This section is from the "Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1917" book, by Aristo Motto. Also see Amazon: Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1917.
Stains may be due to several causes, and vary accordingly in their nature and color. Thus, we may have red or blue ink stains, iron stains, pyro and iron stains, silver and dichroic fog stains, oxidation stains and others less common. Since yellow stains are most frequently met with, we will deal with them first.
The two commonest yellow stains in photography are oxidation and silver stains. Oxidation stains are caused by oxidation of the developer by oxygen from the air. Thus we may have Elon, Pyro, Hydrochinon and other developer stains, which may be either local or general.
Local stains are the result of careless handling of the negative or print, being caused by incomplete immersion in the developing- or fixing-solutions. A slight curl of a film or print, a negative not entirely covered, or too many prints in one tray, will leave some part of the surface exposed to the air, oxidation will take place and a yellow patch will appear the size of the portion of negative or print exposed to the air.
The necessity of completely submerging the plates, films or prints in the solutions is obvious, and after being placed in the fixing-bath they should be moved about. A precaution that is a great trouble-saver is the use of an acid stop-bath between developing and fixing. The effect of this is to neutralize or destroy the effect of the alkali in the developer that is carried over in the negative, thus reducing the tendency of the developer to oxidize.
Artura Print, From An Eastman Portrait Film Negative By Moffett Studio Chicago, Ill.
There is also a danger that if the acid fixing-bath becomes neutralized through the carrying over of alkali from the developer, stains may be produced. It is therefore a wise precaution to add further amounts of hardener to the fixing-bath at intervals, to make sure that it remains acid.
Local yellow stains produced in this way act just as if pieces of yellow filter were placed over the negative, and the print produced will be weaker in those places where the stain is present, hence the necessity of avoiding or removing it.
General oxidation stain extends over the entire surface of the negative or print, and is caused by the use of an old or discolored developer, or by a developer not containing a sufficient amount of sulphite. Pyrowill give this stain, especially if the solution has been allowed to stand for any considerable time before use. In cases where the general stain is uniform it will have no other effect than to prolong the printing-time of the negative.
In the case of a pyro-devel-oped negative, in addition to the slight general yellow stain above, there is usually a yellow stain image present along with the silver image, the presence of which may be revealed by removing the silver image in a bath of Farmer's reducer. This image is an oxidation product of the developer produced in those places where the metallic silver is formed during development. This oxidation stain has the effect of increasing the contrast of the negative, and explains the fact that a thin-looking pyro-devel-oped negative will often give a contrasty print.
The other common stain that is likely to occur is silver stain. It is difficult to distinguish this from oxidation stain by ordinary observation. Like the latter, it can be either local or general, and it arises from one or several of the following causes :
(a) The first cause is the use of an old and exhausted fixing-bath, containing an undue amount of silver in solution. If the negative or print is not sufficiently washed, some of the silver salt remains. This is colorless, but is changed to yellow silver sulphide after some time. This first cause is easily prevented.
(b) Incomplete Fixing. This can occur even with a new bath if the print or negative is taken from the bath too soon. While the plate is fixing, the silver halide in the emulsion changes first to colorless silver thiosul-phate, which is comparatively insoluble. At this point the milki-ness of the plate or film disappears. By leaving the plate in the bath this soluble and colorless compound is changed to a more soluble double thiosulphate of silver, which can be easily washed out. When the film is removed from the fixing-bath immediately after the milkiness has disappeared - the first stage of fixing - no amount of washing, later, will remove the insoluble silver salt, and this will in time be changed to yellow silver sulphide stain. The only safe rule is to leave all prints and negatives in the fixing-bath for double the time required to reach the end of the first stage, which is marked by the disappearance of the milkiness.
(c) If prints or negatives have not been completely covered while in the fixing-bath, they may appear completely fixed; but in spots they may have fixed only as far as the first stage, with the result that on exposure to the air yellow stains will appear.
Local silver stains may be caused by leaving a negative in contact with damp gelatine proof paper. This paper contains a soluble silver salt which is more or less absorbed by the negative and produces the stain.
When using printing-out paper care should be taken to see that the paper and the negative are perfectly dry. When there is a possibility of the negative and paper being in contact over night, owing to failing light, a sheet of Kodaloid should be placed between them before exposing.
As previously stated, it is difficult to detect slight silver stain in the presence of oxidation stain by observation. While a stain may be either pure silver stain or pure oxidation stain, it is more likely to be a combination of the two.
From the above it is evident, therefore, that yellow stain may consist of one or more of the following compounds:
Metallic silver, silver sulphide, silver thiosulphate, silver photo-halide, together with an oxidation product of the developer.