PROTECTIVE SURFACE COATINGS FOR DAGUERREOTYPES
M. Susan Barger, A.P. Giri, William B. White, William S. Ginell, & Frank Preusser
THE JUDICIOUS USE OF protective coatings on certain daguerreotypes can be a useful step in their long-term preservation. In particular, coatings offer a way to break the harmful cycle of repeated corrosion and cleaning that has been used to treat so many daguerreotypes.
The most important characteristic of any daguerreotype coating is that it must not modify the optical properties of the image. A daguerreotype should not be regarded solely as a metal object containing a photographic image. The image on a daguerreotype is unique and quite unlike any other kind of photographic image. This image is the result of visible light scatter from the image microstructure. As such, it is a very narrowly defined physical phenomenon. In this set of physical constraints, very small physical changes can result in large changes of the image appearance.14 Any coating that overpowers the optical properties of the image is not acceptable.
Additionally, a coating should offer protection from corrosion over and above that offered by the daguerreotype package alone. A coating should have a high degree of long-term physical and chemical stability. It is also desirable that coating structure and deposition should be controllable and predictable.
Several modern coating materials were tested for use on daguerreotypes. Of those tested, the best coatings were the sputtered coatings, silica and boron nitride, and the polymer coating, Parylene C.
The inherent sphere-like structure of SiO2 coatings makes them the least corrosion resistant of the final three coatings. These films are slightly more colored than would be acceptable for daguerreotype coatings, although this coloration can be eliminated by using thinner films.
The Parylene C films tested modified the optical properties of the daguerreotypes and the films were more absorbing in the visible than either SiO2 or BN. These effects are due to the relatively thick films (2–4 μm) deposited on the test samples. Reduction of film thickness to the 0.1–0.2 μm range should alleviate this problem. The variability in film structure from one run to the next is a processing problem that requires closer control of the deposition variables. Although the long-term stability of organic coatings is of some concern, the reversibility of Parylene C allows the removal and replacement of the coating as required.
BN produced a dense, highly corrosion-resistant coating. The film structure was consistent and controllable from sample to sample. Initially, BN films have a gold or rose-gold color, but the coloration disappears over time. Controlling film thickness can eliminate this effect altogether. Most importantly, the BN coatings do not interfere with the optical properties of the image. The UV-VIS-near IR reflectance data show that anti-reflectance coatings were achieved by the BN as seen by the close match in the reflectance spectra. BN is inert and should not cause any long-term damage to a daguerreotype. It cannot be considered a removable film, and this must be accepted if BN is used. The FTIR data show that BN does not distort the corrosion and past treatment information contained in the 4000-500 cm−1 region of the spectrum. Hence, the presence of a BN coating does not preclude future analysis of a daguerreotype by SEM, x-ray, UV-VIS-near IR spectroscopy, or FTIR.
On the whole, BN, of all the coatings tested to date, has the most of the desirable film properties required for protection of daguerreotypes. However, because two of the materials studied have the potential for providing acceptable coatings, additional work should be done on stoichiometric TiO2 and on uncontaminated, much thinner Parylene C films.
THE PORTION OF THIS WORK performed at the Materials Research Laboratory of The Pennsylvania State University was funded by the Andrew W. Mellon Foundation. A Lucretia V.T. Simmons Project Renew Grant from the State College (PA) Branch of the American Association for University Women to one of the authors (MSB) provided for the purchase of the nineteenth-century daguerreotypes used for testing.
The Parylene C work is part of a more extensive study being conducted under Zdravko Barov at the J. Paul Getty Museum Antiquities Conservation Department on the use of this material for conservation purposes. We wish to thank John Thomas and the McDonnell Douglas Astronautics Company for making the Parylene C deposition equipment available for this study.