We now take up that more important branch of the collodion process, vis., that for negatives.
The formulae and instructions already given for positives might enable one to make a negative, but the negatives so made would bear even a remoter relation to one made by the formula about to be given than would a positive made according to these formulae bear to a good one made by the appropriate means. Important modifications are necessary to reach the highest excellence in either process.
To produce a good negative a collodion is necessary, the film of which is more permeable than that for positives, so that a heavier deposit of iodide of silver may be carried in the film, to the end that in the development a denser deposit of metallic silver may be secured on all parts of the image, but especially on the high lights, which should be sufficiently dense to prevent the transmission of light almost altogether, or at least to exclude the light sufficiently to allow the shadows and intermediate shades to print to the proper depth, while the high lights of the face and such parts of the clothing as are white shall be only slightly discolored.
There are very many formulae for such collodions, some of which have proved to be eminently satisfactory, and among the best are classed the following:
Ether and alcohol, equal parts. Climax cotton (Anthony's), 3 grains to the oz. Negative cotton (Anthony's), 1 grain to oz. Place the cotton in the ether first, then add the alcohol and sensitize with
Iodide of Ammonium.........3 grains to ounce
Iodide of Lithium...........1 " " "
Bromide of Cadmium..........2 " " "
Collodion prepared after this formula gives very pleasing results and is rather more rapid than the average.
In many cases it is well to have two samples of collodion differing in formulae, mixing them in varying proportions, according to the effect desired.
A good formula for mixing with the above is as follows. It can be used separately if desired, but is not primarily intended to be used alone:
Plain collodion, prepared as in the preceding formula, but with a different cotton, Anthony's snowy Cotton, for instance, to each ounce of the plain collodion add:
Iodide of Potassium...............2 ½ grains
Bromide of Cadmium..............2 "
Iodide of Cadmium................2½ "
The iodide of potassium is insoluble in absolute alcohol and ether, therefore it must be first dissolved in the smallest quantity of water possible and then added to the collodion. A portion will even then most probably be precipitated; if it is not, it would be an indication that the alcohol or ether used were either one or both of a higher specific gravity than they should be.
It is more important that the ether should be pure than the alcohol, as the former is often contaminated or adulterated with water as well as with alcohol, and sometimes contains impurities of an acid nature.
Ether, for photographic purposes, should not have a higher specific gravity than 720, and the alcohol should never be used for collodion when below 95 per cent, as the presence of water in the collodion makes the film weak and glutinous.
Most formulae for collodion prescribe alcohol and ether in equal parts, but these may be varied with advantage during the hot season; for instance, the alcohol may be used in somewhat larger proportion, as the tendency to evaporation is much more with a high temperature, and alcohol evaporates less rapidly than ether. Consequently the flowing quality of the collodion and the evenness of the film would be promoted by a not too rapid congelation or setting of the film. And again, during the cold season, the ether may be used in larger proportion, to promote a more rapid evaporation and setting of the film. By a moderately close observation of the action of collodion under varying temperatures, the operator might soon learn how to modify his formulae to suit all conditions of heat and cold.
It should be mentioned that it would not be correct to vary the proportions of ether and alcohol very greatly, as an over proportion of alcohol would render the collodion weak and glutinous, and, being very soluble in water, would the sooner injure the silver solution. On the other hand too great a proportion of ether would make the collodion less sensitive.
Alcohol and ether, equal parts.
Anthony's Negative Cotton... .....3 grains to ounce
Anthony's Climax Cotton..........1 " " "
Put the cotton into the ether and let it become saturated, then add the alcohol. Excite with
Iodide of Ammonium......3½ grains to ounce
Iodide of Cadmium........2 " " "
Bromide Cadmium.........2½ " " "
Collodion sometimes, from long keeping, loses sensitiveness and becomes of a deep red color. When such is the case, it should be set aside and fresh samples prepared, which would be improved in working quality by the addition of small quantities of the old, and thus old stock may be used up without deterioration in the work.
Negative collodion should not be so heavy bodied or thick as that for positives; neither should it be so highly excited. Good positive collodion should have as many as 8 grains of the iodides and bromides to the ounce, requiring a silver solution not lower than 50 grains strong and going somewhat higher in cold weather.
The standard silver solution for negatives is 45 grains of silver nitrate to the ounce of water and the collodion from 5½ to 6½ grains to the ounce, with a tendency to less in cold weather.
Collodion for negatives should be permitted to ripen a day or two before using, unless it is brought to that state by mixing with old.
Iodide Ammonium................ 192 grains
Bromide Cadmium................ 128 "
Bromide Potassium............... 96 "
Ether and Alcohol.................3 2 oz. each
Cotton......................... 320 grains
The preceding formulas are for portrait work in the studio. For other work, such as outdoor views, landscape or architectural, or for copying engravings, etc., certain modifications of the collodion, to produce greater intensity or more contrast, are desirable. Collodion suitable for such purposes can always be purchased from the photo stock dealer, in quantities desired, and for that reason it is not best for the amateur nor the professional photographer either, to prepare small samples for special purposes. If it should, however, be found at any time necessary to prepare such a collodion, the second formula of Negative Collodion A will be found suitable for views of buildings, copies of engravings and such like work.
It was at first thought to be unnecessary to give a formula for the preparation of pyroxiline or gun cotton, as no individual not in the business can possibly produce an article that can be in any way compared to that made by manufacturers of skill and experience.
For the benefit of any one wishing to experiment, this formula is given: