PhotoHistory

From: PHOTOGRAPHY OF TO-DAY

By H. CHAPMAN JONES, 1913

A “dry plate,” that is a plate that could be prepared at home, and that would keep in good condition long enough for it to be taken where desired for exposure and then brought home for development, was clearly a great desideratum. The wet collodion plate could not be dried, for then the nitrate of silver would crystallize on its surface and render it useless, and the silver salt could not be washed away to permit of its drying, for that would render the plate very much less sensitive. The search therefore was for some substance that would effectively replace the nitrate of silver on the plate, not crystallize when it was dried, and at the same time form a protective film. Many substances were found to serve this purpose to a greater or less extent, and the want of a discriminating knowledge as to their constituents led various persons from time to time to succeed with and to recommend such things as tea, coffee, wine, stout, porter, gin and water, ale, jelly, raspberry syrup, raisins, eggs, rice, tapioca, honey, sugar candy, gallic acid, tannin, gelatine, &c.

These collodion dry plates were prepared as described for a wet plate, but allowed to remain rather longer in the silver bath to ensure a complete action, then instead of at once exposing them, the plates were washed, coated with the preservative and dried. The photographer would, as a rule, prepare as many as he wanted the day before they were to be used. Such plates were generally less sensitive than a wet plate and might require as much as thirty times as long an exposure.

It will be observed that in the earliest silver methods, Wedgwood’s and Fox Talbot’s, the paper that was to carry the silver compound was impregnated alternately with the two substances that would produce the desired silver salt. When albumen or collodion was used the photographer himself prepared the film at the time required, and he added one of the two necessary compounds, the iodide of potassium, for example, to the bulk of the material of which he prepared the film. After the glass was coated and the film produced, it only remained to immerse the film in the other solution, that is, the nitrate of silver, to produce the required silver compound. It seems a most obvious step further, to add the nitrate of silver solution to the film-producing substance while it is still in bulk, so that the sensitive silver salt may be produced in the bulk of the material instead of in each plate separately. This clearly means a saving of labor, and a simplification of work at the time when the photograph has to be taken, for as soon as the material was poured on to the glass and the film produced, the sensitive compound would be there without further manipulation, and the need for the silver bath with all its attendant difficulties and unpleasantnesses would be done away with. There were several attempts to bring about this simplification, but it seems that the first practically successful formula was published by B. J. Sayce and W. B. Bolton in 1864, and shortly after improved and modified in many ways by these and other workers.

A liquid prepared as stated, which contains in it the sensitive compound and also the material that is to form the film, is called an “emulsion,” in such a case a “photographic emulsion,” because the word emulsion is of general applicability and merely indicates a milky liquid. In milk it is the little globules of butter-fat that render the liquid non-transparent or “milky,” while in a photographic emulsion it is the little particles of silver salt that cause a similar appearance.

Messrs. Sayce and Bolton found that bromide of silver was better than the iodide, and this was one of the chief elements in their success. The coated plates were washed and the preservative applied as if they had been prepared by the use of a silver bath. Some ten years afterwards the washing was done by Mr. Bolton on the bulk of emulsion instead of each single plate being separately washed. This was another considerable advance, for every operation that can be carried out on the bulk of material instead of upon the individual plate, means not only a saving of time and an economy of material, but a greater uniformity in the properties of the several plates prepared from the batch of emulsion. The washing is effected by adding water a little at a time to the emulsion, which it will be remembered is a solution of pyroxyline (gun-cotton) in a mixture of alcohol and ether containing also the sensitive silver salt, and as the pyroxyline is not soluble in water it is gradually thrown out of solution in a curdy form, but still retains the silver salt. Or the alcohol and ether may be evaporated away, leaving the non-volatile constituents in the solid form. By whatever means the solvents are got rid of, the solid residue is washed in water, dried, and then redissolved in alcohol and ether.

Collodion processes of photography are still employed to a considerable extent in trade works, because they offer many special advantages, and the bulk of the apparatus required and the extra manipulations become negligible in such establishments when economy and efficiency are considered. But the general tendency to replace collodion with gelatine, which has for nearly thirty years been complete so far as amateurs and portrait photographers are concerned, seems to be at work, surely if slowly wherever collodion remains in use.