(sometimes called Alternative Processes) describes a number of traditional photographic processes including Salt Prints
or Palladium Prints
, amongst others. Of these the most common are Cyanotypes
and Salt Prints
, while Platinum Prints
are prized for their tonal subtlety and longevity (a platinum print will typically last as long as the paper it's printed on without any sign of fading).
were "developed" by William Henry Fox Talbot around 1835. They involve soaking the paper in a salt solution, then coating the dried paper with a solution of silver nitrate (for the chemically minded, the silver nitrate reacts with the salt - sodium chloride - to produce silver chloride, which is sensitive to light). The paper is then exposed through a negative to ultra-violet light producing a reddish brown colour.
process was discovered by Sir John Herschel in 1849 and involves coating a suitable medium (usually watercolour paper) with a sensitising solution, then exposing it through a negative to ultra-violet light, producing a distinctive Prussian Blue colour. The shade of blue can be varied by staining with different materials - including tea, which contains tannin, a natural staining agent.
For an informative discussion on the relative merits of Alternative Processes and their digital counterparts see the article Alternative Processes - are they worth it? on the SWPP & BPPA web-site