The next step will be to remove the plate from the printing frame, and immerse it in clean cold water for five or six hours, or for such a time as suffices to remove the whole of the bichromate from the film. The plate is then placed in the rack to dry spontaneously, and should be allowed to stand for at least twelve hours afterward before printing from, so as to give the film a proper chance to become thoroughly hard.

We now have a collotype plate which has undergone the following operations, viz.:

Fig. 17.

A plate of thick glass ground with fine emery until of an even matt surface. Then it is washed and coated with the pre­lim­i­nary coating of beer and sil­i­cate of soda and dried. Then rinsed in cold water and again dried. Next placed on the warm slab of the drying oven until just warm enough to hold on the palm of hand. Then coated with the sens­i­tive mix­ture, either No. 1 or No. 2, and placed in the oven until dried. Then ex­posed to the light under a re­versed neg­a­tive in a print­ing frame until the de­tails of the image are well out. Then the back of the plate is ex­posed to the light for a short time. Then it is placed in clean cold water until the bi­chro­mate is removed. At last the plate is dried, after which it is ready for the printer.

Now, before pro­ceed­ing furth­er, it will be as well to touch upon various points of pro­ce­dure, and also to des­cribe what may be reck­oned the most im­port­ant piece of a col­lo­type plant, viz., the drying oven. Figure 17 gives a drawing of the oven as it stands on the table for use, the {168} dimensions being from the top to the bottom of the frame 5 feet by 3 feet 4 inches wide by 3 feet deep. The top A, and the back, are covered with matched boards well seasoned; the sides and front, B, B, B, B, are made of frames 2 inches by 1 inch, covered with canvas sufficiently close to exclude light, but still open enough to let out the heat slowly. The door C is also a frame covered with canvas, 24 inches square, sliding upward in grooves, and balanced by means of weights. D is the jacket of iron upon which a copper tank rests, the jacket serving to carry the tank and to protect the wood-work from the gas flame used to warm the water in the tank. E is a sliding door in the jacket for the purpose of lighting the gas burner. F F are the projecting ends of a levelling arrangement forming the base of the oven, these ends being fitted with good, strong screws working upon iron plates let into the top of a strong table upon which the oven is placed. The distance between the two lower rails or styles is 6 inches, which is also the height of the iron jacket. D G is a tube let into the jacket to carry off the products of combustion from the gas. H is a tube with a screw nozzle, for filling the tank.

Fig. 18.Fig. 19.
Fig. 20.

Figure 18 is the base of the oven, A A A being the three points where the levelling screws are placed; this base is formed of good 4 by 3 inch quartering, the other dimensions being as marked. This base stands on the table, the oven on the top, without any fastenings. {169}

Figure 19, the jacket of sheet iron well wired, with a tube at one corner, for carrying off the products of the combustion of the gas, and a hole at the other corner to carry the projecting tube from the tank, by which it is filled.