Farmer's attenuator
The Farmersche attenuator is in classical chemical engineering photography and photolithography to attenuate continuous tone and halftone films used baths approach. It is considered to be the most reliable and most widely used attenuator in practice.
It consists of two partial solutions:
- A: 50 g potassium ferricyanide (red blood liquor salt), dissolved in 1 liter of water
- B: 100 g sodium thiosulphate , dissolved in 1 liter of water
For the application, A and B are mixed in equal parts to a working solution, which, however, only has a short shelf life. The effect is based on the fact that the metallic image silver is oxidized again to silver ions . This reduces the blackening or, if necessary, completely eliminates it. The shelf life of the solution can be increased significantly by adding soda.
The Farmer's attenuator attacks all parts of the treated image equally and thereby increases the contrast at the same time. This solution can of course also be applied selectively to individual parts of the image by applying it to the appropriate areas with a brush and watering the image again when the desired weakening is achieved. The application was widespread in conventional offset reproduction in connection with peelable or washable masking varnish, with which the parts of the film that were not to be changed were protected.
In addition to the well-known Farmer's method, other chemicals such as ammonium peroxodisulphate are also used for weakening .
See also
literature
- Helmut Kipphan (Hrsg.): Handbuch der Printmedien. Technologies and production processes. Springer, Berlin et al. 2000, ISBN 3-540-66941-8 , p. 519 f.