PROTECTIVE SURFACE COATINGS FOR DAGUERREOTYPES
M. Susan Barger, A.P. Giri, William B. White, William S. Ginell, & Frank Preusser
2 EXPERIMENTAL DETAILS
Fifteen nineteenth-century daguerreotypes from the MRL working collection7 were used for the major analysis of protective coatings. These samples were chosen because they are representative of the commonly found variations in daguerreotypes caused by past handling, storage, and aging. Each of these daguerreotypes was arbitrarily cut into three portions. One portion was left uncoated, one received a sputtered coating, and one was coated with Parylene C. This allowed a direct comparison of the various coatings on the same daguerreotype. In addition, some tests were done on modern daguerreotype step tablets and on polished, unexposed daguerreotype plates. Experimental results using step tablets are not substantially different from those on nineteenth-century images. However, it is very difficult to access the visual effect a coating will have on the optical properties of images using step tablets. Further, modern step tablets give no information about the physical and chemical interaction of coatings with the variety of corrosion products found on daguerreotype surfaces. For these reasons, the tests on modern step tablets are not discussed here.
2.2 Sputtered Coatings
The sputtered coatings were deposited in an rf diode sputtering unit, MRC model number SES 8632. Each daguerreotype was placed in the sputtering chamber without any special preparation. A small glass cover slip was placed on the surface so that the coating thickness could be measured using a stylus profilometer. The chamber was closed and pumped down to a base pressure of ∼3 × 10−7 Torr, which increased to ≃3 × 10−6 Torr after throttling and before sputter gas admission. Various gases were injected into the sputtering chamber using leak valves. Gas flow and pressures were monitored using an MKS Flow Ratio Controller Model 254. Sputtering conditions for the various coatings are given in Table 1. All of the targets were two inches in diameter, except the SiO2 and the aluminum targets which were five inches. Aluminum nitride (AIN) was obtained by reactive sputtering of Al-metal target in an Argon (Ar)—Nitrogen (N2) atmosphere. Different coatings (i.e., different compositions, physical structures, or thicknesses) obtained by either changing the sputtering conditions or the time of deposition.
Table 1. Sputtering Conditions
The coating materials were chosen for their optical properties, for their chemical inertness and stability, and for corrosion resistance. The optical properties of all the coating materials are given in Table 2. The composition and structure of the coatings were controlled by the range of gas partial pressures and total pressures used. This, in turn, allowed the optical properties and corrosion resistance of the coatings to be controlled.
Table 2. Optical Properties of Coating Materials
2.3 Vapor-Deposited Polymeric Coating
Parylene C is an unusual plastic film material that is used extensively in industry as a protective coating for electronic and biomedical components. Its principal advantages are: non-toxicity, very low water and gas permeability, high strength, insolubility in all common solvents, transparency in the visible region, and ability to form adherent, pinhole-free conformal coatings. Parylene C films are highly crystalline and are produced by polymerization of a diradical species, di-monochloro-paraxylylene, directly onto a surface.8 The diradical is obtained by vaporization of the solid dimer and pyrolysis of the vapor at high temperatures and low pressures. The highly reactive gaseous diradical will deposit directly from the vapor and will polymerize on any cool surface. Because deposition occurs at very low pressures, films formed by this process follow the contours of the substrate, as well as being uniform and continuous. Parylene C films can be removed from surfaces with orthodichlorobenzene.9 It has no effect on either silver, the image microstructure, or the assorted corrosion products found on daguerreotype surfaces.
The daguerreotypes used in this study were coated in a Union Carbide Corporation (UCC) deposition system, Model 1050. Prior to coating, the samples were washed in ethanol, vapor degreased in Freon 113, and primed with UCC A-174, a substituted silane. The use of a silane primer is usually recommended to improve the adhesion of Parylene C to metallic surfaces. Priming does not affect the visual appearance of the coated daguerreotype. Various coating thicknesses ranging from 1 to 5 μm were tried and deposition conditions altered to determine the effects of these variables on the visual appearance of the daguerreotype image.
The absorption spectrum of Parylene C is featureless from about 300 to 2000 nm except for interference fringes. Transmission in the visible is ∼100% at the interference maxima for a 1.0 μm film. Only slight traces of interference colors were observed on daguerreotypes coated with Parylene films thicker than 2–3 μm.
2.4 Film Characterization Methods
All of the coatings were examined and characterized after deposition to determine film structure, composition, and the optical effect of the films on the daguerreotype image. Film structure and composition were analyzed using an ISI DS 130 scanning electron microscope (SEM) with a Kevex Energy Dispersive X-Ray Detector (EDX). The optical properties of the films in the near IR-visible-near UV regions of the spectrum were assessed using a Beckman DK2A Reflectance Spectrometer with a lead sulfide detector for λ = 2000-500 nm and a photomultiplier tube detector for λ 550-300 nm. The spectrometer has a barium sulfate coated integrating sphere, and barium sulfate was also used as the reference material. The spectrometer was used to measure total (diffuse + specular) reflectance. For each portion of the sample daguerreotypes, the reflectance of a uniform section (¼″ diameter) of the image background (a midtone) was measured.10 This type of spectroscopy gives information about the optical effects of the coating on the daguerreotype and on the physical state of the image microstructure.
Fourier Transform Infrared Spectrometry (FTIR) in the region of 4000-500 cm−1 was used to evaluate the interaction of the coating materials with daguerreotype corrosion products. The mid-IR spectra of daguerreotypes contains information concerning corrosion and past treatments of daguerreotypes. It was felt that, especially for permanent coatings such as the sputtered films, the absorption spectra of the coating should not mask this region of the reflection spectrum of the daguerreotype.
Both a Talysurf 10 stylus profilometer and an interferometer were used to try to measure the thickness of the sputtered films. Neith of these techniques was successful because the daguerreotypes were not flat enough for accurate measurements. The film thickness of all the sputtered films was estimated on the basis of coating deposition conditions and varied between 0.1 and 0.2 μm. The thickness of the Parylene C films was determined by measurements made on copper witness plates that were coated along with the daguerreotype samples. An eddy current probe was used for this purpose. Table 3 lists the measured film thicknesses for all the samples produced.