Chester F. Stiles
When a plate has not been developed far enough, we find in printing that the light penetrates even the densest part of the negative before the shadows of the print are printed deep enough. Such a plate is thin all over, with detail in abundance, but density is lacking. The fault is called " under development." Opposed to this fault is the lesser one of "over development, " where the tones seem all right and make excellent prints provided one prints long enough. Some plates have been seen by the writer which seemed almost opaque. They take hours to print by strong light with ordinary paper, and a number of minutes with rapid bromide papers.
Of course the above remarks refer to errors of development only. The exposure itself has been assumed to be fairly correct. The error of under-development is remedied by "intensification"; that of over-devel-ment is corrected by "reduction". In this article only the first process will be treated.
In the early days of photography, intensification was done with nitrate of silver. With the advent of the gelatine dry plate came identification with mercury. The base of this intensifier is bichloride of mercury (corrosive sublimate,) which is placed in hot water till no more of the salt will dissolve, thus making what is known as a saturated solution.
The plates to be intensified must be thoroughly fixed and washed, so that no developer is left in the film. If not, the careless worker reaps his reward later in the form of indelible yellow spots. No satisfactory method has been found for correcting stains. To avoid them, washing from half to three-quarters of an hour is none too long.
The chemistry of the process of intensification is quite simple. We place the plate in a diluted solution of the corrosive sublimate and the black silver image turns white. The silver image has been replaced by a white compound of mercury and silver. Various agents are known which blacken this compound. Among these is ammonium sulphide, which forms an intense black compound of mercury, much blacker than the original silver image. Hence the intensification.
In practice we place the well washed plate in the diluted mercury solution and bleach till white throughout. A thorough washing is necessary to remove the mercury. Next apply the blackening solution and wash again when sufficient density is secured. If only a partial intensification is desired, bleach the negative to a grey appearance instead of white.
For ammonium sulphide may be substituted a solution of sodium sulphite or of ammonia. The latter gives the strongest intensification of all. It also helps contrast by blackening the darkest parts in a greater degree than the lighter portions.' It is, therefore, a gain in quality of intensity, a flat plate resulting from over-exposure.
The time for intensification is immediately after washing. The film is porous and will take the inten-sifier easily. If an acid hardening and fixing bath has been used, the film is very hard to penetrate. It is well to double or even triple the washing times in the process, or, better than that, avoid acid hypo and hardeners. You will be surprised to see the harden-ing and lack of frilling which plain freshly made hypo baths will promote.
Notice an over-exposed plate carefully. You will see a wealth of detail from highest light to weakest shadow. ;The plates have a brownish cast and pass light in all parts, so that the prints are weak and flat. You can usually improve these plates by selecting a contrasty intensifier.
On the contrary, an under-exposed plate will not stand intensification to advantage. In this case, the contrast is wrong on the plate in the first place, since the shadows did not develop at all, while the high lights have piled on density by the prolonged development. Now, if an intensifier is applied, it still further increases contrast. A double error in tones is then present. To diverge a little from our subject: - In every case that you can duplicate an under ex-posed plate, do so in preference to intensifying. The best place for an under-exposed plate is the ash barrel.
The bleached mercury image may be developed in an old developer if desired. Use an old tray and keep it for this purpose alone. Discarded developer is better than the stronger and fresher solutions.
The mercury chloride will dissolve better in water if some ammonium chloride (sal-ammoniac) is added. Equally good is muriatic acid. Mercury chloride dissolves very slowly, and the additions above quicken the speed of solution. Besides, the muriatic acid will destroy any traces of hypo which may cling to the plate from the fixing. The mercury seems to cling to the gelatine film and is hard to wash away. It is wise to use a weak solution of citric acid after washing from the bleaching solution, and before the darkening solution.
All the above methods are two solution processes,. They have the disadvantage that one does not know the density until the process is finished. Yet, as in all photo operations, experience teaches, and a careful worker will have few real difficulties.
We will now take up an intensifier much used abroad, on account of its simplicity. It is known as the Lu-miere intensifier, and consists of a mixture of mercury iodide and sulphite of soda. In this intensifier hypo matters little. A slight washing suffices, after which we intensify directly, watching the density as it increases. The application of an old developer at the end of the process will make the image permanent.
The great advantage of this intensifier is its direct method. If only a partial intensification is desired, one stops at the density desired. No fixing is necessary after the development, although the developer is allowed to act. The product is common in this country under the name of "Scarlet Intensifier," from the brilliant color of the mercury iodide, and in common with all mercury compounds, is poisonous internally.
The uranium intensifier is one which has gained much ground of late. Since the deposit is red, density is improved greatly by its use. Make two solutions of eight ounces water each, using 25 grains uranium nitrate in the first, and 3 grains red prussiate potash in the second. Mix 1 1/2 ounces of each with 1/2 ounce glacial acetic acid. Skilful workers may intensify locally, for the uranium deposit is soluble in ammonia. It may be removed by painting with a brush dipped in the ammonia solution. Take a hint from the experience of others and try local intensification on an old plate until you get the hang of the process.
"Agfa" intensifier, made by the German photo chemists, is obtainable at most photo supply stores. It is a colorless liquid, containing a mercury salt in solution, which adds density directly to the plates. One part of the solution is added to nine of water, making working solution. It works directly, and the plate slowly gains a deposit of grey tint, which is quite non-actinic.
Many other processes are known, a large portion being variations of the mercury baths. It is possible to bleach in copper sulphate with bromide of potash, after which silver nitrate is the blackening agent. Potassium bichromate is used to bieach the image in another process, and the blackening follows by redeveloping.
In general, we may remark that cleanliness is the key to perfect intensification. Whenever one process is succeeded by another, have the intermediate washing complete. This is the greatest secret in intensification.