The collodion emulsion sets very rapidly ; in warm weather about one minute, in winter three to four. When the superfluous collodion has run off, the paper is removed from the frame or support and hung up to dry by means of clips attached to a string.
Right here we come to another very important consideration, which must on no account be overlooked. We allude to the temperature in which these various operations of coating and drying are to be carried on. Paper prepared in a moist room and dried slowly gives flat prints, while if dried at too high a temperature will not tone at all.
The coating of the paper should be carried on in a cool, dry atmosphere, the room thoroughly well ventilated, as the fumes of ether and alcohol are exceedingly obnoxious and injurious to the health. Directly the paper is coated it should be taken and hung up in a room heated to about 6o degs. Fahr. After drying, the edges are trimmed with a pair of shears and the paper laid down flat under a heavy weight. It should be well preserved from light and moisture. If rolled with film outward if can be kept in an air-tight tin case, and when required for use will fall into a flat position if unrolled.
Coating by is one advantage in coating collodion-emulsion paper—a smaller room only is necessary than with gelatine—as the paper dries very rapidly, and, conse quently, does not require to be hung up in festoons.
In Fig. 132 we give an illustration of part of a collodion-emulsion coating machine. The paper coming from below passes round one roller, then down round another turning in the trough containing the collodion emulsion, then up and round another roller to a level position. It is very important that the paper should be brought to a level position as quickly as possible after it has received the collodion coating. For this reason, the two principal rollers should be as close to each other as possible—much closer in proportion than in the sketch, which is more to snow the pnwapie upon which the paper is coated. When the paper comes over to a horizontal position, it passes into a long tunnel box about one hundred feet long. The paper runs along a number of rollers at convenient intervals, every third or
fourth being made to turn and drag the paper along. The tendency of the paper, when coated, to curl up at the sides, necessitates the arrangement of a number of bars fitted across about an inch or less above the rollers. This will effectually prevent the paper from curling up. If the tunnel be the length stated, and if a current of warm air is made to pass continually through it, the paper will be set and dry before it reaches the other end, arriving at which it can then be cut up into the sheets required as it comes out.
It is necessary that the current of warm air should be as even as possible without consid erable variations of temperature. It should be made to pass through the tunnel and out through a pipe outside the building, as the alcohol and ether fumes are very dangerous and inflammable.
Printing and considering the depth that the print should be carried, much depends on the paper and on the subsequent operations. Many toning baths, for instance, have a strong reducing action.
Collodion paper is particularly sensitive to finger marks, which will produce red spots on the paper, as they stop the toning action on those parts. Handling the paper should, therefore, be avoided as much as possible.
In washing prints, previous to toning, we often have a difficulty to contend with that is not met with in gelatine papers—we refer to the curling of the paper. Everyone is aware that paper expands very considerably when moistened; the gelatine film also undergoes the change, so that no apparent effect is produced. The collodion film, however, refuses to expand with the paper, hence it curls up inwardly, a process which causes considerable annoyance to the operator, espe cially in toning, as the action of the gold is then unequal. We must consider the various methods of avoiding this. If only small quantities are made, it is not a difficult matter to coat the back of the paper (either before or after coating with the emulsion) with an alcoholic varnish or with a plain collodion.