When the season for outdoor work closes, amateurs begin to look about for means of employment during the dark evenings. One of the most pleasing occupations is the production of transparencies for the lantern, by artificial light, with Beachy's dry collodion plates.
It may be interesting to some to know the formula by which the emulsion is made, as the making of it is by no means a difficult operation. The formula is as follows: - In 8 oz. absolute alcohol dissolve 5 dr. anhydrous cadmium bromide. The solution will be milky. Let it stand at least 24 hours, or until perfectly clear; it will deposit a white powder. Decant carefully into an 8-oz. bottle, and add to it 1 dr. strong hydrochloric acid. Label this " bromide solution"; and it is as well to add on the label the constituents, which will be found to be nearly -
Alcohol ...... 1 oz.
Cadmium bromide .. 32 gr. Hydrochloric acid .. 8 drops
This solution will keep for ever, and will be sufficient to last 2-3 years; with this at hand, you will be able in 2 days to prepare a batch of plates at any time. In doing so, proceed thus: - Make up your mind how many plates you mean to make, and take of the above accordingly. For 2 doz. 1/2-plates or 4 doz. 3 1/4 by 3 1/4, dissolve by heat, over, but not too near, a spirit lamp, and by yellow light, 40 gr. silver nitrate in 1 oz. alcohol '820. Whilst this is dissolving in a little Florence flask on a retort stand at a safe distance from the lamp - which it will do in about 5 minutes-take of the bromised solution 1/2 oz., of absolute ether 1 oz., of guncotton, 10 gr.; Put these in a clean bottle, shake once or twice, and the guncotton, if good, will entirely dissolve. As soon as the silver is all dissolved, and whilst quite hot, pour out the above bromised collodion into a clean 4-oz. measure, having ready in it a clean slip of glass. Pour into it the hot solution of silver in a continuous stream, stirring rapidly all the while with a glass rod. The result will be a perfectly smooth emulsion without lumps or deposit, containing, with sufficient exactitude for all practical purposes, 8 gr. bromide, 16 gr. silver nitrate, and 2 drops hydrochloric acid per oz.
Put this in your stock solution bottle, and keep it in a dark place for 24 hours. When first put in, it will be milky; when taken out, it will be creamy; and it will be well to shake it once or twice in the 24 hours.
At the end of this time, you can make your 2 doz. plates in about 1 hour. Proceed as follows: - Have 2 porcelain dishes large enough to hold 4 or 6 of your plates; into one put sufficient clean water to nearly, fill it, into the other put 30 oz. of clear, flat, not acid, bitter beer, in which you have dissolved 30 gr. pyrogallic acid. Pour this through a filter into the dish, and avoid bubbles. If allowed to stand an hour, any beer will be flat enough; if the beer be at all brisk, it will be difficult to avoid small bubbles on the plate. At all events, let your preservative stand while you filter your emulsion. This must be done through perfectly clean cotton-wool into a perfectly clean collodion bottle; give the emulsion a good shaking, and when all bubbles have subsided, pour it into the funnel, and it will all go through in 5 minutes. The filtered emulsion will be found to be a soft smooth creamy fluid, flowing easily and equally over the plates. Coat with it 6 plates in succession, and place each, as you coat it, into the water. By the time the sixth is in, the first will be ready to come out.
Take it out, see that all greasiness is gone, and place it in the preservative, going on till all the plates are so treated.
A very handy way of drying is to have a flat tin box of the usual hot plate description, which fill with hot water, then screw on the cap; on this flat tin box place the plates to dry, which they will do rapidly; when dry, store away in your plate box, and you will have a supply of really excellent dry collodion plates.
Just a word as to the preparation of the glasses before coating. It is very generally considered that it is better the glasses receive either a substratum of albumen, or very weak gelatine. After your glasses are well cleaned, place them in, and rub them with a weak solution of hydrochloric acid of the strength of 2 oz. acid to 18 oz. water.
Prepare a solution of gelatine, 1 gr. to the oz. of water, rinse the plate after removal from the acid mixtures, and coat twice with the above gelatine substratum; the first coating is to remove the surplus water, and should be rejected. Rear the plates up to drain, and dry in a plate rack, or against a wall, and be careful to prevent any dust adhering to the surface while wet.
If we take a negative, and in contact with it place a sheet of sensitised paper, we obtain a positive picture. Substitute for the paper a sensitive glass plate, and We obtain also a positive picture, but, unlike the paper print, the collodion or other plate will require to be developed to bring the image into view. Now this is, what is termed making a transparency by contact. It often happens, however, that a lantern slide 3 1/4 by 3 1/4 has to embrace the whole of a picture contained in a much larger negative, so that recourse must be had to the camera, and the picture reduced with the aid of a short focus lens to within the lantern size; this is what is called making a transparency by reduction in the camera. Both cases are the same, however, so for as the process being simply one of printing.
Those who have never made a transparency will have doubtless printed silver prints from their negatives, and when printing, how often do you find that to secure the best results you require to have recourse to some little dodge.
Now, let us bear this in mind when using such a negative for the printing of a transparency. Although we cannot, when using a sensitive plate, employ the same means of dodging as in the case of a silver print, still we are not left without a means of obtaining the same results in a different way, and a deal more depends on the manipulative skill of the operator than in the adoption of any particular make of plate, or formula; and not only does this manipulative skill show itself in the exposure, development, etc, but likewise comes into play in a marked manner even in the preparation of the negative for transparency printing.