The customary method of iodizing a collodion is to omit half the quantity of alcohol in the plain collodion, and use it to dissolve the iodide or bromide which is then added. The addition of the iodide or the iodide and bromide to the plain collodion is termed iodizing, and the collo dion is said to be iodized.
All good workers with collodion adopt the method of writing upon the label of the iodized collodion the date of its manufacture, the date when iodized, and the various ingredients and proportions used in both processes. This will be found of much use, and a convenient guide for future operations.
It may be as well to state that both time and trouble may be saved by those who object to lengthy processes by purchasing the iodized collodion sold in the market ready for use.
Coating the next operation is to coat the glass with the collodion. It is one that requires no little care. The iodized collodion is kept in a suitable bottle, guarded from dust, and free from solid particles of dry collodion. The form of the bottle should be similar to that depicted in fig. 13o, which permits of the collodion being poured out evenly, and prevents the formation of bubbles and solid particles from settling on the film.
The glass, cleaned and polished, is held at one corner by the thumb and first finger of the left hand by allowing the extreme corner to press into the ball of the thumb, and the under side of the glass to rest upon the first finger bent into position to receive it. By this means touching the upper surface is avoided, and the collodion can be coated over the whole of the surface of the glass save a very small corner nearest the thumb. A still better method is to employ a pneu matic holder, which can be attached to the back of the glass and allow the whole surface to be coated. The glass plate is first dusted over with a broad camel-hair brush, scrupulously clean, and a pool of the collodion, of more than sufficient quantity to cover the glass plate, is poured on towards the top right-hand corner. The plate is then slowly tilted to allow the collodion to first run in and fill up the top right-hand corner without overflowing. The collodion is then made to run along the plate to the top left-hand corner, and then to the bottom left-hand corner. An
other movement of the plate serves to bring it to the bottom right-hand corner, the whole of the plate having been covered. From this corner the superfluous collodion is poured off the plate into another bottle, rocking the plate from side to side to prevent the collodion leaving traces of its downward flow towards the bottle. As soon as the collodion is " tacky " it is placed into the "nitrate " or " silver bath." The coating a glass plate with collodion requires considerable practice; it will therefore be as well for the operator to make his first attempts with spoilt collodion, glycerine, or thin cream.
The Silver Bath is one of the most important solutions used in wet-plate photography, and upon its purity and strength a great deal depends. The salt invariably used is the silver nitrate, and this should be of the purest quality obtainable. The strength of the solution varies. With a collodion which contains iodides and bromides the greatest strength permissible is 5o grains of silver nitrate to each ounce of water. According to Abney, even this proportion is too large, since the silver nitrate in solution will dissolve up a certain amount of silver, the quantity depending upon the silver solution and on the temperature. If, therefore, it were not that the solution becomes saturated with silver iodide on the immersion of a collodion film, the iodide would be dissolved out, either partially or wholly, according to the time of the immersion. Now, as it is easier to saturate a dilute than a strong solution, and as a variation in the temperature will cause a less marked difference in the former than in the latter, it will be evident that the less the amount of silver salt in solution, the greater the probability that the solution will not show signs of under or over-saturation of iodide.
Another point to be observed is the acidity or alkalinity of the bath, as upon this the sensi tiveness of the plate partially depends. If a collodion be used containing an iodide only the bath should be either neutral or very slightly acid, but with a collodion containing bromide and iodide it should be distinctly acid unless a considerable amount of free iodine be present.