Bleach bath
The bleach bath converts the metallic silver that is produced during development in analog photography and is responsible for the blackening into silver salt ( reverse development ) or into re-exposable silver halide (e.g. during redevelopment ).
The easily soluble silver salts can be removed from the layer in a clarifying bath and / or a fixing bath with subsequent rinsing .
The bleach bath used in black and white reversal development is used to remove the silver from the negative image that was created during the first development. As a rule, a mixture of potassium dichromate , a salt of dichromic acid , and sulfuric acid is used (5 g of potassium dichromate dissolved in 1000 ml of water, then addition of 5 ml of concentrated sulfuric acid). Chromium salts are poisonous and damaging to the skin and require careful handling. Caution is also required when handling the concentrated sulfuric acid. It is rinsed and then treated in an aqueous solution of sodium sulfite as a clarifying bath. With this and with a brief subsequent rinse, all silver and chromium salts are removed from the film layer . From now on the film can be further processed in the light (second exposure, second development, fixing, watering, drying).
Using the same principle, the bleach bath is used with color films to convert the silver that is not required from the film material back into silver salts , since the fixer can only wash the silver salts out of the carrier material. These baths mostly contain potassium hexacyanoferrate (III) as a bleaching substance. If light hits an undeveloped color film, the exposed silver salts are first converted into metallic silver, as is the case with black and white film . The existing latent image is completely developed in the development bath . The multi-layer color film emulsions contain various so-called color couplers in the individual layers . These react (“couple”) with the oxidation product of the developer substance created during development and thus form the desired dye in the individual layers. If the silver is bleached out and the film is fixed, only the pure negative dye image is on the film.
The procedure for color reversal film is similar to that used for black and white reversal development. The first development takes place in a black and white developer . Then the previously unexposed silver halide is made developable by a second exposure. Only now does the color develop. All the silver is then bleached out and the film is fixed. What remains is the pure positive dye image.
In today's common color processing processes, it is hardly customary to bleach and fix in two steps. Instead, a bleach-fix bath is used. These solutions contain the iron (III) complex of ethylenediaminetetraacetic acid (Fe III -EDTA) as an oxidizing agent and the conventional sodium thiosulfate as a fixing agent .
A modification of the normal development process is the so-called bleach bath bridging , in which the silver is left in the color film .
In analog photography, bleach baths are also used when it comes to toning black and white images. This can be paper prints, negatives or slides. The bleach baths are designed in such a way that the resulting silver salts remain in the layer. During the subsequent treatment in a clay bath , these react with the substance contained in the clay bath to form a colored silver compound. The most well-known toning is the brown toning. The clay bath contains sodium sulfide and the result is brown, very poorly soluble silver sulfide.
The most versatile method for creating different colors on black and white photos is chromogenic development . It is the basic principle behind the development of color films and images described above . Black and white images or slides are completely bleached and then restored in a color developer. One or two different color couplers have been added to this according to the desired color . In this way, a pure dye or a mixture of dyes is created during development. Then, depending on the desired effect, it is completely or partially bleached and then fixed. So z. B. by combining three different color couplers (yellow, purple, blue-green) any desired color.
Bleach baths can also be used to save negatives or slides that are much too dark. Here, in a mostly very diluted bleach bath, only enough silver is bleached out until a good image impression is achieved. It is then fixed in order to completely remove the silver salts that have formed. This process is known as attenuation in analog photography . The opposite is also possible: through so-called intensification , images that have turned out to be too bright can be improved. Here, too, bleach baths are sometimes involved.
Of course, bleach baths are also used in other areas, for example when treating materials such as cloth or paper . For reasons of environmental protection , the baths that were previously used extensively for this purpose, some of which are also highly toxic, are being replaced by safer chemicals such as hydrogen peroxide or oxygen ; the paper industry sometimes completely dispenses with bleaching, which is why so-called environmentally friendly paper can rarely be pure white.
literature
- Edwin Mutter: The technique of negative and positive processes . Ed .: Josef Stüper. Springer-Verlag, 2013, ISBN 978-3-7091-8029-7 .