(B) cause : May be due to insoluble impurities in the chloroplatinite of potassium. These spots have a black nucleus, with an extension like the tail of a comet, of lighter color. In such a case filter the sensitizing solution.
Platinotype prints may be made upon many other materials besides paper. Linen or other fabrics are treated in the same manner as paper. To keep smooth it should be stretched on a suitable frame after the second coating of the gelatine sizing solution. To print on wood it should be planed perfectly smooth and then coated with the gelatine solution. Thin sheets should be kept between two boards to prevent them from warping.
Sepia Platinotype is a special paper prepared by the Platinotype Co. for giving sepia-toned pictures. It is believed that this result is obtained by the use of a salt of mercury with the platinum. With a few exceptions all the operations are similar to the ordinary kind of platinotype paper. The following special instructions are given :— The sepia paper is more easily affected by faint light, and, therefore, increased care must be taken during the printing operation.
To develop, add to each ounce of the potassic oxalate solution one or two drachms of a special solution supplied for the purpose, and proceed as described for ordinary platinotype paper. The temperature should not exceed iso degs. to 16o degs. Fahrenheit. The developing bath should be kept in the dark, and must on no account be used for black prints. Discoloration of the white of the image is due to one of the following causes —( I) Want of sufficient " special solution " in the developer ; (z) too much exposure of the developing solution to light ; (3) use of a dish is which the enamel is cracked so as to expose the iron ; (4) paper kept too long : (5) exposure of prints to too much light while clearing. The print are cleared with an acid bath of one part of hydrochloric acid (sp.gr. 1•16) to 6o parts of water. As the sepia prints, unlike the black ones, may be affected by light when in the acid bath, the lights being stained and degraded, the prints at this stage should be manipulated in a very weak light. The prints are damaged by being left long in the acid baths. The subsequent operations are the same as for the other kind of paper. Dishes used for sepia prints must not afterwards be used for developing black toned
prints. Black and sepia prints must not be washed together in the same dish.
Process —A new method of platinotype printing introduced by Mr. Willis. The novel feature about this is that the platinum salt is used in the developing solution instead of in the paper. The following notes concerning it are given by the inventor : Paper is coated with ferric oxalate and a small quantity of mercury salt, then exposed to the light, and afterwards developed on a cold solution containing potassic oxalate and potassic chloroplatinite. The solution of ferric oxalate employed is the same as that used in the present process, both as to the strength and acidity. In each ounce of this ferric salt is dissolved from i to grains of salt of mercury, preferably the chloride. It is then dried perfectly, exposed to the action of light beneath a negative, then developed on a cold solution containing from 3o to 20 grains of oxalate of potash and from 5 to 15 grains of potassic chloroplatinite. The develop ment proceeds sufficiently, slowly to allow of its being watched and stopped by immersion of the print in the acid clearing as soon as the desired strength of the deposit has been the attained. The following are the instructions given for working the paper as supplied by the Platinotype Company : General Treatment of the presence of moisture in the paper, either during its exposure to light, or afterwards and before development, is important. Excessive moisture is neither desirable nor useful. In England the moisture absorbed from the air of a cold room in winter during fifteen minutes is usually sufficient ; or, in summer, about the same time in a dampish room. Sufficient moisture will be present when the paper has lost its crispness, but if allowed to become limp the moisture will be excessive. The best results are usually obtained when the paper has been damped before its exposure to light. But for reasons explained in the next section, beginners will find it better to expose in the dry state. If damped before exposure such damping should not long precede the printing.