THREE-COLOUR PHOTO-CERAMICS It is quite possible to obtain a burnt-in picture in natural colours by the exact superposition of a blue, a red, and a yellow picture printed from three negatives obtained through suitable screens, as described in the section on " Photography in Colours.'' The powder process appears to be the most suitable for this purpose, although the photo-mechanical method may be used. The yellow film is first transferred and fired ; this is then allowed to cool, and the blue image very carefully superposed on this in exact register and burnt in. After again cooling, the red film is finally transferred and fired, when the tricolour ceramic picture is complete. The greatest attention must be paid to securing colours properly adapted to rendering their individual portion of the picture while combining well with each other. Transparency and depth are especially necessary. A certain amount of retouching will be required at each stage, though as little as possible should be done. The introduction of colour effects, it goes without saying, renders the whole process far more difficult ComEINATIoN EFFECTS.
A good deal of scope will be found for the production of artistic effects by a judicious use of tinted grounds, fired before the image proper is laid on. A
method known as "colouring behind" may be sometimes used to advantage. This consists of obtaining a faint print in a warm colour to act as a ground, and, after firing this, to roughly tint in the desired final colours, using, of course, enamel powders. This must. be done rather deeper than it is desired to be when finished, somewhat after the fashion of the back tinting in a crystoleum pic ture, taking care to leave the high lights untouched. This roughly-coloured picture is now fired, and, when cool, the final transfer, which is to supply the definite outline and shading, is applied. A suit able tint for this is a rich sepia, or any other dark brown ; black will hardly look so well. Care should be taken to obtain a good glaze in the last firing. If it is desired at any stage to remove small portions of colour in the lights after firing, this is readily done by applying a dilute solution of hydrofluoric acid to the intended part with a fine camel-hair brush, immediately placing under the tap. It is better to do this gradually, with repeated washings, than run the risk of taking off too much colour at once.