DEVELOPING, ETCHING, ETC. When the plate is considered to be sufficiently dusted, it is carefully drawn, and heated over a Bunsen burner. The heat applied must be only just enough to melt the bitumen, and cause it to adhere to the plate ; on no account must the melted dust be allowed to run together. The carbon print from the transparency is now developed on the grained plate. When this is washed and dried, the edges and back of the plate are protected with bitumen varnish, or other suitable resist, and the etching can be proceeded with. Five solutions of ferric chloride adjusted to the following strengths with a Beaume hydrometer 43°, and required.
These must be used at a temperature of about F., the room being also kept warm. The strongest solution is first allowed to act on the plate for about one minute, or until the shadow portions are etched. This is then poured off, and the solution of the next strength is employed ; this being done in turn with each solution, finishing tip with the weakest, until the highest lights commence to show indica tions of etching. The action must then be at once stopped by placing the plate in a solution of caustic soda. After this, the plate is washed, and the bitumen removed with henzole, turpentine, and methylated spirit. It is now ready for inking-up and printing from.
THE WOODBURrrYPE PROCESS.
In this process a relief negative is made iu bi,Airomated gelatine, and from that a metal intaglio or mould is obtained by heavy pressure. Melted pigmented gela tine is poured into the mould, the paper laid down on this, and pressure applied, when a picture exactly reproducing the gradation of the original is secured, formed by the different thicknesses of pigmented gelatine ; the relief, neverthe less, being so slight as to escape notice.
A specially bright and vigorous negative having an edging or safe edge is required. For the relief plate, sheets of plate glass are well cleaned with French chalk and coated with plain collodion. When per fectly dry, the sensitised tissue is poured on. There are various formula' for this, the following being very suitable :— Nelson's sheet gelatine ... 1 0.Z6.
Glycerine ... 30 minims Sugar ... 4. oz. Indian ink Carbolic acid .„ minim Ammonium bichromate „. 150 grs.
Ammonia ... 30 minims Water 6 ozs.
The gelatine is soaked in three-fourths of the water, and melted by heat, the glycerine, sugar, carbolic acid, and ammonia being then added. The Indian ink is dissolved in the remaining water and added gradually, and the bichromate, well powdered, is finally stirred in. The temperature of the solution should be raised to about 140° F., when the warmed and carefully levelled plates are coated by pouring a sufficient quantity of the mix ture on, from a warm measure, and spreading it over with a glass rod or by tilting the plate. About half an ounce will be sufficient for a quarter-plate. The plate is then rapidly dried.