THE OZONE FADING OF TRADITIONAL NATURAL ORGANIC COLORANTS ON PAPER
Paul M. Whitmore, Glen R. Cass, & James R. Druzik
THE AUTHENTICITY OF THE natural pigments obtained from museum collections was tested by a variety of analytical techniques. Solubility tests and color reactions8–11 were usually performed first, for not only did these tests provide a great deal of information (often sufficient for a definitive identification), but the solutions generated in these tests could also be used in subsequent analyses. Transmission spectra of pigment solutions were recorded in the 380 nm–700 nm range on a Diano Match Scan II spectrophotometer and were compared to literature results12 or to spectra of reliable samples obtained from suppliers of artists' materials (Windsor & Newton, Fezandie & Sperrle, Cerulean Blue, A. F. Suter Co.), chemical supply houses (Aldrich, Sigma), and from Prof. Helmut Schweppe. Thin layer chromatography on polyamide plates was performed according to published procedures,11 and comparison to authentic reference samples permitted differentiation of closely related colorants, such as cochineal and lac lakes, or various yellow lake pigments derived from different plants. The results of these analyses indicate that most of the pigments tested which bear the names of natural colorants were genuine. Only these natural colorants whose chemical compositions could be established were included in the ozone exposure test. The complete list of the natural organic colorants tested is shown in Table 1, along with the Color Index designation for each. Also shown are the number of different examples of each colorant which were tested. These multiple samples reportedly represented various shades, manufacturers, or dates of manufacture for a single coloring agent, but this information could not be substantiated in most cases.
Table 1 The traditional natural organic colorants tested in this experiment. Also shown are the Color Index designation and the number of different examples of each colorant.
Because of the limited availability of many of the natural colorants tested, a technique was developed which allows preparation and study of very small (milligram) quantities of pigment on paper in the absence of any binder. A suspension of pigment in about 1 ml of volatile solvent was airbrushed (Iwata HP-A) onto a 1″ × 2″ piece of watercolor paper (cut from 22″ × 30″ sheets of Arches 140 lb. hot-pressed paper) through a stainless steel mask with a 1 cm diameter hole. This produced an even, matte 1 cm spot in the center of the paper, and the pigment particles were embedded into the paper securely. Repeated color measurements of a number of control samples were made prior to exposure to verify that routine handling did not produce a measurable color change due to loss of pigment from the samples. (Four colorants—curcumin, dragon's blood, gamboge, and saffron—were applied as dye solutions rather than as pigment suspensions.) An effort was made to produce samples whose reflectance at the wavelength of maximum absorption was about 40%, for at this initial reflectance the color change has been found to be most sensitive to changes in the colorant concentration.13
Reflectance spectra were measured using the Diano Match Scan II, with the spot size of the light beam limited to 7 mm using the small-area-view option. This small sample size reduces the signal-to-noise ratio of a measurement and also increases the sensitivity to inhomogeneity of the pigment samples, so a jig was made to hold the paper samples in a reproducible position in front of the sample port of the spectrophotometer. This allowed precision of a few percent in reflectance measurements, or an uncertainty in ΔE of less than about 1 unit. The instrument was calibrated with a standard white tile (referenced to an NBS standard) and, owing to the translucence of the paper samples, this tile was used as a backing during reflectance measurements. The calibration and reflectance spectra were all recorded with the specular beam excluded. Reflectance measurements were made at 2 nm intervals from 380 nm to 700 nm and were stored on floppy disks. Chromaticity coordinates and color differences (using the CIE 1976 L∗a∗b∗ formula) were calculated for the CIE Illuminant C.14 Munsell color notations were subsequently calculated from these chromaticity coordinates.15
The ozone exposure apparatus has been described in detail elsewhere.1 Briefly, an airstream was pumped through a series of filters to remove existing pollutants. Following humidification, the air flow was irradiated by ultraviolet light from a mercury vapor lamp housed in a commercial ozone generator (Ultra Violet Products SOG-2) and then admitted to the exposure chamber through a perforated Teflon tube. The ozone concentration inside the chamber was measured continuously with a UV photometric ozone monitor (Dasibi Model 1003 AH), and a computer interfaced to the ozone monitor automatically collected and stored the average ozone concentrations during the course of the exposure.
Samples of all the colorants on paper were prepared and mounted on anodized aluminum panels. These panels were hung in the exposure chamber adjacent to the chamber walls, with the samples facing the interior of the box. The colorants were exposed to 0.397 ± .007 parts per million (ppm) ozone at 72°F and 50% RH in the absence of light for 12 weeks. The samples were removed and reflectance spectra recorded after 1, 2, 3, 4, 6, 8, and 12 weeks of exposure.