BAS-RELIEFS Plaster or wax casts in low relief produced from photographs ; formerly they were popular for brooches, cameos, etc. Generally, the relief obtained is very small. The process depends primarily upon the hardening action of light upon gelatine impregnated with potassium bichromate. A negative should be specially taken for the purpose of the first experiments, a suitable subject being a modelled bust or a sculptured bas-relief. Landscapes are prac tically impossible subjects ; while portraits should not be attempted unless the sitter's face and hair have been powdered to imitate a bust, great care has been taken with the lighting, and the photograph has been taken in the brightest possible light against a perfectly black back ground. Any heads attempted should not exceed 3 in. in height, as the relief obtained is not sufficiently strong for larger sizes. Success depends upon the contrasts in the negative, and if not sufficient these should be increased by local intensification or by working on the back of the negative.
Having obtained a suitable negative, a sheet of No. 4 gelatine is soaked in a solution oft drm. of potassium bichromate in 6 oz. of water. The swollen gelatine is next squeegeed on to a well waxed glass plate and dried slowly in the dark, it being then stripped from the glass and its polished side printed upon from the negative.
An alternative method of preparing the bichromated gelatine is to soak 5 oz. of gelatine (Nelson's No. i) and 2 oz. of powdered gum arabic for four hours in a mixture of oz. of acetic acid and 18/ oz. of water ; at the end of the four hours dissolve by gentle heat, strain through clean linen, and coat some polished glass plates with it, avoiding air-bubbles and dust. Dry the plates in a well-ventilated dark room as rapidly as possible, maintaining the temperature at from 50° to F. (io° to 24° C.).
Either gelatine sheet or plate is printed by daylight in contact with the special black and white negative. Exposure depends upon the strength of light and density of the negative, the average duration being thirty minutes. The gelatine shows a faint image, which is a slight guide.
The gelatine sheet, after printing, is cemented firmly, face upwards, to a piece of glass, by means of liquid glue or similar adhesive, and the whole is then soaked in cold water for several hours, at the end of which time the parts acted upon by light are found to have lost their power of absorbing water, while the other parts swell considerably. If the relief is not sufficiently
pronounced, it may be increased by soaking in a solution oft oz. of citric acid in 4 oz. of water and transferring to water. When swelled as much as possible the superfluous water is drained and removed with blotting-paper, some oil poured on and drained off, and it is then ready for casting from.
In the case of a glass plate instead of the gelatine sheet, after printing it is soaked in a solution of 2 oz. of powdered alum and 3o drops of glacial acetic acid in 40 oz. of water. At the end of two hours a fairly good relief should be secured. If the relief is not enough the negative may have been unsuitable, the exposure under the negative not long enough, or the plate dried too slowly after sensitising. The fluous moisture is blotted off and the relief oiled.
Whichever method has been employed, the result so far is an oiled gelatine mould, from which a cast is now to be made in plaster-of-paris or wax. The mould is placed in an old printing frame or a tray ; or, instead, wooden sides are built up round it to form a receptacle for the plaster. All surfaces which the plaster will touch, except the relief itself, should now be smeared with vaseline. Mix up some perfectly fresh plaster-of-paris to about the consistency of cream, immediately strain through muslin, and without losing time pour on to the oiled relief to a depth of -} in. When the plaster has set it may be separated from the relief and the latter used again after soaking in water and oiling.
In addition to plaster, stearine, spermaketi, and even heavy brown wax, make excellent casts. Coloured waxes may be made according to the following formula; (given by H. E. Black burn) : Red.—Wax 50o grs., India red 64 grs., carmine lake 90 grs. Sepia.—Wax 50o grs., sepia 5o grs., lampblack to grs. Green.—Wax Soo grs., cobalt blue io grs., Indian ink 5o grs. Blue.—Wax 50o grs., Frankfort blue loo grs., alizarin blue 15 grs., Indian ink 5o grs. Warm black.—Wax 50o grs., lampblack 5o grs., burnt umber 6o grs., indigo 32 grs. If desired a thin layer of wax of one colour may be brushed on the mould, allowed to set, and the mould then filled up with wax of another colour. The plaster gives more permanent results.