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Platinotype

solution, drachms, oxalate, water, paper, ferrous and potassium

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PLATINOTYPE PROCESS.—A process of positive printing from the negative, giving pic tures of remarkable softness and artistic quality. As far back as 1832 Sir J. Herschel gave an account of his experiments on the action of light upon salts of platinum. Later on in 1844 Hunt pointed out the fact that if a piece of paper be dipped in a solution of a platino-cya nide of potassium, and hung up to dry in the sun, no change was perceptible ; but if after a short exposure it be treated with mercurous nitrate, a weak positive image was produced.

The first practical platinum printing process was, however, invented and patented by Mr. Willis, of the Platinotype Co., and who supply all the necessary materials and the paper ready sensitized. Other processes are hereafter described, however, which may be worked by anyone.

The principles of the process may be briefly stated as follows:—Papar is coated with a mixture of potassium chloroplatinite and ferric oxalate. The ferric oxalate is sensitive to light, becoming converted into ferrous oxalate; we therefore get a faint grayish orange-colored image of ferrous oxalate. Now, ferrous oxalate possesses the power when in solution of reducing potassium chloroplatinite to metallic platinum. It, therefore, only becomes necessary to dissolve the ferrous oxalate in a suit able liquid, when the potassium chloroplatinite will be re duced to the metallic state as metallic platinum.

A solution of potassium oxalate possess this requisite power of dissolving the ferrous oxalate. The paper prepared with the chloroplatinite of potash having an image on it of ferrous oxalate is, therefore, floated on this solution, and a picture consisting of finely divided metallic platinum is the result obtained. It then only becomes necessary to dissolve away the remaining iron salts by any suitable acid. The following reactions (according to Berkely) take place +2 +6 KC1+3Pt Metallic platinum being one of the most stable substances known, it is probable that prints by this process are absolutely permanent.

Preparation of the paper is first treated with a size to prevent the sensitizing solution'from sinking too deeply into it. A good stout paper is required of even texture and pure in color.

Iso grains of moderately hard gelatine are dissolved in 3o ounces of water, and 45 grains of alum, together with seven ounces of pure methylated spirit, are added. This is then filtered

into a conveniently large dish, and the sheets passed through it one by one and hung up to dry. When dry it may be passed through a second time and dried. If arrowroot be used as the sizing browner tones are obtained in the finished prints.

Coating the next operation is the coating of the paper with the sensitive solu tion. This should be done in a darkened room, care being taken that the sensitized paper be exposed for the shortest possible time. Lamplight, owing to its yellow color, is not suitable, as the coated parts cannot be easily distinguished from the uncoated.

Captain Pizzighelli and Baron Hubl in their work on the subject* give the following methods of preparing the paper, the variations being made to suit the different class of negatives. These two solutions are prepared— No. 1.

Ferric oxalate .120 grains Water 1 ounce Oxalic acid 8 grains No. 2. No. 1 solution 1 ounce Chlorate of potassium 2 grains Considerable care must be taken that both these two solutions are protected from actinic light, otherwise the ferric salt will be speedily reduced to the ferrous salt.f The sensitizing solution is prepared as follows: Potassium chloroplatinite solution (8o grains to i ounce of water) 24 drachms No. i solution 22 drachms Distilled water .... 4 drachms This should give very soft and deep black prints. If greater brilliancy is required the following is recommended: Chloro-platinite solution 24 drachms Solution No. z i8 drachms • No. 2 4 drachms Distilled water 4 drachms The next solution is recommended when results corresponding to silver images are required Chloro-platinite solution 24 drachms Solution No. I 14 drachms • No. 2 & drachms Distilled water 4 drachms For very weak negatives, reproductions of engravings, etc., use Chloro-platinite solution 24 drachms No. 2 22 drachms Distilled water 4 drachms The addition of the No. 2 or chlorate of potash solution increases the contrast, as it reduces a portion of the platinite into a platinic salt. It will be obvious, therefore, that by a judicious use of it brilliant prints may be obtained even from weak negatives. If the pictures possess no black shadows; as, for instance, in the reproduction of pencil drawings, the above mixtures may be diluted with half or even equal volumes of water. Distilled water should always be used.

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