JAIC 1981, Volume 21, Number 1, Article 3 (pp. 49 to 64)
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Journal of the American Institute for Conservation
JAIC 1981, Volume 21, Number 1, Article 3 (pp. 49 to 64)

MONITORING THE FADING AND STAINING OF COLOR PHOTOGRAPHIC PRINTS

Henry Wilhelm



2 DIRECT MONITORING OF COLOR PRINTS

OF THE TWO METHODS described in this article for monitoring the fading and staining of color prints, most institutions will choose to measure image changes directly on the prints. Separate “fading monitors” are useful chiefly for research in cases where the fading monitor's unique ability to distinguish between light-induced changes and dark-storage deterioration can provide important information in the design of better storage and display conditions. The difficulties involved in preparing and using fading monitors, as well as the fact that monitors cannot be made for print materials which are no longer commercially available, will limit their routine institutional use. In some cases it may even be impossible to identify precisely the type of color material that was used to make a print, so that it will not be possible to use a fading monitor with that print. There may also be uncertainty as to whether or not an original print has been correctly processed and washed; to be accurate, a separate fading monitor must be made with materials processed and washed exactly as was the print being monitored. These and other requirements will generally limit the use of monitors to contemporary color prints, where the monitor color patches can be made at the same time as the print. In most situations, a color print should be monitored directly even when a separate fading monitor is being used.

Suitable color reflection densitometers for print monitoring are available from a number of companies.7Kodak Wratten filter numbers 92 (red), 93 (green), 94 (blue), and Visual 106 are recommended for this application in preference to Kodak Certified Status A filter sets because the Kodak Status A filters are more likely to become obsolete in future years than are the traditional Wratten filters; the Status A filters were designed primarily for Kodak chromogenic materials and may not be well matched to the dye sets used in other types of color photographs. However, some high-quality photographic densitometers are now being supplied only with the Status A filters, and these filters should be used if the Wratten filters cannot be readily installed. Densitometer filters should be replaced by the instrument's manufacturer every 3 or 4 years.8 To ensure maximum life of the instrument, the densitometer should not be used for other museum applications such as photographic process control. In general, readings are taken with the red-green-blue filters; use of the Visual 106 filter is not necessary during the monitoring of most types of color prints, although it should be used in addition to the color filters for the monitoring of monochrome prints. Density data should always be recorded in the standard red-green-blue sequence to avoid confusion.

It is not possible to measure image density accurately when a print is framed with glass, so the print should be removed from the frame when density readings are to be made. To avoid direct contact of the densitometer head with the surface of the print, and to provide an exact record of the densitometer reading locations on the print, a thin matte-surface polyester overlay sheet9 must be prepared for each print (cellulose acetate or polyvinyl chloride sheets are not satisfactory because curling and dimensional stability are potential problems), with the print image locations marked on the sheet so that the densitometer head may be registered accurately on the image during each series of readings (Fig. 1). The matte surface of the polyester sheet accepts ink and pencil lines readily; ordinary clear polyester should not be used, as ink will not firmly adhere to the surface and may smear or wear off. The author has found that polyester sheets with one matte side and one glossy side are more satisfactory for this application than sheets both sides of which are matte. The sheet should be cut about 5 cm (2 inches) larger than the print in both dimensions; space on the edges can be used to write identification data, date of preparation, and other information. The polyester sheet material should be 3 mil or 4 mil (.003 or .004 inch) in thickness for general applications; thinner material may be used for prints 20 25 cm (8 10 inches) and smaller.

Fig. 1. Print and matte polyester overlay sheet marked with densitometer head reading locations. The photograph (original is a Kodak Dye Transfer color print) has an image size of 25 25 cm (10 10 inches) and is of the John F. Kennedy family; it was taken August 4, 1963, by Cecil Stoughton. The original color negative is part of the John F. Kennedy Library Collection.

Densitometer head locations must be precisely marked on the overlay sheet using a technical pen with a medium point and a suitable stable black ink.10 The glossy side of the polyester sheet is placed down, against the surface of the photograph. The matte side of the sheet is on top, and all ink markings and notations should be on the matte side of the sheet. Great care should be taken to keep ink away from the photograph. It is usually satisfactory to mark the densitometer head locations by tracing the outer edges of the densitometer base plate (Fig. 2). After the densitometer head locations have been determined and marked, the polyester sheet should be removed and a hole about 1.5 cm ( inch) in size should be cut at each densitometer head reading location.11 All ink markings on the overlay sheet should be completed before cutting the holes; otherwise, the pen point might accidentally slip through a hole and deposit ink on the photograph. To avoid confusion, the overlay sheet should be marked (e.g., “Top,” “Bottom”) to indicate its position. The sheet should bear a serial number that identifies the print, and densitometer calibration data should be recorded in a notebook made of reasonably stable paper. The overlay sheet must remain in exact registration with the print during all readings; smooth-surfaced weights placed on the sheets will help keep them in position. Because of localized density variations in most color prints, subsequent readings must be taken at precisely the same locations as the original readings if the measurements are to be meaningful. When densitometer heads are changed, a method must be devised for positioning the new head using the old tracings. The importance of taking future readings at the exact locations used for the original readings cannot be overemphasized: accurate data cannot otherwise be obtained. The person taking the readings should test his or her technique by seeing whether readings taken by a second person produce the same results.

Fig. 2. Densitometer head located in the reading position by an ink tracing of the base of the head on the polyester overlay sheet.

Work areas should be clean, and to avoid putting fingerprints on the photographs, cotton gloves should be worn by the operator. A sheet of bright white matte board or opaque white glass should be placed on the work table and prints placed on this white surface while densitometer readings are made. As most print support materials transmit a significant amount of light, the reflectance of the surface beneath a print may have a significant effect on densitometer readings made in low-density areas of a photograph. For example, readings taken on a dark work surface will usually indicate somewhat higher densities than readings taken on a white work surface; such discrepancies may be quite large with albumen prints and other types of photographs made on thin paper supports. The same type of work surface should be used for making all readings, and densitometer readings of the work surface itself should be recorded each time the work surface is cleaned or changed.

Density readings through the red, green, and blue densitometer filters should be taken at minimum-density locations, low-density locations of about 0.35, and maximumdensity locations. Normally, readings in each density range should be taken in at least three locations (for a total of at least nine reading locations), and the readings should include near-neutral colors if possible. Some of the readings should be taken near the top of the print, where the intensity of illumination during display is usually somewhat higher than it is in the lower areas. Fairly large areas of uniform tones should be selected if possible for reading locations. However, in many photographic images such areas may not be available, so a large number of readings will be required for accurate representation. After readings have been taken, the polyester overlay sheets should be stored flat in a protective polyester sleeve12 or in a high-quality paper envelope.

When prints are displayed and are subject to light fading, it is particularly important that minimum-density and low-density (approximately 0.35) areas be carefully monitored, as dye losses in the low-density areas will be much greater proportionally than dye losses in high-density areas (Table II). This relationship between dye loss and density is typical of many types of color print materials; however, some types of prints, such as Polaroid SX-70 prints, may develop stains which alter minimum density behavior. In dark-fading, dye losses tend to be proportional throughout the density range of the color image. Prints which have faded in dark storage do not suffer from the type of highlight detail loss that is characteristic of prints which have faded because of exposure to light.

TABLE II


Copyright 1981 American Institute of Historic and Artistic Works