The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Cyanotype
a 2 per cent, solution of silver nitrate, washing well, and de-veloping the pale yellow image with ferrous oxalate developer. Instead of using a mixture of ferridcyanide and ammonio-citrate as the sensitive mixture, the latter solution alone may be used for sensitising the paper, and the ferridcyanide solution used as a developer ; or development may be with a chloride of gold solution. (See Chrysotype.)
Positive Cyanotype, or Pellet's Process. This process is the opposite to the last - that is, it gives blue lines on a white ground from a plan, or, in other words, where the light acts no image is formed; only on those parts where the light does not act is a precipitate of Berlin blue formed by the action of the ferro-cyanide of potassium with a ferrous salt. This process was also suggested by Herschel, but no satisfactory progress was made till Pellet patented in 1877 his process of adding a viscous substance to the sensitising liquid. Pellet used
Oxalic acid ...        ...        ...        ...        ...        5 parts.
Ferric chloride         ...        ...        ...        ...      10 ,,
Water ...        ...        ...        ...        ...     100 ..
Gum Arabic...        ...        ...        ...        ...     9*5 ..
In 1880 Collache patented a similar process, and used
Gum Arabic ... ... .... .. ... 7-10 parts.
Citric acid ... ... ... ... ... 2-3 ..
Ferric chloride solution 450 Baume ... 4-6 ,, Water           ...............81-87 .,
Pizzighelli in 1881 gave good working formulae for this pro-cess : -
I.
Gum Arabic ... ... ... ... ... 20 parts.
Water* ............... 100 ..
II. Ammonio-citrate of iron ... ... ... 50 parts.
Water           ...............100 ,,
III. Ferric chloride         ... ... ... ... 50 parts.
Water           ... :........ ... 100 ,,
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