1 INTRODUCTION

THE IDEA OF USING protective coatings and varnishes on daguerreotypes is nothing new. There are many references in the literature of both the daguerreian era and after which recommend various coatings for daguerreotypes. However, the known incidence of coated nineteenth-century daguerreotypes that have survived is scant. The oldest surviving American daguerreotype, Joseph Saxton's “View of the Central High School,” is coated with a copal varnish.1 Of the nearly two hundred daguerreotypes analyzed at the Materials Research Laboratory, only one other was found to be coated. It appears to have been coated with a sugar solution. These two are the only experimentally confirmed examples of coated ninteenth-century daguerreotypes.

The first use of coatings was to improve the mechanical stability of the image of ungilded daguerreotypes. In that instance, the image is made of a variety of silver amalgam crystals. Characteristically, these crystals are somewhat unstable solids that gradually gain mechanical strength over time due to mercury loss or more complete amalgamation by a process called hardening.2 For a time, daguerreotypes made without gilding remain quite fragile and the image can be removed by the slightest touch. Daguerre himself tried several of the standard organic coating materials in common use at that time—amber, copal, caoutchouc, wax, and other resins—to protect the freshly made images.3 He observed that all of these coatings considerably altered the appearance of the daguerreotypes by making the highlights appear less white and by dulling the black tones. The aging of these organic coatings caused the images to become increasingly less distinct. Others also recommended coatings, such as dextrose.4

The damaging optical effects from the coatings appear to have outweighed any advantage that they may have offered and they were not adopted for general use. Instead, what has become the traditional daguerreian package consisting of daguerreotype, mat, cover glass, and tape seal in a frame or case was adopted to protect the image. In addition, the discovery of gilding, in 1840, improved both the mechanical and optical strength of the image. The universal use of this processing step removed the need for a varnish to protect the image from immediate physical damage.

From time to time, other coatings to protect daguerreotypes from corrosion have been suggested, but again there is little evidence that these were used. The corrosion-resistant coatings include all of the varnishes listed above and also materials such as heated egg albumen.5 As with the varnishes used for mechanical protection, the alteration of the daguerreotype appearance was an unacceptable loss, disproportionate to any prophylactic qualities of the coatings.

Despite this dismal history, there may be occasions when a carefully chosen protective coating is desirable for the preservation of a daguerreotype. The prime function of such a coating should be to prevent corrosion. It is unlikely that any thin layer will have much effect on the inherent ductility of silver and copper in the daguerreotype plate. Most appropriately, a coating can be used to stop the repeated cleaning and corrosion cycle that has been used to treat so many of our most valuable daguerreotypes. Certain coatings may also provide some enhancement of the optical properties of a daguerreotype. For instance, an anti-reflectance coating could be used to improve the apparent contrast6 of an over-cleaned daguerreotype. The traditional daguerreotype package provides protection from mechanical damage, and it is also a very effective means of inhibiting corrosion. Although protective coatings provide an additional corrosion barrier, they should not be used as a substitute for casing these images.

Of course, the decision to coat a daguerreotype with anything is extraordinary and should not be done without the careful consideration and advice of a curator familiar with the value and importance of a particular daguerreotype. Coating should be viewed as a drastic measure to be used only when the corrosion-cleaning-corrosion cycle must be absolutely halted. While we recognize that the widespread use of protective coatings may not be warranted, we have tested a variety of modern coating materials that might be useful at times when such coatings are desired. The coatings used in this study all require some degree of specialized equipment. All of the coatings are used in various industries, and it is possible to make arrangements for coating applications on a fairly routine and relatively inexpensive basis. The coating materials tested include sputtered coatings of insulating metal oxides and nitrides—silica (SiO2), titania (TiO2), boron nitride (BN), and aluminum nitride (AIN); sputtered coatings of metallic lanthanum hexaboride (LaB6); and vapor-deposited coatings of Parylene C, a polymeric film. The sputtered coatings were applied at the Materials Research Laboratory (MRL) of the Pennsylvania State University. The Parylene C coatings were applied at McDonnell Douglas Astronautics Company in a cooperative effort with the J. Paul Getty Conservation Institute and the J. Paul Getty Museum. All physical characterization was done at the MRL.