Friday, June 27, 2008

Further Processing

For black and white emulsions both negative and positive, further processing may react the silver with other elements such as selenium or sulfur to produce a more permanent image.

In these cases the silver in the image is changed to silver selenide or silver sulfide, which are more resistant to oxidising agents (pollutants) in the atmosphere.

Successful selenium or sulfide toning requires complete fixation. In other cases, the silver may be chemically bleached using a potassium hexacyanoferrate (III) solution and then re-developed in a range of toning formulations.

This two-step technique is collectively called "indirect toning." Depending on the toning solution, sepia, red and blue colors may be obtained.

If color negative film is processed in conventional black and white developer, and fixed and then bleached with a bath containing hydrochloric acid and potassium dichromate solution, the resultant film, once exposed to light, can be redeveloped in color developer to produce an unusual pastel color effect.

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