Sheet Gelatine........5 oz.
White Sugar........2 ,,
Water . ........14 „
Soak the gelatine in the water till soft, add the sugar and apply heat sufficient to dissolve, stirring frequently. Coat as thickly as possible on glass plates or paper. (Winstone's thick, lithographic, photo-transfer paper will give considerable relief, but the skilful operator may get a thicker coating of the gelatine on glass, especially if he glues a margin of paper round the edges.) Sensitise in a 7 per cent. solution of potassium bichromate, to which enough ammonia has been added to give a light yellow colour. To accelerate drying, methylated spirit may be added. Dry in a warm room away from light and expose, under a suitable negative, till the details are well out.
Development is accomplished by prolonged soaking in water until the maximum relief is reached. In winter tepid water is necessary, and the transfer paper will generally require warm water with a little ammonia before the gelatine becomes highly absorbent. When ready the swelled print is laid face upwards on a piece of level plate glass, superfluous moisture blotted off, and a wooden frame placed round it to act as a rim.
Take a suitable quantity of Brucciani's fine plaster of paris, stir in water till of the consistency of a thin cream, and continue stirring till the mixture shows signs of beginning to set, when it must be poured quickly into the mould formed by the wooden frame, and flatted over with a palette knife or glass plate. After about half an hour the plaster cast may be lifted off and dried. This cast is a reverse, and the final relief must be made from it. To accomplish this the plaster matrix is warmed on a hot plate or in an oven for some time, and the surface made to absorb a considerable amount of paraffin wax, the best method being to dip it into a vessel containing the melted wax. A new rim of wood is laid on it, and the plaster poured on. In making the permanent cast a spoonful of sodium silicate or 50 per cent. of alum solution may be used in the mixing water.
More delicate detail is attained, if the first cast on the gelatine is made of soft wax, by melting up ordinary wax candles in a saucepan. On this wax matrix the final plaster is poured, and the wax detached by the application of heat. Negatives with plenty of contrast are the only ones suitable for plaster relief work.
At The Fountain.
Dr. W. M. Daniels.