The first phase of our lightfastness testing aimed 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. (See earlier posts to explain the project plan and selected dyes. Future posts will explain the results of this testing, which is currently underway.)
The second phase of testing uses accelerated aging and periodic color measurements to look at how lightfastness is affected when Orasol dyes are applied to aged, faded furs as they are used in a recoloring treatment. We expect to see substrate impacts on lightfastness for several reasons:
- As furs age, they produce reaction products that may affect the chemical behavior of an applied dye.
- Compared to quartz, dyes deposit very differently onto hair. There is also significant variation among fresh and aged fiber surfaces, and among fibers from different animals.
- The optical properties of dyed fibers differ from those of dye films on quartz. Differences in how the sample reflects and/or absorbs incident light affects its total light exposure dose and its apparent color.
Several considerations played an important role in our selection of animal fur substrates. First, we sought furs that are light in color. The principle reason for this was that a light-colored substrate was needed to control our dye application and keep reflectance spectra minima in the range of 30–40 percent (see earlier post for reflectance discussion). For consistency of samples, the furs also needed to be as uniform in color as possible. To facilitate sample mounting, longer fibers were preferred over short. The fibers themselves should present minimal color change upon exposure to accelerated aging, ensuring that the dye (and not the fiber) is the primary contributor to any color change observed in a dyed fiber. Finally, we aimed to represent some of the naturally occurring variation in mammal furs, from hollow guard hairs and bristles to awns and underfur.
We ultimately chose furs from two animals with naturally white coats: an arctic fox and a white-tailed deer. The latter is fully white but not a true albino, a variation selectively bred to be whiter than the closely related “spotted” piebald. The arctic fox represents fine, smooth-haired fur-bearing mammals, while the deer offers hollow guard hairs.
However, there is an important downside to using white furs such as these. As we pointed out in our discussion of backing materials for our samples on quartz, light-colored substrates reflect and scatter proportionately more light than dark ones. Transparent dye films applied to highly light-scattering substrates will be exposed twice: once directly by the lamps, and a second time by reflected light from below, increasing the light-exposure dose. While faded historic taxidermy may be light in color, most examples are generally still darker than these bright white furs.
Consequently, light-scattering plays a larger role in the fading of our samples than is expected in the treatment that we are modeling. While it could be argued that our samples represent a worst-case scenario with respect to the impact of specimen color on the longevity of recoloring treatment, we acknowledge that the use of white furs is a compromise needed to consistently produce the most light-sensitive dye application possible.
The impact of different tanning methods on the dyes under investigation is unknown and offers an interesting avenue for further research, but this is not addressed as part of our current project. Nevertheless, in order that they more closely model taxidermy in the American Museum of Natural History collection, the fox and deer pelts for this project were tanned according to methods representative of those historically used at the Museum.
When considering the production and acquisition of historical taxidermy at the Museum, particularly for use in dioramas, the period of interest spans from approximately 1925 to 1965. Though we do not have a complete understanding of all the tanning methods in use at, and for, the Museum during this 40-year time frame, we were able to partner with a local tanner trained by Sinclair Clark, a renowned tanner who was on staff at the Museum around 1924–1927. Clark later set up tanneries in other locations, but maintained his relationship with the Museum tannery over a long period of time.
In general terms, Clark’s method involves the following:
- The skin is salted to remove moisture and stabilize it prior to tanning
- Tanning begins with rehydration in a saltwater bath until the skin is soft and pliable
- It is next soaked in an acid pickle until swollen, and then shaved down on a fleshing machine or by hand
- The skin is returned to the pickle, and, if needed, shaved again
- The skin is then removed from the pickle and the acid is neutralized
- Warmed oil is applied either by hand or with a “kicking” machine
- The skin is left to sit overnight or for one day before being tumbled in hardwood sawdust until dry and soft
The upcoming Part 2 post will describe how we are mounting the furs to run as research samples.