Oil of turpentine . i oz. i,000 „ Oil of turpentine oxidises by exposing for some time to light and air. The mixture should be filtered and kept from the dust. When dry, the plaque coated with this medium should appear uniformly dull. Any white spots on the picture may now be filled in with a little of the powder colour mixed with the medium above mentioned. If the plaque is given a light firing, just sufficient to attach the powder to the surface, it can be spotted or worked upon with facility.
Firing is the next process. The work will be fired at any pottery for a moderate charge ; but by the aid of a gas muffle the worker can easily do his own. For tiles and plaques the heat must be applied very carefully, gradually increasing the consumption of gas. When the furnace is fully heated the gas is immediately turned out, the chimney covered up, and the whole left to cool, or, instead, the work can be transferred to an annealing chamber. Enamel plaques or medallions on a metal base need not be cooled with such extreme care.
Dust is especially to be avoided in ceramic work ; it is advisable to sprinkle the floor of the room with water, which will probably suffice to lay the floating dust, and on no account should any sweeping or dusting be done immediately preceding any of the manipulations. Charging the air with moisture by means of a spray diffuser or vaporiser prevents the dust nuisance, but makes the room too moist for developing.
It may be said that the image may be printed direct on perfectly flat surfaces, instead of using a temporary support.
There are many possible modifications of the above process. (i) Instead of using the dusting on process, a film prepared with ferric chloride may be employed. The film is exposed under a negative to obtain a plate of various degrees of tackiness, which is then brushed over with powdered enamel colour as before. No trans parency is required. The picture so obtained is coated with collodion, stripped, and treated as before described. (2) Prints obtained by the carbon process, simply substituting enamel colours in the preparation of the film, may be developed upon porcelain as a final support, forming excellent pictures for firing. (3) A
method that is useful where numbers are re quired is to obtain a photo-mechanical impres sion in the usual manner on a special transfer paper prepared with a collodion substratum. A suitably tacky ink is used for printing, and the ceramic powder is dusted over this. The paper is then moistened and removed, while the film and picture, collodion side downwards, are transferred to the porcelain support or plaque, which is first treated with an adhesive medium. (4) The following procedure may be taken as representative of the substitution pro cess. A collodion positive is obtained in the camera by the wet process, fixed, washed, bleached in a 5 per cent. solution of mercuric chloride, again washed and placed in the sub stitution bath, which is to replace the original image, unsuitable for firing, with one of platinum or iridium. A gold and platinum bath, as follows, gives a purple-brown colour : Potassium chloro platinite . . 8 grs. T.5 g.
Gold chloride (i in 6o) drms. 43 ccs.
Water (distilled) . 44- oz. 345 ,, Lactic acid . . 5 mins.
'9 )7 An iridium and gold bath gives a warm black tone :— Iridium chloride . 8 grs. i.5 g.
Gold chloride (i in 6o) 4i drms. 43 ccs.
Water (distilled) . 44- oz. 345 Lactic acid . . 6 mins.
,, After the deepest shadows of the picture have been toned through, wash for a few minutes to remove the free toning solution. Cut round the margins of the film, immerse the plate in a i per cent. solution of sulphuric acid, wash the film after it strips off, transfer to the final sup port of porcelain, collodion side up, and dry. Then remove the collodion film by gently rub bing with a sponge moistened with ether and alcohol, again dry, and dust the image with flux, when it is ready for firing. (5) It is 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. The yellow film is first transferred and fired, 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 transferred and fired.