It is absolutely essential to successful intensification that the negative be completely fixed and completely washed after fixing, for any trace of hypo left in the film will give rise to brown stains. It is also important, in order to prevent stains of another sort and to secure uniform action, that the mercuric chloride solution be mixed with a small quantity of hydrochloric acid. Too much acid will cause frilling. If the negative has been dried it must be immersed in water for, as a rule, not less than half-an-hour, in order that it may be thoroughly and uniformly wetted.
Mercuric chloride 1 oz. or 5 parts Hydrochloric acid 1-1/2 drachms or 1 part Water to make up to 20 oz. or 100 parts
When uniform intensification is required the negative is allowed to remain in this solution until it is completely bleached. If, however, it is desired to intensify the shadows more than the high-lights, the plate should be removed from the solution as soon as the shadows have bleached, and should be rapidly washed in order to stop the action on the more opaque parts of the image.
In either case the negative must be thoroughly washed after bleaching, and the water used must be soft water. Hard water tends to produce a precipitate of the mercury salt in the film, which may subsequently lead to stain or fog.
Perhaps the best plan of all, when constant results are desired, is to treat the bleached negative with the ferrous oxalate developer, which will gradually convert the white image into a black one, after which the plate is thoroughly washed and dried. It is recommended that the first water used for washing should be slightly acidified with oxalic acid.
Instead of using ferrous oxalate the bleached plate may be treated with a weak solution of ortol or metol to which some sodium carbonate (soda crystals) solution has been added, but _no sulphite_. After the image has blackened completely the plate is washed.
With any of these methods if the first intensification is not sufficient, the plate may be again bleached with the mercury solution and the process repeated.
An old method, frequently used, is to treat the bleached plate with dilute ammonia, which converts the white image into a dark brown one of very considerable printing opacity. The results are often very good, but are somewhat uncertain, since the precise effect obtained depends on the strength of the ammonia solution and the time during which it is allowed to act. With somewhat strong ammonia, allowed to act for a fairly long time, part of the intensification first produced is removed. This affects the shadows more strongly than the lights and the result is to increase the contrast of the negative, which is very useful for certain purposes.