Oh, golly, more. It may also be that Eder is refering to the
production of a stain image by Pyrogallol developers (and some others)
along with the silver. The stain is a reaction product of development
and is exactly proportional to the silver. In fact, the silver can be
bleached out leaving the stain. The stain is a pigment, rather
yellowish in color, and is quite permanent. Pyro also causes a
proportional hardening of the gelatin resulting in a relief image which
can be seen by reflected light. The relief image can be used to produce
prints by differential absorption of dyes or inks. When this is desired
the proportions of the developer are modified to exagerate the effect.
This allows silver-gelatin emulsions to be used in some of the processes
othewise depending on dichromate sensitization of gelatin or other
colloids, such as gum arabic, which have only a fraction of the
sensitivity to light. This was the basis of the Dye Transfer process
sold by Kodak for color printing and, indeed, the Technicolor "dye
imbibition" process for printing color motion pictures. However, most
often physical development refers to the deposition of silver on the
exposed silver grains as they are being converted to silver in a
conventional developer. It is generally supposed to take place in
slow, high sulfite, developers. In fact, pyro developers usually
contain only modest amounts of sulfite if they are intended to produce
either a stain image or a substantial amount of differential hardening
since sulfite generally interferes with the production of reaction
products in development. That is one of their main purposes.
On 7/15/2015 12:15 AM, Brian Smith (Redacted sender smithcbrian2@yahoo
for DMARC) wrote:
Hi folks - I wonder if someone can explain this to me please?
I'm reading an old book on the history of photography (by JM Eder). He
distinguishes between physical development and chemical development, but I
don't understand his explanations (or they are contradictory). Both seem to
involve a developing agent, e.g. pyrogallol.
Can anyone elighten me please?
Regards - Brian Smith
