As earlier posts have mentioned, the first phase of our research aims to establish the lightfastness of the Orasol® and similar Sorasolve metal-complex solvent dyes in isolation‒ that is, in the absence of a binder, and without a chemically active substrate that could potentially influence the behavior of the dyes or interfere with the measurement of color change. To accomplish this we needed a substrate for the dye film that would not be altered physically or chemically by the accelerated aging exposure and would not interfere with our reflectance measurements (reflectance measurements will be discussed further in up-coming posts). The required substrate would need to be:
1. Optically pure.
Any color or visual irregularities in a substrate would affect our color measurements, and impede an accurate calculation of the dyes’ lightfastness.
The substrate needs to withstand a degree of handling throughout its use. For example, in one test cycle, the samples are measured for color change 8 times, in addition to before-, during-, and after-treatment photography.
3. Chemically inert.
The substrate must not be altered physically or chemically by the accelerated aging process, to ensure that it does not discolor (impeding accurate measurement of color change in the dye itself) or produce reaction products that could potentially alter the behavior of the dye.
Before making our final choice, we considered and rejected several candidates:
Wool textile: inexpensive, readily available, and resilient to handling. It has the great benefit of accepting the dye in a manner very similar to the fur/hair on our taxidermy specimens because it is made from the same material, keratin fiber. However, it is not chemically inert, and is known to bleach or yellow during accelerated aging.
Acid-free paper: durable and inexpensive, but, like wool, is not chemically inert and may undergo color change or influence the stability of the applied dye. In addition, it accepts the dye solutions in a manner quite different from hair/fur, as they are readily absorbed deep into the sheet.
Glass slides or plates: much more chemically stable than wool or paper, though they may still produce reaction products in high temperatures and humidity. Dye applied to glass sits on the surface, similar to keratin fiber. Glass is inexpensive, but could be cleaned and reused if necessary. It is, of course, more fragile than wool or paper.
Ultimately we decided to use thin quartz plates, because they best fulfill our optical, physical, and chemical requirements for a substrate. Quartz plates consist of pure silicon dioxide (SiO2). They are not only optically pure (being transparent and colorless) and have a high damage threshold; they are also non-absorbent and reusable. However, as we came to appreciate, they are typically used in very sensitive research applications, which require high standards of dimensional exactness and chemical/optical purity. As such, they aren’t cheap. Finding affordable quartz plates was very challenging, and eventually took us to a Chinese company that was able to provide us with custom-cut plates that easily met the needs of our project, without the cost that accompanies the exacting standards required by other research applications.
Drawing on the procedure outlined in the ASTM D4303 standard, our testing of each dye solution and set of lighting conditions uses three replicate samples (A, B, and C). Two plates (A and B) are exposed in the aging chamber during the test cycle, and the third (C) is held in dark storage as a control. Throughout each test cycle, color measurements are taken periodically from each replicate. Each plate is measured in three locations, and these results are averaged to produce an overall measurement for that sample at that time index.
Having selected a transparent substrate, it was necessary for us to then identify a material to place behind our samples during our reflectance measurements and accelerated aging cycles. For the former, a standard white backing material was needed — something opaque (to eliminate any influence on the measurement from the tabletop or work surface behind this backing), and either durable or easily replaced. It would also need to be extremely white (i.e. with a uniform high reflectance throughout the visible spectrum) so that it would reflect the light transmitted through the sample back up into our reflectance spectrophotometer without distorting the spectrum of the dye. We chose to use plain white nitrocellulose membrane filters stacked three layers deep to ensure opacity. While a white ceramic tile could also have been used, we felt that the membrane filters would be more easily incorporated into the wooden jig that we are using to standardize our measurement locations.
For the same reason that a white backing material is helpful in color measurement, it would be problematic in our accelerated aging exposures: a white surface would act as a mirror, reflecting light from the xenon lamp that had already passed through the sample back up toward the plate. This would be exposing the dye films twice, once by direct exposure of the lamps, and a second time by reflected light from below, increasing the light exposure dose by some unknown percentage. This would make our determination of a rough equivalence between accelerated and real time aging periods in our dioramas more difficult. Eliminating the backing altogether would have the same effect: the plates would rest directly on our reflective aluminum sample holders. We needed to identify a material that would absorb the light transmitted through the sample plates but also would withstand the extreme environment within the accelerated aging chamber. We would need to use a black backing inside the chamber.
To realize this light-absorbing backing, we initially lined our aluminum sample holders with black archival matte board. We also added bumpers along the edges of the backing to create a small space for air circulation between the board and the plates, which helps with heat dissipation. Unfortunately, we discovered that the matte board generated a sticky condensate on the underside of the plates during aging. We altered our design to instead use black textile masks and corrugated blue board. These have proven to be free of the condensate problem of the matte board. The masks slowly fade during our exposures that include UV radiation, so we designed our sample mounts in such a way that new masks can be inserted when replacement is needed.
Part Two will describe how we prepare the quartz plate dye samples.