The collodion being ready we take a glass measure capable of holding about 4 ounces. Into this place half an ounce of nitrate of silver and half an ounce of water. Thoroughly dissolve the silver, applying heat if necessary, and add ounces of alcohol, the addition to be made very slowly. Into another small measuring glass we dissolve so grains of citric acid in r$ ounces of alcohol, and in another 48 grains of chloride of strontium are dissolved in ounces of alcohol.
Instead of chloride of strontium many other chloride compounds have been recommended, and may be employed, all varying in their effects. The principal advantage gained by the use of chloride of strontium lies in the color of the print ; when toned it is possible to get with this emblem those rich purple-brown tints so much admired.
We have now four vessels, containing respectively the collodion, the silver solution, the citric acid solution, and the solution of strontium chloride.
The next operation, when all the ingredients are thoroughly dissolved, is to add the citric acid solution to the strontium solution, and after well stirring together to place in another vessel containing 4 ounces of the plain collodion.
All the operations already described can be performed in strong daylight without any injury, but as we are now about to form the sensitive emulsion it will be necessary to remove our solutions, etc., to a room lighted only by a yellow light. Although a moderately non-actinic light is sufficient it should be remembered that the emulsion is more sensitive than sensitized albumen paper.
We now have the collodion containing the acid and the chlorine, and the silver solution. We mix the two together by dropping the silver solution into the collodion, stirring rapidly the whole time. The dropping is better effected by placing the silver solution into an ordinary dropping bottle. Too much care cannot be taken with this part of the process. It will be noticed that directly the drops of silver solution enter the collodion it takes a white, milky appear ance. This is due to the formation of chloride of silver, and other compositions of minor importance, which remain suspended in the collodion. It being very necessary that the silver and the chloride be well distributed and thoroughly combined, the gradual addition of the silver solution and continual and violent stirring during the process are absolutely essential. If this is not observed an unevenness in the prints will be the result.
It must be mentioned that after the addition of the alcohol to the silver solution it very often happens that a portion of the silver crystallizes. As soon as this is observed the vessel
containing it must be placed in warm water until the crystals are re-dissolved, when it must be immediately mixed with the collodion after the manner described.
We have now a sensitive collodion emulsion. It must be well shaken and allowed to stand for two or three hours. It must now be filtered in a collodion filter, and is then ready for use.
Our other method gives equally good results, but differs slightly in preparation.
Weigh out 2 drachms of silver nitrate and place it with x drachms of water in a thin glass vessel that will stand heat. Place this in a pan containing hot water until the silver is dissolved; then remove it and add by dropping 4 ounces of alcohol. If the alcohol is added too rapidly the silver, will all crystallize in the solution. Should this happen, return the vessel to the hot water till the silver is again dissolved. It is much better, however, to carefully avoid the crystallization of the silver by adding the alcohol slowly and shaking it up well after each addition, for we have found that if allowed to crystallize and is remelted, there is always a tendency afterward for the silver salts to deposit themselves at the bottom of the collodion : Next add about a quarter of an ounce of As Simpson stated, the quantity of pyroxyline can only be determined by experiment, as it differs so materially in quality. In summer, too, a smaller quantity is required than in the winter. With ordinary commercial qualities and in mild weather, one-quarter of an ounce will usually be found about right.
After the gun cotton is added it is stirred round and round to get quite soaked through with the alcohol. This done, the ether is next added in the quantity of si ounces. The gun cotton or pyroxyline will immediately be dissolved and a collodion containing the silver nitrate is formed. We have next to add the chloride and other matter. In r ounce of alcohol dissolve xs grains of lithium chloride, xs grains of tartaric acid, rs grains citric acid. When these are all dissolved the solution is gradually added to the silver collodion. Add it little by little, either with a dropping bottle or by blowing it in in a fine spray with a bottle such as is used for spraying perfumes. Shake the sensitive collodion up thoroughly so as to distribute the silver salts well over the emulsion and allow it to stand for several hours, when it is ready for coating.