The only thing that calls for particular attention in these ovens is the copper tank, which should (especially the top) be made of good, stout sheet copper, otherwise the water on being heated has a tendency to make the copper bulge, and elevate the slab. By that means the care which should be taken in levelling the slab will be lost, as this elevation will put it all out. Before putting the superstructure in position, the slab must be cemented to the top of the copper tank by means of plaster-of-Paris; this is important, as a much more even heat is thus obtained.
Now, whichever oven is decided upon, it must be erected in a room free from draught, where it can be kept at a temperature of 70° F., as the quality of the collotype plate is much influenced by the temperature of the room in which it is prepared. So, too, is the oven in which it is dried, especially if below 60°, for then it is very difficult to get good results, as the plate chills when taken from the oven, and the film is almost sure to prove rotten during the printing. {175}
In the summer time, if the room gets too hot, the gelatine solution is apt to get frothy, and then dry hard and glossy, instead of with a nice half-mat surface. It is then best to prepare the plates only in the early morning.
The temperature at which to dry the collotype plates will vary according to the subject to be printed upon them, as some subjects will require a finer grain than others. But a few intelligent experiments will soon enable the operator to judge of the temperature required; 75° F. is the lowest, and 120° F. the highest temperature giving good, bright blacks.
One of the great secrets in collotype is to give a double coating to the plate. I find it really a great advantage so to do. I found this out quite by accident. One day, when I went to use some plates, I found that they had not been properly levelled, and that the coating was thin at one end, and thick at the other. This happened to three out of a batch of four, the other one being all right; so, in a fit of laziness, or hurry, or something, I decided on recoating the three, and did so. When I came to print from the three twice coated and the one once coated, I was agreeably surprised at the difference; the pictures on those twice coated were much more vigorous. Since then I have tried the experiment exactly, and always get the same uniformly excellent results. The second coating is applied as soon as the first is dry; but this second coat cannot be applied so well if the plate be held on the hand, therefore a levelling stand should be used, and the gelatine poured on the centre and coated over the surface by means of a glass rod.
The proper quantity of gelatine solution requisite for a plate 16 by 13 inches is 2 ounces for each coat. Note well that all of it is left on the surface of the plate, not some on the table.
When once the coated plates have been placed in the oven, and the lid is closed down, they must not be disturbed until they are dry; this will be in two or three hours, according to the thickness of the coating of gelatine. If removed before they are dry, there is danger of marking the film.
After the second coating of gelatine is dried, the plate will be all the better if kept a few hours before being printed upon, care being taken to store it in the dark, or somewhere free from damp and gas fumes.
By the bye, the oven must be placed in a room illuminated by yellow light—i. e., daylight filtered through yellow glass, cloth, or paper; or else the room must be lighted by gas or oil light.
The negative to be printed from should have all the parts that are intended {176} to be quite white in the finished print, stopped out with India ink or Gihon’s opaque, and the limit of the picture should be masked by very thin tinfoil, cut quite straight and gummed upon the film side of the negative. The rest of the margin of the plate must be protected by a brown paper mask.