Gallic/pyrogallic/amidol

TERRY KING (101522.2625@compuserve.com)
06 Mar 96 09:59:09 EST

Sil

Wedgwood's photograms in the 1790s worked more effectively because the leather
contained tannin which came from oak galls.
In other words, for our purposes here, gallic acid..

Pyrogallic acid was prepared from gallic acid which was prepared from tannic
acid which was prepared from nut galls. Pyro is the greek for fire. Pyrogallic
acid is heated gallic acid.

The ' ? ' in the 'subject' was an indication of a query as to which one was
meant; pyrogallic acid being the one more associated with photography for the
past one hundred and forty five years.

Pyrogallic acid, metol, hydroquinone,amidol and other developers are related to
benzene and their structure is based upon the benzene ring. This does not
necessarily imply that they are derived from coal or oil which are themselves
derived from living matter.

It does give rise to the question of to what extent the current fuss over
benzene in super unleaded petrol being a carcinogen, is relevant here.

There are very good explanations of these relationships in the 1864 edition of
'The Silver Sunbeam' and the Focal Press book, 'Photographic Materials and
Processes' 1986 edition.

Pyro is very much in fashion at the moment for use as a staining developer which
gives almost grainless negatives.

My own preferred developer for silver gelatine papers and film for alternative
process negatives is amidol which gives good detail and tonal separation across
the range of tones.

As has been said before the good negative is the sine qua non of the good print.
Amidol gives good negs.

Terry King, FRPS