The Collodion, having been iodized some hours previously, is allowed to settle down and become clear in this bottle; then by gently blowing at the point of the shorter tube, the small glass siphon is filled, and the fluid drawn off more closely than could be done by simply pouring from one bottle to another.
When the Collodion is properly cleared from sediment, the operator takes a glass plate, previously cleaned, and wipes it gently with a silk handkerchief, in order to remove any particles of dust which may have subsequently collected. If it be a plate of moderate size, it may be held by the corners in a horizontal position, between the forefinger and thumb of the left hand. The Collodion is to be poured on steadily until a circular pool is formed, extending nearly to the edges of the glass.
By a slight inclination of the plate the fluid is made to flow towards the corner marked 1, in the above diagram, until it nearly touches the thumb by which the glass is held: from corner 1 it is passed to corner 2, held by the forefinger; from 2 to 3, and lastly, the excess poured back into the bottle from the corner marked No. 4. It is then to be held vertically over the bottle for a moment, until it nearly ceases to drip, and then, by raising the thumb a little, the direction of the plate is changed, so as to cause the diagonal lines to coalesce and produce a smooth surface. The operation of coating a plate with Collodion must not be done hurriedly, and nothing is required to ensure success but steadiness of hand and a sufficiency of the fluid poured in the first instance upon the plate.
In coating larger plates, the pneumatic holder, which fixes itself by suction, will be found the most simple and useful.
The Proper Time for immersing the Film in the Bath.—After exposing a layer of Collodion to the air for a short time, the greater part of the Ether evaporates, and leaves the Pyroxyline in a state in which it is neither wet nor dry, but receives the impression of the finger without adhering to it. Photographers term this setting, and when it takes place it is a sign that the time has come for submitting it to the action of the Bath.
If the film be lowered into the Nitrate before it has set, the effect is the same as that produced by adding Water to Collodion. The Pyroxyline is precipitated in part, and consequently there are cracks, and the developer will not always run up to the edge of the film. On the other hand, if it be allowed to become too dry, the Iodide of Silver does not form perfectly, and the film, on being washed and brought out to the light, exhibits a peculiar iridescent appearance, and is paler in some parts than in others.
No rule can be given as to the exact time which ought to elapse: it varies with the temperature of the atmosphere, and with the proportions of Ether and of Pyroxyline; thin Collodion containing but little Alcohol requiring to be immersed more speedily. Twenty seconds in the common way, or ten seconds in hot weather, will be found an average time.
When the plate is ready, rest it upon the glass dipper, Collodion side uppermost, and lower it into the solution by a slow and steady movement: if any pause be made, a horizontal line corresponding to the surface of the liquid will be formed. Then place the cover upon the vertical trough[35] and darken the room, if this has not already been done. As the presence of white light does no injury to the plate previous to its immersion in the Bath, it is not necessary to exclude it during the time of coating with Collodion.