur to prepare his own paper, the number of preliminary solutions being very numerous and needing much variation. The simplest formula is one given by Pizzighelli and Von Hubl:

A. Ferric Oxalate....... 120 gr.

Oxalic Acid....... 8 ,,

Water........ I oz.

B. Potassium Chlorate...... 2 gr.

Solution A....... 1 oz.

Which must be mixed and kept carefully in the dark, all the ferric salts being exceedingly unstable.

C. Potassium Chloroplatinite.....240 gr.

Solution A.......2 3/4 oz.

Water........3 1/2 „

A sensitiser for normal prints of good deep black colour.

D. Potassium Chloroplatinite..... 240 gr.

Solution A....... 1 3/4 oz.

Solution B....... 1 „

Water........ 3 1/2 „

A sensitiser producing brilliant prints from fully exposed negatives.

E. Potassium Chloroplatinite.....240 gr.

Solution B.......2 3/4 oz.

Water........3 1/2 „

For weak negatives.

Potassium chlorate has the effect of increasing contrast by oxidising a small proportion of the platinite into a platinate. A paper well sized, either with gelatine or arrowroot, is sensitised with one or other of the above solutions C, D, or E. Arrowroot tends to give brightness to the image, but gelatine acts somewhat to the detriment of the tone. It is usual to brush the sensitiser over the paper for three minutes or more, after which it is allowed to dry hung up in a room warmed with a laundry stove.

Another formula was given by Mr. A. J. Jarman, in the Camera, for Whatman's paper sized with arrowroot:

A. Citrate of Iron and Ammonia .... 350 gr. Water........ 5 oz.

B. Potassium Chloroplatinite..... 225 gr.

Hot Water........ 5 oz.

C. Saturated solution of Oxalic Acid.

D. Lead Nitrate....... 300 gr.

Hot Water........ 5 oz.

E. Potassium Chlorate . . . . . 60 gr. Water........ 5 oz.

F. Take of A solution...... 1 oz.

E solution...... 2 dr.

Sensitising is accomplished with : A, 1 oz.; B, 1 oz.; C, 4 drops; D, 1 dr.; F, 2 dr. with four or five drops of gum-arabic solution. Brush on two coats with drying between, and print rather deeply.

Printing

The paper is packed in tin calcium tubes, and must always be kept in these, as the slightest exposure to damp before using will spoil it. Some papers which have accidentally become damp may be partially restored by baking in an oven at 100° Fahr., but prevention is better than cure. Even when placed under the negative a waterproof pad, such as a piece of old celluloid or oil sheet used for letter-copying books, is advisable behind the usual paper padding. CC heavy stout paper is the best quality for beginners, and the appearance will greatly be improved by a margin masked with black paper, although this is, of course, not necessary for success with platinum paper. Printing is complete in about a third to a half the time required for a silver print, the image appearing of a greenish colour on the yellow ground, and a little practice is therefore essential before the exact degree to which exposure should be carried can be ascertained. Unlike silver printing, very little difference in the character of the image is secured whether the light be full sunlight or comparatively dull. Hard negatives gain nothing from sunlight, and feeble negatives printed in subdued light do not gain in value. On account of the great sensitivity the progress of printing should always be examined in artificial or at least subdued light.

Development By Hot Bath

The hot bath is not very widely used nowadays, except for some sepia papers or for prints that are lacking in contrast. It consists of oxalate of potash in a solution of 120 grains to the ounce of water, which is heated at time of development to about 140-1600, and the print brought face downwards over the surface of the water. Draw it rapidly backwards and forwards over the solution to prevent bubbles, and then lift it up. Development has taken place at once like a lightning-flash, and the print must be rapidly transferred to the fixing bath -

Hydrochloric Acid.......1 oz.

Water.........60 „

- where it must remain for ten minutes and then pass into a second fixing, or rather clearing, bath of the same composition, and lastly into a third. These acid baths serve to remove the superfluous iron salts, and the paper may afterwards be rinsed for a short time and dried. The developing solution may be employed for several prints in succession.

Sepia papers frequently contain a mercury salt, and are often much more sensitive than the black. They continue to be sensitive even after development until thoroughly cleared; and they must be protected even more carefully from damp. The developer recommended by the manufacturers should be used with these papers, or a little mercury chloride added to the usual developer. Development is carried on in the hot bath at a temperature of 1600.

Cold-Bath Development

The colder the bath, the colder the image, and conversely the warmer the bath the warmer the image. So, even with cold-bath papers, it is occasionally worth while to warm the developer. But given a negative of reasonable gradation, the best platinum prints are those developed cold. Compared with these, hot-bath prints often have a dull, flabby appearance, which is partly due to the size soaking off the paper.