WOODBURYTYPE A photo-mechanical process invented by Mr. W. B. Woodbury in 1865, and patented by him on July 24, 1866. A relief image is obtained on bichromated gelatine, covered with lead, and the two forced together in a hydraulic press, which produces a reverse or mould in the lead. The mould is then inkqd with pigmented gela tine and printed from under pressure. A modi fication of the process was afterwards published under the name of " Stannotype," in which no hydraulic process was necessary. Woodbury type is quite unlike any other process in its working details, which, based on the inventor's own words, are as follow : The production of pictures, either on white paper, upon glass as transparencies, opal, etc., by this method of printing is based on the principle that layers of any semi-transparent material seen against a light ground produce different degrees of light and shade, according to their thickness, as the carbon process, for example. Therefore, by having a mould in intaglio produced by the action of light upon bichromated gelatine, and filling the intaglio so produced with a semi transparent material, there is obtained a mould in which the parts that are the thickest give a dark colour ; while the thinner the layer becomes it gradually merges into white. By pouring a mixture of coloured gelatine on to the intaglio mould, and placing a piece of white paper thereon, and squeezing the whole between two perfectly true planes, the superfluous colour is squeezed out, and the gelatine, having set, ad heres to the paper and leaves the mould quite clean. When the picture leaves the mould it is in slight relief, but during drying the gelatine contracts and leaves hardly any perceptible relief. To make the gelatine relief, wet several pieces of talc, and affix them to a large sheet of glass ; squeeze out the superfluous moisture, and polish the whole of the pieces. Next prepare the bichromatised gelatine as follows :—Soak 4 oz. of opaque gelatine in 28 oz. of water ; dissolve by heat, clarify with white of egg, and filter. To 4 oz. of this solution add 6o grs. of ammonium bichromate dissolved in oz. of warm water and a small quantity of Prussian blue ; the latter serves to give the finished relief a colour by which to judge of its printing quali ties, and does not interfere with the action of the light in penetrating the gelatine. When
well mixed, the solution is filtered through mus lin, and is then ready for coating upon the talc covered glass, placed upon a levelling stand, and dried. When set, each piece of talc is cut round the edges with a sharp knife and stripped from the glass. Lay the sensitised talc upon blotting paper and clean the talc side ; then place in contact with the negative, and having placed a piece of glass behind, fasten all together with rubber bands, and place in the light of a con denser of 6 in. to 9 in. in diameter, at a distance of about 2 ft. ; after exposing for r or z hours lay in a dish, and pour hot water over it until no soluble gelatine is left ; then allow to dry by gentle heat. Having obtained the relief, it is ready for taking the mould.
The above is Woodbury's original process. A later method of making the relief plate is to clean sheets of plate glass with French chalk and coat with collodion. When dry, the sensi tive mixture, made as follows, is poured on, and laid on a level place to set :—Sheet gelatine, I oz. ; glycerine, so drops ; sugar, f oz. ; Indian ink, x gr. ; carbolic acid, i drop ; ammonium bichromate, 15o grs. ; liquor am drm. ; water, 6 oz. The gelatine is soaked in three-fourths gf the water, melted by heat, and the glycerine, sugar, carbolic acid, and ammonia added. The Indian ink is dissolved in the remaining water, and added gradually, and the bichromate, well powdered, stirred in. Exposure and development are the same as for carbon printing, hot water at a temperature of r io° F. (43° C.) being used to begin with, afterwards raising it to 160° F. (71° C.). Develn ment—that is, the dissolving away of the unacted upon bichromated gelatine—may take two hours or longer, but the process must not be hurried. When the image is developed, it is immersed in a 4 per cent. solution of chrome alum for a few minutes, washed, soaked in methylated spirit for an hour, and then dried.