2.1 MATERIALS AND METHODS

2.1.1 Experimental Group

The 20 19th-century albumen photographs selected for this study typify the mature albumen printing process used between the mid-1860s and the 1890s. They have a moderate to high sheen. Ten of the photographs were mounted to heavy paperboard; 10 had never been mounted. All appear to have been coated with albumen by large-scale, commercial manufacturers. Prints were selected to avoid additional variables, such as tinting with aniline dyes or silver mirroring. An attempt was made to secure photographs with diverse origins. Of the 10 never-mounted prints, 7 are by various photographers active in the American West, and three originate from the same photographic publishing house in Paris. The seven American photographs came from the same institutional collection. Of the 10 mounted photographs, 3 are German, 2 are American, 2 are English, and 1 is of French, 1 of Italian, and 1 of Spanish origin. All mounted prints are from two private collections.

2.1.2 Dimensional Characterization

Prior to treatment the photographs were measured along the machine direction and cross-machine direction of the paper base. Albumen thickness was measured using a Leitz Component Measuring Microscope (McCormick-Goodhart 1989). With this device the top layer of the albumen is brought into focus. The microscope is then focused through the albumen layer to the paper fibers below. The resulting focal length displacement is digitally recorded. This technique is capable of accuracy up to 0.3 μm. There is, however, no compensation for the index of refraction of the albumen, which makes the albumen coating appear very slightly thinner.

A drawback to this technique, when applied to albumen prints, is that albumen thickness is widely variable across the surface of the same photograph. To help compensate for the irregular nature of the coating, 10 measurements of randomly selected areas were made for each photograph. The 10 measurements were then averaged.

2.1.3 Characterization of Albumen Cracking

After considering many techniques to quantify albumen cracking, the simplest alternative proved to be the best. A representative area on each photograph was chosen. This area was always well away from the edges of the photograph. Areas of abrasion or other damage were avoided. The area (1.1158 mm2) was photographed through a microscope at 44.5�. The film used was Polaroid PolaGraph High Contrast 35 mm film, chosen because the enhanced contrast emphasized crack delineation. The samples were lit with a raking light, which was kept constant for all photographs. A polyester template with cutout openings was created so the areas examined could be returned to after surface cleaning and washing.

The resulting slides were projected onto a smooth, reflective surface. Cracks were measured at their widest point using a ruler in mm. To ensure the same projected magnification for each slide, a calibration slide was made. The calibration slide showed a micrometer with a measured distance of 0.1 mm also at 44.5�. Prior to making measurements from the projected slides, the magnification on the slide projector was adjusted so that the 0.1 mm distance equaled 40.0 mm when projected. This technique also had the advantage that each sample had the same conversion: 1 mm projected image equaled 2.5 μm of actual distance.

The slides were projected onto the reflective surface in treatment sequence. Characteristic cracks from the first slide were traced on the reflective surface. After measurements were taken, the same cracks in the after treatment slides were superimposed on these markings. This technique ensured that the same cracks were measured across the same points before treatment, after surface cleaning, and after immersion. The number of cracks measured for each photograph was dependent upon the extent of cracking before treatment. Following treatment the number of cracks had measurably increased in all but two prints. Only cracks present in the before-treatment slides were used to calculate change in crack width. Since new cracks had smaller widths, their incorporation into crack width calculations would have biased the average.

Crack population was calculated as a function of the crack width measurement process. The average number of cracks before treatment was 20.6, based on the 1.1158 mm2 measured area. Therefore, for a square inch there was an average of 12,000 cracks; for an average 8 � 10 in. print there were approximately 1 million cracks.

2.1.4 Characterization of Gloss

Print gloss has always been a defining, essential photographic attribute. Gloss, with its associated benefits of image sharpness and detail, was very much an aesthetic consideration during the 19th century. As the albumen printing process developed, thinly coated albumen prints were gradually replaced by glossier prints with thicker albumen coatings. A further transformation took place as the pictorial aesthetic evolved in the late 19th and early 20th centuries. At that time, matte albumen papers (made with starch or arrowroot added to the albumen) were introduced. These papers constitute the last large-scale commercial production of albumen paper.

As a physical attribute, gloss can also be used to gauge the character of a surface coating. A rough, nonuniform surface will scatter light and appear matte. On the other hand, a smooth, regular surface will reflect incident light, resulting in higher reflectance and higher gloss.

Specular gloss measurements were taken with a Dr. Lange Labor Reflektometer using the 60� incident light–reflected light geometry. The ASTM Standard Method D523-85 for specular gloss was followed (ASTM 1985).

The glossmeter is a relatively simple device. Measurements are based upon the difference between a known quantity of incident light projected by the instrument and the amount of reflected light detected by a photocell within the instrument. These quantities are scaled from 0 to 100.

Three (30 mm2) areas across the individual photographs were measured. These areas were measured three times each. This procedure was repeated a total of three times, resulting in a total of 27 measurements per photograph. The data were averaged to yield a single, overall gloss measurement for each photograph before treatment, after surface cleaning, and after immersion. The resulting data meet the ASTM criteria for reproducibility and repeatability. Areas of potentially high wear, such as the edges of the photographs or abraded areas, were avoided when making measurements. A polyester template with cutout openings was made for each photograph so the area measured before treatment could be returned to after treatment.

2.1.5 Characterization of Albumen Print Color

The 20 albumen prints were analyzed for color using a Hunterlab Ultrascan integrating sphere spectrocolorimeter. Diffuse illumination of the sample was achieved with a simulated CIE D65 spectral distribution light source. Data are reported as both spectral reflectance and in the CIE L∗a∗b∗ colorimetric scale.

Spectral measurements between 375 nm and 750 nm were made at 5 nm intervals. Data are reported as percent reflectance (%R) of a theoretical white, which is approximated by fresh fumed barium sulfate or microbeads of a solid chlorofluorocarbon (Halon). Measurements were taken in the specular-included (SIN) mode, which comprises, in part, incident reflected light. In planning the experiment, it was anticipated that different colored mounts would influence the color of print highlights. However, during initial trials it was discovered that the specular-included mode virtually eliminated the influence of colored photographic mounts on measurements of albumen print highlights.

Colorimetry was chosen over the more familiar densitometry for a number of reasons. Print color can be correlated to human perception when using colorimetry. Changes in print color due to treatment could, thus, be related to human perception. A difference of 0.2 color units is generally considered to be the limit of perception for the standard observer. Colorimetry also yields spectral reflectance data across the visible spectrum. Similar to human perception, the colorimeter is more sensitive than the densitometer to changes in light colors. A slight color change in the print highlights could, therefore, be measured more accurately with the colorimeter than with the densitometer (Popson 1989). Since aqueous treatment was thought to have the greatest potential impact on areas of minimum image density, colorimetry emerged as the measuring tool of preference.

Data from the colorimeter were quantified using the CIE 1976 L∗a∗b∗ (CIELAB) color scale. The scale is defined by a vertical axis and two intersecting perpendicular axes. The L∗ scale is the vertical axis measuring lightness-darkness. Along the L∗ axis L∗=0 is black, L∗=100 is white. The a∗ scale measures redness-greenness. Positive values for a∗ measure the amount of red; negative values measure the amount of green. Similarly, b∗ values measure the amount of blue or yellow. Positive b∗ values measure the amount of yellow; negative b∗ values measure the amount of blue.

L∗a∗b∗ data were gathered for the albumen print maximum density areas, middle density areas, and highlights. Polyester templates with cutout openings were used so the exact area measured (0.375 in diameter) before treatment could be measured after surface cleaning and immersion. It was determined that five individual readings for each area were sufficient to produce repeatable results with acceptable standard deviation.

2.2 EXPERIMENTAL DESIGN

Once fully characterized, the 20 photographs were treated. The treatments were designed to best approximate common conservation practice. The experimental philosophy was to follow the effects of a common treatment protocol rather than the independent effects of surface cleaning or immersion. The latter experimental design would have required twice the number of prints if performed at the same level of accuracy.

Note that the data used in a previous report on this work (Messier and Vitale 1993a) have been recalculated. The previous report concentrated only on gauging the width gain of pre-existing cracks. Following that report, it became apparent that means existed to gauge the formation of new cracks. This assessment of new crack formation required a repetition of crack measurements for the never-mounted photographs. Measurements were also collected for the mounted photographs (these measurements were not previously reported). The current data for crack width increase for the never-mounted photographs are lower by approximately 10–12%, a range within the 95% confidence intervals previously reported.

The photographs were first surface cleaned with deionized water applied with cotton swabs. After surface cleaning, the mounted photographs were air-dried, while the unmounted photographs were dried between smooth photographic blotters and under weight (0.1 psi) to minimize planar distortion. Once dry, color, gloss, and crack width and population data were collected again.

The photographs were immersed in baths of deionized water. The never-mounted photographs were immersed for 1 hour, then transferred to a final bath of fresh deionized water for 15 minutes. Photographs with mounts were immersed until they could be safely separated from their mounts with gentle manipulations of a Teflon spatula. Length of immersion time varied from approximately 1 hour to up to 4 hours. Immersion times were noted, though it became clear that the amount of time spent in water had no correlation to the amount of change measured after treatment. Once unmounted, each photograph was placed in a fresh bath of deionized water for 15 minutes. Following immersion, both sets of mounted and never-mounted photographs were allowed to air-dry face up until the surface gloss was gone (approximately 10–15 minutes). They were then placed between smooth photographic blotters and under light pressure (0.1 psi) until dry. The blotters were changed once after 15 minutes. The photographs remained between blotters for approximately 48 hours before the effect of treatment was assessed. There was no evidence of albumen adhering to the blotters.