Dissolve the tannin in a small quantity of water, mix with the silicate, allow to stand for twenty-four hours. Both solutions will keep some time separate. For use take
No. i.................. i part.
No. 2.................. 2 parts.
Water ............... 7 ,,
Filter through cloth. This must be used at once as it will not keep. The plate should be placed on a levelling stand, and after brushing, plenty of the solution should be poured on and spread evenly over, if necessary using a small piece of paper, then drain off and place in a rack to dry; when dry the plates should be well rinsed to get rid of the free silicate, and again dried, and the plates are then ready for coating. The plates may also be dried in the oven, being placed on edge, and leaving the door open, and regulating the heat to about iooo F. The sensitive coating is bichromated gelatine prepared as fol-lows : - Special hard gelatine, 80 parts, are allowed to soak in water for a quarter of an hour, the water poured off and as much as possible squeezed out; the gelatine is then placed in a graduated measure, and water added to 1000 parts. The gelatine is melted by the aid of a waterbath, and then 16 parts of bichromate of potash finely powdered are added and dissolved by careful stirring. The mixture is then filtered through fine muslin. The glass is now carefully levelled in the oven and heated, and the plate carefully and evenly coated, from 100 to 200 minims being allowed for about 4 ins. square of glass. The plate or plates having been coated, the oven is closed and the temperature allowed to attain about 1150 F., and this temperature must be kept even during the whole time the plates are drying; and the oven should not be opened till the plates are quite dry, which at the above temperature will be in two or three hours. At the end of this time the gas under the oven should be turned out and the plates allowed to cool. The plates when cool are ready for exposure, and this is effected in a heavy printing frame with plate-glass front, and with screw-bars to obtain pressure _ The duration of exposure is sometimes judged by examining the plates from the back, but the safest way is to use an actinometer, using, as the sensitive actinometer paper,, a piece of paper coated with the same mixture as used for the plates. A little