A Horse of a Different Color – researching colorants for recoloring taxidermy

For the 2011-2012 renovation of the Bernard Family Hall of North American Mammals dioramas, we were limited by the need for high lightfastness (resistance to fading), in situ treatment considerations, health and safety requirements, and reversibility/retreatability, so the project conservators looked to materials already in use in our profession (see previous posts for more details on this project). The Orasol® dyes seemed like a good alternative to the acrylic paint favored by the project taxidermist. Orasol® dyes (Ciba-Geigy; currently marketed by BASF for coatings and printing inks) are commercially available metal complex dyes that cannot be dissolved in water, but can be dissolved in many common organic solvents such as ethanol. They can be applied without salts or peroxides (which can be acidic), and require no rinsing, making them useful for in situ treatment. They are manufactured in a palette of 26 colors, which was enough to mix the range of tones needed for the North American Mammals. They are also generally approved by the conservation community and are commonly used in furniture and wood stains, as well as for tinting epoxy and other resins used for fills.

A word on dyes:

A dye is a substance that has an affinity to the substrate (in our case, the hair of the taxidermy mount) to which it is applied. In contrast, a pigment is generally insoluble and has no affinity for the substrate.

A typical dye molecule is composed of different chemical groups, each responsible for a specific property of the dye:

  1. Chromophore – color producing portion
  1. Auxochrome – influences the intensity of the color and provides the site at which the dye chemically bonds with the fiber (hydrogen, ionic, or dipole-dipole interactions with the substrate).
  1. Solubilizing group – allows the dye molecule to be soluble in a given solvent so it is capable of reacting with the fiber.
Typical dye molecule (from Synthetic Dyes for Natural Fibers. Knutson, Linda. 1982)

Typical dye molecule (from Synthetic Dyes for Natural Fibers, Linda Knutson. 1982)

Metal complex dyes are made of a transition metal ion such as chromium, copper, or cobalt complexed to two symmetrical dye molecules (chromophore and auxochrome). In general, members of this group have better lightfastness (resistance to fading) compared to other dyes due to the stability of the chelated complex and their large particle size; this, along with their working properties, has been the basis for their past use in conservation.

To determine if the dyes met all of our needs for the project, we developed some tests to compare the acrylic paints to the Orasol® dyes. The conservators worked closely with the taxidermist and partnered with outside conservation scientists to test the materials against these necessary criteria: minimal physical alteration to the hairs, retreatability/reversibility, and high lightfastness.

The physical appearance of bison hair samples colored with acrylic paint and ones colored using Orasol® dyes was examined using normal light as well as scanning electron microscopy (SEM). At first look, all of the colored samples appeared somewhat similar. The acrylic paints produced a matted and stiff feel, whereas the Orasol® dyes produced a more natural look and feel. The SEM images showed us that the binder in the acrylic paints covered the hair cuticle unevenly, creating a non-cohesive coating around the hair with some visible peeling and lifting. The Orasol® dyes were only just visible on the hair fibers and did not appear to cover or coat the hair shaft.

The surface of bison hairs imaged using scanning electron microscopy. From left to right: control (no colorant), acrylic paint, Orasol dye.

The surface of bison hairs imaged using scanning electron microscopy. From left to right: control (no colorant), acrylic paint, Orasol dye.

The taxidermist also found that the Orasol® dyes could be reduced or removed entirely when wiped or rinsed with ethanol after application. Although initially taken aback by this revelation, we quickly recognized the potential for reversibility, as well as the ability to produce special effects, such as localized reduction or removal of color to achieve special effects.

Project taxidermist reduces dye from a practice skin.

Project taxidermist George Dante reduces dye from a practice skin.

Metal complex dyes are generally regarded as having a high lightfastness. However, chromophore and auxochrome structures vary, making some dye colors more susceptible to fading. In their product literature, the manufacturer reports a definite range in both lightfastness and solubility from color to color.

Orasol dye lightfastness ratings according to manufacturer.

Orasol dye lightfastness ratings according to manufacturer.

These ratings are based on their use in printing ink, and the tests were conducted with two different binders (nitrocellulose and vinyl acetate) which produced significantly different results. To be able to judge the lightfastness of the Orasol® dyes when used on taxidermy, it was necessary to conduct our own tests without any binding media.

 The lightfastness of the dyes was tested using accelerated light-aging following the ASTM D4303 Standard Test Methods for Lightfastness of Colorants Used in Artists’ Coloring Materials. Test swatches were produced by airbrushing a 1% solution of dye dissolved in ethanol onto unbleached 100% wool as well as swatches of bleached bison fur. As a flat substrate of keratin fibers, the wool textile was chosen as a reasonable substrate to measure color changes using a spectrophotometer. Details about the testing procedure will not be listed here, but will be covered in subsequent posts as continued lightfastness testing is part of the current research program.

Conservation Fellows Julia Sybalsky and Rebecca Pollak prep swatches of Orasol dye for testing.

Conservation Fellows Julia Sybalsky and Rebecca Pollak prep swatches of Orasol dye for testing.

The ASTM protocol requires exposure of test materials to a broad spectrum of light that includes ultraviolet (UV) radiation. Since new lights in the diorama are filtered to exclude this particularly damaging light, the test helped us develop a worst-case scenario for assessing the lightfastness of the dyes.

Real-time monitoring of cumulative light exposure is continuing inside the bison diorama. Orasol dyes are exposed in situ alongside Blue Wool reference standards out of sight of visitors. At the same time, portions of the bison have been sprayed with dye mixtures and masked to permit direct comparison of exposed and unexposed areas of restoration.

Strategically placed dyed swatches and a blue wool card were placed inside the bison diorama (hidden from view), and sections of the dyed bison were covered for side-by-side comparison.

Strategically placed dyed swatches and a blue wool card were placed inside the bison diorama (hidden from view), and sections of the dyed bison were covered for side-by-side comparison.

Our lightfastness tests identified the most stable of the dye colors, allowing us to be picky about which shades could be used for the diorama taxidermy. The reversibility and retreatability of the dyes also played a major factor when deciding to use them on specimens in dioramas with higher light levels, with the understanding that fading will occur in time. The AMNH conservation and exhibition team was able to successfully recolor many of the faded specimens in the Bernard Family Hall of North American Mammals.

Coyote, before and after recoloring.

Coyote, before and after recoloring.

The dye research we did as part of the renovation generated numerous new questions, which fell into three main topics:

Lightfastness – While our previous testing did effectively establish a lightfastness ranking among the Orasol colors, it didn’t answer one of the questions we were asked the most often – “Ok, we know that the dyes in the brighter dioramas are eventually going to fade… how long will they last?” By developing a test procedure that mirrors the UV-filtered lighting environment in the dioramas , we will get enough information to be able to create a probable lifespan for these materials when used in treatment.

Penetration/Fixation – While the ease of dye removal was beneficial in the 2011-2012 recoloring treatments, in other contexts you might want a greater fixation or penetration of the dye. We want to understand ways we could achieve and manipulate that “fixability”, as well as knowing if those variations impact the dyes working properties and lightfastness.

Substrate Degradation – Finally, we want to better understand whether or not the use of recoloring materials like the Orasol dyes causes the colored animal hairs to degrade faster than normal, or do they act as a protectant and slow down light damage?

These topics acted as a starting point for the research plan for the current project: Recoloring Faded Taxidermy: Research into the Properties and Applicability of Dye Materials for Conservation Treatment, the details of which will have to wait until the next post!

Advertisements

Renovation of the Bernard Family Hall of North American Mammals, Part I: What is a habitat diorama?

In 2011-12, the American Museum of Natural History undertook an ambitious program of renovation to the 45 habitat dioramas in the Bernard Family Hall of North American Mammals.  It was important that the unique features of each diorama be preserved throughout the renovation project, which played a role in deciding what kind of materials could be used for recoloring the faded taxidermy. This post summarizes how the dioramas were constructed – full details on the creation and significance of the Museum’s dioramas are beautifully illustrated in Stephen Quinn’s, Windows on Nature: The Great Habitat Dioramas of the American Museum of Natural History.

Some of the earliest collections at the Museum were taxidermy, and the evolution of the diorama was a natural development in the tradition of using art to teach science. Within the museum, dioramas were created to promote the awareness of wildlife and so-called ‘primitive cultures’ as finite. They were also used to stir concern for the populations and habitats that were threatened by unregulated development and hunting. Fusing art and science, these habitat dioramas depict specific locations and exhibit anatomically correct mounted specimens in their natural habitat.

The wolf diorama in the Bernard Family Hall of North American Mammals ©AMNH/D.Finnin

The original creation of the dioramas was a group effort, combining the skills of artists and scientists to illustrate the complex relationships between animals and their environment. Along with the Museum’s curators, they conducted extensive research, visiting each site to draw reference sketches, take photographs, and collect specimens for exhibit. Once back at the museum, every detail of the scene was painstakingly recreated. Careful positioning of specific lighting combinations illuminated the background paintings to create a particular season and time of day in the depicted location, with naturally posed taxidermy among site-specific plants and foreground materials.

 Fred Scherer and James Perry Wilson painting the background of the bison and pronghorn diorama, while George Mason installs grass in the foreground. (S.Quinn, Windows on Nature) ©AMNH/Library 296655


Fred Scherer and James Perry Wilson paint the background of the bison and pronghorn diorama, while George Mason installs grass in the foreground. (S.Quinn, Windows on Nature)
©AMNH/Library 296655

The dioramas at the American Museum of Natural History are the ultimate mixed-media artifact.  They are made of three main elements: a background painting, foreground materials, and mounted taxidermy specimens. The partial dome-shaped enclosure is created using vertical angle-iron beams and heavy wire mesh. This framework supports layers of plaster, onto which the canvas background painting is attached using a white lead and oil mixture as adhesive. Access to the dioramas is difficult and can only be achieved by removing the front panes of glass or, in some cases, is attained by narrow ladders on the interior extending from the light box to the diorama floor.

The background paintings in the habitat dioramas were examples of the highest form of wildlife artistry in their day, and many consider those in the Bernard Family Hall of North American Mammals to be the most accomplished of their kind. The curved background painting is essential to the overall illusion of space, distance and environment.  It draws on Renaissance techniques such as under-painting, plotting perspective and transferring images with grids. Painters of note included James Perry Wilson, Frances Lee Jaques and Charles S. Chapman. Wilson described his diorama work as “art to conceal art’, in other words, art intended to imitate nature so closely that the artist’s role is not visible (Quinn, 2006).

 James Perry Wilson field sketching at Devil's Tower, Wyoming (1942) ©AMNH/Library


James Perry Wilson field sketching at Devil’s Tower, Wyoming (1942)
©AMNH/Library

The floor of the diorama was built up to the needed height with an underlying structure of wooden beams, over which wire screening was formed to create the desired topography. Features of the landscape were constructed over the wire screening with a mixture composed of plaster, dextrin, whiting, and asbestos fibers.  Plant materials were generally made from painted cotton or paper, sometimes flocked or modeled with wax.

 George Mason laying down wire mesh over the wood structure to create the ground terrain in the bison and pronghorn antelope diorama, 1942. (S.Quinn, Windows on Nature) ©AMNH/Library


George Mason laying down wire mesh over the wood structure to create the ground terrain in the bison and pronghorn antelope diorama, 1942. (S.Quinn, Windows on Nature)
©AMNH/Library

Broad leaves were made from vacu-formed acetate sheet. Snow was created using combinations of plaster, sand, cotton batting and plastics.  A limited number of real botanical specimens, such as grasses, evergreen branches, mosses, and leaves for ground litter, were collected, sometimes chemically treated with preservatives, and then installed. The mammal specimens were mounted in the museum following procedures developed by Carl Akeley in earlier decades, details of which are also discussed in Windows on Nature (Quinn, 2006). The larger taxidermy specimens are mounted into the diorama floors, and thus cannot be removed for treatment.  Many of the smaller specimens, however, are able to be removed.

 The plant-making process for dogwood flower production, 1933. (S.Quinn, Windows on Nature) ©AMNH/Library


The plant-making process for dogwood flower production, 1933. (S.Quinn, Windows on Nature)
©AMNH/Library

A large glass panel serves as the front face of the diorama, and is angled slightly to prevent reflection.  A separated light box with fixtures for interior illumination is located above the enclosure. The original lighting scheme from the early 1940’s is known to have included large theatrical lights.  Lighting revisions shortly thereafter, in the 1950s, resulted in a combination of fluorescent and incandescent fixtures.  This scenario caused a number of unsurprising problems.  Temperature inside the dioramas was elevated, often reaching the high 80’s °F.  The relative humidity was low, with daily and seasonal fluctuations.  Light levels were far higher than is recommended for museum collections and, until recently, lamps were not screened for ultraviolet emissions.  These conditions resulted in deterioration, desiccation, and fading of most exhibit materials.

 Lighting technician Arthur Scharf adjust lights in the Timberline group diorama in the Hall of North American Forests, 1954. ©AMNH/Library 323121


Lighting technician Arthur Scharf adjust lights in the Timberline group diorama in the Hall of North American Forests, 1954.
©AMNH/Library 323121

Informed by a 2003 conservation survey of the dioramas in the Museum’s Akeley Hall of African Mammals undertaken at the museum, testing had demonstrated that it was possible to reduce heat and light levels inside the dioramas while maintaining the desired visual appearance through the use of energy-efficient lamps.

In 2010, as a result of the American Museum’s participation in a citywide effort to decrease energy consumption, funding was provided to replace the diorama lights with more energy-efficient fixtures. The goal of the re-lamping project was to achieve a 50% reduction in electric power consumption. Retrofit fixtures were researched and chosen by an outside lighting design firm to reproduce the visual appearance of the original design, using a combination of energy efficient fluorescent bulbs for indirect lighting and LED flood lights and metal halide spot fixtures as accent lights.  All new lighting fixtures are filtered for UV emissions.

The re-lamping project provided the impetus for a broader renovation of the Hall of North American Mammals.  The renovation team began to explore possible methods of restoring naturalistic color to specimens that had become faded and desiccated in the previous damaging lighting environment in hopes of extending their exhibit life.

Just as the dioramas’ fabrication was necessarily achieved through collaboration, this conservation effort also involved a diverse team of participants, including curators, objects and paintings conservators, exhibition department staff, outside scientists, and a master taxidermist.

The next post will examine the many factors that influenced how we decided what recoloring materials to research and test.