Since the introduction of gelatine processes, collodion has become of less importance, though it possesses some advantages, and has undergone some improvements of late years.
To make this emulsion with ammonia citrate instead of citric acid as the organic body necessary to combine with silver, which is to give vigour to the printed image, is rendered very easy, by a little artifice introduced by Capt. Abney. Ammonia citrate is insoluble in alcohol, and therefore rather difficult to introduce into an emulsion in the ordinary manner; but it can readily be introduced into collodion by the following procedure. Take 10 gr. pyroxyline and cover it with 1/2 oz. alcohol in which 20 gr. citric acid are dissolved, and then add 1 oz. ether. This forms collodion containing citric acid. In order to get ammonia citrate into the collodion in a very fine state of emulsion, ammonia (gas) dissolved in alcohol is added to the collodion. This is effected by inserting a bent tube in a cork in a test tube, which is a quarter filled with liquor ammonia. Placing this in warm water - in fact, nearly boiling water - the ammonia is given off rapidly, and can be made to pass through alcohol contained in another test tube.
The alcohol absorbs the ammonia and takes up a large proportion of gas, as those who use sal-volatile may be aware.
This ammoniacal alcohol is next added to the collodion containing the citric acid, little by little, with shaking and stirring, and sufficient is added til] reddened litmus paper shows a very slight trace of alkalinity. A very fine emulsion of ammonia citrate is thus formed, the grain of which is indistinguishable by the naked eye, and, like other emulsions when first mixed, is orange-coloured when spread upon a glass plate. The emulsion is again rendered slightly acid by the addition of a few drops of a solution of citric acid in ammonia. If an emulsion of silver citrate be required, there are 2 ways of effecting it - one by dissolving (say) 10 gr. silver nitrate in the least possible quantity of water, to which is added 1 dr. alcohol, and gradually dropping it into the collodion containing the citrate. It sometimes happens that this gives a granular emulsion. If, however, the silver nitrate be coarsely powdered, and added to the emulsion, a very fine emulsion of silver citrate is produced by shaking. This may be washed in the usual way, or may be precipitated by pouring in a fine stream into water. Another method of forming the silver citrate is to pour out the emulsion of ammonia citrate into a fiat dish, and when well set, to cover it with a solution of silver nitrate.
It is then drained from the silver, washed, and dried as usual. When redissolved, the emulsified silver citrate should be. excessively fine.
To prepare a collodio-citro-chloride emulsion, 2 plans may be adopted: either to dissolve 20 gr. dry calcium chloride in a small amount of alcohol, and add it to the ammonia citrate emulsion, and then to add 80 gr. silver nitrate to it in the usual way; or to make a collodio-chloride emulsion separately, and then to mix the silver citrate emulsion with it, according to taste.
To make a pure collodio-chloride emulsion, dissolve 20 gr. calcium chloride in 1/2 oz. alcohol; add to it 5 gr. pyroxy-line, and then 1/2 oz. ether. To 1 oz. plain collodion made similarly, add 60 gr. silver nitrate dissolved in the smallest quantity of water, to which is added 1 dr. warm alcohol. This produces an emulsion of silver nitrate in the collodion. To this the chlorised collodion is added drop by drop with stirring or with shaking in a bottle, and a perfect emulsiou of silver chloride should result. This can be poured out to set in a dish as usual, and be washed, dried, and redissolved; or can at once be poured out in a fine stream into a large bulk of water, squeezed, soaked in alcohol twice, wringing out in a cloth all excess of alcohol each time. It can then be redissolved in the 1 oz. ether and 1 oz. alcohol, and should give a good emulsion. The 2 emulsions may then be mixed together as before stated. It is well to dissolve about 5 gr. silver nitrate in water and alcohol, and add to the emulsion in order to increase the rapidity of printing. (Photo. Neas.)