The primary drawback to the use of thiourea as stabilizer
is that the silver-thiourea complexes formed in these
treatments degrade to silver sulfide. Additionally, in
the presence of oxygen, thiourea is strongly corrosive to
to silver. Thus, when thiourea is used as a stabilizer
for monobath processing, the resulting image eventually
disappears because of the corrosive action of thiourea on
silver, the formation of silver sulfide on the surface of
image particles as the result of the thiourea breakdown
cycle, and the ultimate conversion of the image to silver
sulfide.
--greg schmitz
=============================
Edmundson, Thomas M. and M. Susan Barger. "The Examination, Surface
Analysis, and Retreatment of Eight Daguerreotypes Which Were Thiourea
Cleaned in 1977. TOPICS IN PHOTOGRAPHIC PRESERVATION, VOL 5. American
Institute for Conservation Photographic Materials Group, 1993: pgs
14-26.
I. Background and Introduction
Thiourea and mineral acid silver-dip solutions were first
described by Howard Brenner in 1953 for the cleaning of fragile
silver objects unable to withstand abrasive cleaners. [1] The
original formulas recommended by Brenner were modified and adapted
for cleaning daguerreotypes by Mrs. Ruth K. Field, assistant curator
of the Missouri Historical Society, and her formula was first
published by Charles van Ravensway in 1956. [2] The Missouri
Historical Society formula and its variations were widely used and
were thought to be perfectly safe and fool-proof treatments for
daguerreotype cleaning. However, by the early 1970's some reports
about problems associated with thiourea cleaners began to appear.
These problems included indications that daguerreotypes could not be
repeatedly cleaned in thiourea solutions as had been previously
thought, that some thiourea-cleaned daguerreotypes developed
milky-white coatings over time, and that some daguerreotypes
developed "measles." [3] The increasing alarm over the possible
adverse effects of thiourea cleaners was addressed by the fledgling
Photographic Materials Group (PMG) of the American Institute for
Conservation and in 1979, the membership of the PMG agreed to a
moratorium on the cleaning of the daguerreotypes until more research
could be done.
A few papers were published describing research to improve
thiourea daguerreotype cleaning solutions, [4] however, very little
research was done in looking for alternatives to thiourea cleaners.
Three groups published results on the use of plasmas for cleaning
daguerreotypes. [5] A method of electrocleaning daguerreotypes was
published in 1986. [6] In the latter paper, it was reported that
surface characterization of daguerreotypes showed that
thiourea-silver complexes remain on the surface of thiourea-cleaned
daguerreotypes and that these byproducts are not removed by washing.
[1] Howard Brenner. "Silver Dips," SOAP AND SANITARY CHEMICALS 29
(1953): 161-167, 183.
[2] Charles van Ravensway, "An Improved Method for the Restoration of
Daguerreotypes, IMAGE 5 (1956): 156-159.
The Missouri Historical Society formula is as follows:
Distilled water 500 cc
Thiourea 70 grams
Phosphoric acid (85%) 80 cc
Non-ionic wetting agent (e.g.
Kodak "Photo-Flo" solution) 2 cc
Distilled water to make 1,000 cc
The daguerreotype is uncased and washed in distilled water to remove
any surface dirt. It is then immersed in the thiourea solution until
any discoloration is washed away. Finally, the plate is rinsed in
tap water, possibly washed in distilled or soapy water, rinsed again
in water. A final alcohol dip is sometimes used to facilitate drying
of the plate.
[3] Leon Jacobson and W.E. Leyshon, "The Daguerreian Measles
Mystery," GRAPHIC ANTIQUARINA (Sprint 1974): 14-15.
[4] For example see: T.J. Collings and F.J. Young, "Improvements in
Some Tests and Techniques in Photograph Conservation," STUDIES IN
CONSERVATION 21 (1976): 79-84; Siegfried Rempel, "Recent
Investigations on the Cleaning of Daguerreotypes," in AIC PREPRINTS
(Washington, DC: American Institute for Conservation, 1980): 99-105;
Alice Swan, "The Preservation of Daguerreotypes," in AIC PREPRINTS
(Washington, DC: American Institute for Conservation, 1981): 164-172.
[5] Vincent Daniels, "Plasma Reduction of Silver Tarnish on
Daguerreotypes," STUDIES IN CONSERVATION 26 (1981): 45-49; M. Susan
Barger, S.V. Krishnaswamy, and R. Messier, "The Cleaning of
Daguerreotypes: I. Physical Sputter Cleaning. A New Technique," in
AIC PREPRINTS (Washington, DC, 1982): 9-20; Barger, Krishnaswamy, and
Messier, "The Cleaning of Daguerreotypes: Comparison of Cleaning
Methods," JOURNAL OF THE AMERICAN INSTITUTE FOR CONSERVATION 22
(1982): 13-24; Mogens S. Koch and Anker Sjogren, "Behandlung von
Daguerreotypien mit Wasserstoffplasma," MALTECHNIK RESTAURO 90
(1984) 58-64.
[6] M. Susan Barger, A.P. Giri, William B. White, and Thomas M.
Edmondson, "The Cleaning of Daguerreotypes," STUDIES IN CONSERVATION
31 (1986): 15-28.
>===for PGP Key finger Greg Schmitz <gws1@cunix.cc.columbia.edu>===
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Key ID: A7026B1D Key Size: 1024 bits Created: 1994/11/10
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