THE CLEANING OF DAGUERREOTYPES: COMPARISON OF CLEANING METHODS
M. Susan Barger, S.V. Krishnaswamy, & R. Messier
SINCE THE DAGUERREIAN ERA, the usual method for the removal of tarnish from daguerreotypes has been the use of various solutions of solvent cleaners. Until the mid 1950's, the most widely used cleaning solution was potassium cyanide; after that time, solutions of thiourea acidified with phosphoric acid have been the cleaners of choice. These solvent cleaners alter the appearance of daguerreotypes not only by the removal of silver sulfide tarnish, but also by the removal of other constituents of the daguerreotype: silver, mercury, gold, and copper. The chemical action of these cleaners is not fully understood, and in some cases their use has caused irreversible damage to daguerreotypes.1 The work of Swan2–4 and Rempel5 provides introduction and review of the history of daguerreotype cleaning, the deterioration of daguerreotypes, and the current recommendations for the care of tarnished and otherwise damaged daguerreotypes.
The ideal daguerreotype cleaner, as set forth in an earlier paper6 should selectively remove sulfur from the silver sulfide (Ag2S) tarnish and other tarnish debris, but otherwise leave the underlying daguerreian microstructure unaltered. Since the Ag2S layer is, in general, much thinner than image particle height, removal of the complete Ag2S layer would also be an acceptable solution to the cleaning problem. This paper describes experimental results from a comparison of the effects of traditional daguerreotype cleaning methods using solvent cleaners, and also a new method, physical sputter cleaning. Both the effects on gross appearance (optical properties) and the effects on the microstructure are examined in order to determine the efficacy of these cleaners.
Sputter cleaning refers to a cleaning process carried on in a gas plasma. This cleaning process may entail only the physical removal of material, as in physical sputtering, or the removal of a volatile species formed by chemical reactions in the sputtering gas(es), as in chemical sputtering. Physical sputtering is like a billiard game played on an atomic level in which material is removed due to the momentum transfer from the impact of heavy ions from an inert gas plasma (e.g., Ar+).
There are two types of chemical sputtering: plasma chemical reduction and reactive sputtering. The use of plasma chemical reduction on daguerreotypes has been described in a recent paper by Daniels.7 In that instance, an H2 plasma was used to reduce Ag2S on the daguerreotype surface by apparently forming H2S which was pumped away by the vacuum system [Ag2S (s) + H2 (g) → 2Ag (s) + H2S (g)].8 Plasma chemical reduction does not cause any appreciable removal of material from the daguerreotype since the H+ bombarding ions are very light and thus transfer little momentum to the Ag2S layer.
Other reactive gases (e.g., O2, used in this study) can be substituted for H2. These gases (or gas combinations) are chosen to produce selective chemical reactions on the sample surface and can be tailored to remove unwanted materials while leaving the desired materials intact. Again, a volatile species can be formed and carried away by the vacuum system [e.g., Ag2S (s) + O2 (g) → 2Ag (s) + SO2 (g)].8 In addition, the heavier O2+ bombarding ion can cause a significant physical sputtering.
Inert gas sputtering is purely a physical removal process. Sputtering in hydrogen is essentially a chemical reduction process, and reactive gas sputtering is a combination of both physical and chemical processes.