What’s the Word? A Glossary of Taxidermy Terms

In previous blog posts we described our condition and inventory survey of mammalian taxidermy. In completing that survey, we created this working glossary of terms to ensure that each conservator who participated in the survey shared a common understanding of terminology for taxidermy materials and techniques. We share the glossary below for others who may need to describe taxidermy in the context of its conservation.

If you have experience working with taxidermy and you use terms differently or use different terms altogether, leave them in a comment so we can add to our glossary!


Specimen

Mount – the taxidermy animal; the preserved skin of an animal that is secured/mounted over an internal form (manikin) and arranged in a life-like pose.

taxi_mount

Taxidermy mount of a slow loris. ©AMNH /F. Ritchie

The skin is typically secured via stitching, although nails and tacks have been used (and staples), or a combination of stitching and nails. [Note that study skins and mummies are not technically taxidermy.]

Shoulder Mount – the head and neck of the animal, the body has been mounted at the shoulder of the animal (see image of moose at the end of the post).

Trophy Mount – a shoulder mount, usually of a game animal.

Full Body Mount – full body of the animal is articulated in a life-like pose.

taxi_mount1

Taxidermy full body mount of a guenon monkey. ©AMNH /J. Bloser

Manikin – the internal form of the animal that the preserved hide is attached to. Note the spelling: “manikin” or “mannikin” is used when referring to anatomically correct forms, like research manikins used for CPR training. The “mannequin” spelling refers to fashion and other non-anatomically correct forms.

38b70db111560f095434d175c5df7713

“Alaskan Moose manikin in process, American Museum of Natural History,” Research Library | Digital Special Collections, accessed December 13, 2017, http://lbry-web-007.amnh.org/digital/items/show/25169.

ca0f0b35706d3d7f13dffb2a20a25d7e

“Manikin of Alaskan moose, ready for skin, American Museum of Natural History,” Research Library | Digital Special Collections, accessed December 13, 2017, http://lbry-web-007.amnh.org/digital/items/show/25302.

Stuffed animal – specimen produced using a method common in the 19th century and earlier in which the preserved animal skin was sewn and stuffed until full. This technique did not produce realistic-looking specimens. Once better techniques were developed (such as binding and the dermoplastic method), specimens were no longer stuffed in this manner. Today, museum-quality taxidermy is referred to as mounted, not stuffed (see “mount” above).

Binding method – technique in which wood wool or other loose materials are wrapped with string or thread to bind them in place and form musculature around an internal armature/frame to create the manikin for the specimen.

Dermoplastic method (or Akeley method) – a technique developed in the early 20th century for creating highly detailed, anatomically accurate, lightweight, hollow manikin. Its most famous proponent was American taxidermist Carl Akeley, who began working at the American Museum of Natural History in 1909. The form of the animal is sculpted, and a mold is made taken from the sculpture. The manikin is cast inside the mold using papiermâché or reinforced plaster (or a number of other materials). [For more explanation, see “Care and Conservation of Natural History Collections.”]

manikin

Manikin constructed using the dermoplastic technique. “Indian lion, complete manikin, 1930 ,” Research Library | Digital Special Collections, accessed December 13, 2017, http://lbry-web-007.amnh.org/digital/items/show/47932.

Polyurethane foam – material used to make contemporary mass-produced manikins in common use today by commercial taxidermists. Polyurethane foam manikin are used with varying degrees of custom modification to suit the specific needs of a particular mount. Commercial taxidermists also use dense polyurethane for reproduction skulls/beaks.

Internal armature – sturdy wood, wire, and/or metal frame that provides internal structural support and defines the position of the mount, especially the limbs. The internal armature extends through hands and feet to attach the specimen to the display base (see x-radiograph below).

taxi_rat before treatment

Taxidermy kangaroo rat before treatment. ©AMNH /L. Kramer

taxi_rat BT XRAY

X-radiograph of taxidermy kangaroo rat before treatment revealing internal armature. Note the dense, white area at the head (most likely bone and plaster) and the wires (also appearing as white lines) that extend throughout the body, down into the wooden display base. X-ray taken at the Conservation Center at the Institute for Fine Arts at New York University. ©AMNH /L. Kramer

Wood wool/Excelsior – thin slivers of wood (wood shavings) may be used as stuffing or padding inside the animal, forming part of all of the internal manikin

Straw – thick vegetal material may be used as stuffing or padding inside the animal, forming part of all of the internal manikin

Cotton batting – cotton fibers may be used as stuffing or padding inside the animal, forming part of all of the internal manikin.

Papier-mâché – technique using strips of paper (and/or textile) built up in layers with adhesive to create a manikin

Plaster – gypsum or plaster of Paris may be used to form part or all of the manikin/specimen.

Shellac – natural resin that may be used to seal papier-mâché or plaster manikins to provide strength and prevent water damage.

Wax/pigmented wax – transparent or pigmented wax may be used to recreate areas of supple hairless skin on the nose, lips, and around the eyes. In these areas, the original skin dries out and shrinks once the animal is preserved, losing its natural appearance.

Paint – oil or acrylic (or other) paint may be applied to compensate for the loss of color that occurs once the animal dies, especially to hairless skin (ex, beaks/legs/feet/waddle of birds, faces of primates, etc.).

Earliner – rigid material that is inserted between the layers of skin in the ears of many animals for support. Today they are commonly made out of plastic, although historically lead and papiermâché were also used.

Jawset – recreation of top and bottom of jaw, including teeth and tongue. Jaw sets manufactured today are commonly made out of plastic.

Mouthcup/mouthpiece – recreation of the mouth, including surrounding flesh (cheeks and lips). Mouthcups manufactured today are commonly made out of plastic.

Eyes – eyes were traditionally made out of glass, but many of those manufactured today they are made of plastic.

Teeth – often the original skull and teeth were cleaned and used in historic taxidermy mounts; however, because teeth are prone to cracking and breaking, many contemporary mounts use plastic jawsets.

Antlers and horns – usually the originals from the animal are used in taxidermy mounts, although there may be fills, paints, or varnish added by the taxidermist.


Display Base

Base – an external supportive structure that a specimen is attached to make it stable for display. The base could be a flat piece of wood, a branch, fake rock, etc.

taxi_display base

Taxidermy full body mount of an armadillo on a display base. ©AMNH /F. Ritchie

The base is also sometimes called the mount, but because taxidermy specimens themselves are often called mounts, we prefer to use the alternative term “base”. 

Habitat base – a base with additional components resembling the habitat of the living animal, for example, grass, moss, snow, fern, etc.

taxi_habitat base

Taxidermy full body mount of a star-nose mole on a habitat base. ©AMNH /F. Ritchie

Panel – a wooden backing board that attaches the animal to a wall for display, particularly for trophy/shoulder mounts.

taxi_panel

Taxidermy shoulder mount of a moose, complete with wooden panel. ©AMNH /F. Ritchie

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Case Study: Flying Squirrel Treatment

Another of the specimens treated as part of our preparation for our Care of Historic Mammalian Taxidermy workshop at the Society for the Preservation of Natural History Collections (SPNHC) 2017 annual meeting was a taxidermy giant red flying squirrel. Project Intern Logan Kursh executed the treatment.

TearRepairProcess

Conservation Intern Logan Kursh performing conservation treatment on a flying squirrel taxidermy mount. ©AMNH /F. Ritchie

There was little data available about the date or location where this specimen was collected, or about the taxidermist who prepared it. However, an examination of the specimen indicated that it was likely a nineteenth century mount, prepared using the binding method.

The specimen suffered from a number of structural and aesthetic issues. The treatment of many of these condition issues is detailed in a poster titled, “Strategies for the Conservation and Storage of Taxidermy:  Flying Squirrel Case Study” that was prepared for the SPNHC 2017 annual meeting in June 2017. Access the poster here:

SPNHC_2017_Squirrel Poster_Logan Kursh

The poster did not address the restoration of the squirrel’s snout, however, so that work is explained below.


Skin on the face and around the squirrel’s snout is thin and delicate. Like all skin in taxidermy, this hairless skin dried out and discolored after death. Taxidermists typically address this with the use of “finishing materials” such as wax and paint to add vitality. The finishing materials degrade over time, giving the specimen an unnatural appearance.

flying squirrel before treatment

Flying squirrel before treatment. Note the desiccated deformed nose. ©AMNH /L. Kursh

In the case of the squirrel, the nose appeared flat and desiccated. The skin had discolored, and there appeared to be some local fur loss. Examination of other giant red flying squirrels in our collection and reference images of squirrels in life confirmed that the specimen’s appearance was not an accurate reflection of the species in life. With the approval of the Mammalogy Department, we decided to craft a reversible overlay for the squirrel’s snout from reference images using well-understood conservation materials.

The nose and upper lips of the specimen were restored using an overlay made from Paraloid F-10 bulked with glass microballoons. Paraloid F-10 is a thermoplastic acrylic resin. It was chosen for this application because it adheres well to wax and has known ageing properties. The Paraloid F-10 mixture was spread onto a piece of silicone-release Mylar and toned slightly with dry pigment, then shaped with a microspatula based on reference images. The overlay was allowed to dry for several hours and reshaped as it slumped.

flying squirrel during treatment

Flying squirrel during treatment. The white fill material has been applied on top of the original surface and shaped to reconstruct the nose. ©AMNH /L. Kursh

To ensure complete reversibility, the overlay was attached to the specimen over a barrier layer of Paraloid B72 in acetone. Paraloid B72 is a stable thermoplastic acrylic resin and is a common art conservation adhesive in many different applications. Minor adjustments to the shape of the overlay were made, and the overlay was allowed to dry overnight. Once dry, further adjustments to the shape were made with a scalpel in consultation with expertise from the Mammalogy Department.

flying squirrel during treatment

Flying squirrel during treatment. The nose has been reconstructed and the white fill material toned brown with paint. The next step is to add hairs to integrate the fill with surrounding areas. ©AMNH /L. Kursh

flying squirrel during treatment

Flying squirrel during the final treatment step – the addition of hair on the toned fill of the nose. ©AMNH /L. Kursh

The overlay was toned with acrylic paint based on available references. Rabbit fur was flocked onto the overlay with Lascaux 498HV. Lascaux 498HV is a thermoplastic acrylic resin that dries flexible and is reversible with heat.

After treatment the squirrel’s nose appears more life-like and integrates with surrounding features.

flying squirrel after treatment

Flying squirrel after treatment. Note the reconstructed nose. ©AMNH /L. Kursh

For details about other aspects of the treatment of this specimen, be sure to check out the SPNHC poster. SPNHC_2017_Squirrel Poster_Logan Kursh

Case Study: Lemur Mount Treatment Part 2: Treatment

Before Treatment of lemur.

Before treatment of lemur taxidermy mount. ©AMNH /C. Richeson

In addition to thorough condition examination and photographic documentation, conservation treatment decisions should begin with a clear proposal to be approved by Collections Managers, Curators, or other stakeholders prior to beginning hands-on work. Caitlin submitted the following treatment proposal to the Mammalogy Department for the treatment of the lemur mount. 


Treatment Proposal

  1. Use a HEPA-filtered variable-suction vacuum fitted with micro-attachments to remove dust from the specimen.
  2. Use wet cleaning solution to remove or reduce other surface accretions, as needed after vacuuming.
  3. Attach the detached proper left (PL) ear using an appropriate adhesive system.
  4. Inpaint the repair to visually integrate it with the mount.
  5. Attach the detached PL hand using an adhesive system, with a pin if necessary.
  6. Create fills for the fur losses using compatible materials.
  7. Inpaint and/or fill losses and deformations on the nose and face.

Once the treatment proposal was approved, she carried out the conservation treatment and recorded each step (described below). The reports and photographs generated during conservation treatments are archived in the Museum for future reference for conservators, researchers, and department staff. 


Treatment Record

Treatment began with surface cleaning the specimen overall using a variable speed HEPA vacuum at a low setting with micro attachments and an eyebrow comb.

In the process, hairs around the seam matted with a dark brown/yellow accretion  were exposed. Solvent tests were conducted using cotton swabs dampened with three cleaning solutions: a 1:1 mixture of deionized (DI) water and ethanol; mineral spirits; and a 1% solution of Surfynol 61 dissolved into a 1:1 mixture of DI water and ethanol. Although each solvent removed some of the brown/yellow accretion, mineral spirits was most effective. The accretion removal process began by placing small triangles of cotton blotter paper under the hairs. We then brushed mineral spirits onto the accretion. The blotter paper served to absorb the solvent and the loosened accretion. The white blotter paper also acted as a visual guide to ensure that the hairs were sufficiently cleaned of the dark accretion before moving to the next section. Once the hair was cleaned and dry, the specimen was gently groomed using an eyebrow comb.

during treatment lemur

During treatment of the lemur taxidermy mount. Note the right side of the torso has been cleaned and groomed, while the left side has not. ©AMNH /C. Richeson

Next we humidified the proper left (PL) ear and the detached fragment because both of them were desiccated and curled inward. The humidification was done using a small vapor chamber created from in a layer of Gore-Tex (a semi-permeable membrane that allows vapor through, but not water), a piece of blotter paper dampened with DI water, and a thin polyethylene bag. The Gore-Tex barrier was laid over the skin and the dampened blotter was secured on top of the Gore-Tex using a hair clip. A polyethylene bag was then placed on top of both layers to seal them into a chamber. The skin was checked approximately every 15 minutes for the duration of one hour until it was relaxed and pliable enough to be re-formed. The humidified skin was re-shaped and allowed to dry in proper alignment with the help of a rigid piece of dry blotter paper.

during treatment lemur

During humidification of the broken proper left hand of a taxidermy lemur. The humidification allows the skin to realign for reattachment of the hand. ©AMNH /C. Richeson

during treatment lemur

During treatment of the taxidermy lemur. Detached proper left hand in a water vapor humidification chamber to align the edges for reattachment. ©AMNH /C. Richeson

To reattach the ear fragment we used BEVA 371 film. Small strips of film were tucked between the layers of the epidermis on both the fragment and attached ear. The film was then warmed using a variable heat spatula, which set the adhesive. To protect the skin from direct heat contact, silicone release Mylar was used as an interleave.

Reattachment of the PL hand required use of Japanese tissue paper lining impregnated with BEVA 371. A bamboo skewer bent at a 45° angle was used to help apply pressure to the tissue while applying the heat spatula to the opposite side.

Losses on the repaired hand were filled using a small piece of coyote hide with bleached and toned fur. Faber Castell Pitt artist pens were used for toning, and excess ink was removed with blotter paper. The fill was adhered with BEVA 371 film.

after treatment lemur

After treatment photos of lemur. ©AMNH /C. Richeson

To restore the damaged nose, we sculpted a reconstruction using Paraloid F-10 heavily bulked with glass microballoons. The bulked resin was cast out onto a sheet of silicone release Mylar into the desired shape and thickness. After two days the bulked resin was pliable, but not sticky, and could be shaped without slumping. The reconstruction was formed to fit on top of the existing damaged nose. Once hard, it was smoothed using micromesh and toned with Golden acrylic paint. It it held in position by friction, and can easily be lifted/removed with a microspatula or bamboo skewer.

after treatment lemur taxidermy mount

After treatment of lemur taxidermy mount. ©AMNH /C. Richeson

Finally, we constructed a new storage mount for the specimen. The specimen was previously stored in an upright position with an L-shaped storage base. This orientation was optimum for the the specimen, as it placed the least amount of stress on the specimen and posed the least risk to the disruption of the hair, but the limited space in collection storage required a different solution. After confirming that the habitat base could be altered in a non-visible way, we drilled two small holes into the display branch, inserted brass rods, and positioned the rods into a plywood base, elevating the specimen slightly above the board. An ethafoam block supports the proper right hand of the specimen and branch. The new storage solution allows the specimen to be stored horizontally, while protecting the fur from being crushed.

 

Case Study: Lemur Mount Treatment Part 1: Condition Examination

In addition to testing the stability of metal-complex dyes, we have been studying condition issues facing historic taxidermy collections (see our previous posts on the Mammalogy condition survey) and performing conservation treatments on selected specimens. These treatments stabilized important mounts and served as case studies for a workshop on the Care of Historic Mammalian Taxidermy at the 2017 Denver meeting of the Society for the Preservation of Natural History Collections (SPNHC).

Before Treatment of lemur.

Before treatment of lemur taxidermy mount. ©AMNH /C. Richeson

In one of these Case Studies, project intern Caitlin Richeson treated a damaged red-fronted brown lemur (Eulemur rufus) taxidermy mount. Its catalog number suggests that it was acquired soon after the Museum was formed in the late 19th century. This post and the next one will provide some of the details of that treatment.

All conservation treatments should begin with thorough documentation: an object description, condition examination, and photography. Caitlin’s project provides a good example of what that documentation may look like for a piece of taxidermy.


Object Identification: A full-body taxidermy mount of a female red-dronted lemur (Eulemur rufus) mounted on a habitat base constructed from a tree branch.

 

Description
The specimen is constructed around a composite manikin that is visible in several locations due to previous damages. The manikin is composed of wood wool (thin wood shavings traditionally made of poplar, pine, or spruce) bound to recreate the musculature form of the specimen; cotton batting used to bulk appendages such as the hands; and a metal armature used to provide rigid structural support. Appendages such as the hands and feet still contain original skeletal materials. The nose, snout, and eyelids are shaped from a soft black material, likely a pigmented wax.

The specimen is attached to the base with ferrous metal wires at three points: the proper right palm, the proper right foot, and the proper left foot. The wires penetrate the palm and feet of the specimen as well as the habitat base where they have been secured to the base by bending at a 90-degree angle.  The specimen is mounted in a standing position gripping the habitat base with the proper right (PR) hand and both feet.

There is a paper specimen label tied to the proper right ankle, which contains taxonomic information in two separate campaigns of writing. There is also a metal plate attached to the habitat base located on the front of the branch.  The plate is inscribed with the catalogue number and attached to the branch with two tacks. There is another metal tag tied with a thin metal wire around the PR wrist of the specimen, also stamped with the catalog number.

Condition
The specimen is in fair condition. It is structurally stable; however, there are two detached elements, several areas of fur loss, tears and cracks/splits in the hide, which contribute to the overall instability of the specimen. In addition, the specimen is covered in a layer of light grey dust and grime.

Hide/Manikin:

  • A portion of the proper left (PL) hand is detached from the specimen. The detached hand consists of the hide and four digits, but excludes the thumb, which remains attached to the specimen. The detached hand has several cracks/splits in the hide and a large loss on the palm side, exposing the interior construction. The PL ear of the specimen is also detached and there are small areas of unrecovered loss.

Before Treatment of lemur detached proper left hand.

Before Treatment of lemur detached proper left hand.

Before treatment of lemur taxidermy mount. Proper left detached hand. ©AMNH /C. Richeson

  • On the lower back at both the PR and PL sides there several moderately large stable tears in the skin of the lower torso, which expose the wood wool manikin below. There is also a minor split located at the PR side of the vent. Although the skin is slightly out of plane, it remains somewhat pliable and can be pressed back into place with gentle pressure.
  • There is a small circular area of abrasion on the back of the PR hand.

Finishing Materials:

  • The material used to finish the nose of the specimen has sustained localized losses and a dent on the PL side. The eyelids, which are made from the same finishing material, have also been deformed.
    Before Treatment of lemur. Detail of nose with missing wax.

    Before treatment of lemur taxidermy mount. Note the loss of finishing material on the nose and the loss of skin on the ear. ©AMNH /C. Richeson

    Before Treatment of lemur. Detail of nose with missing wax.

    Before treatment of lemur taxidermy mount. Note the loss of finishing material on the nose. ©AMNH /C. Richeson

Fur:

  • The fur of the specimen has discolored to a light yellow-brown color, having lost its natural variation in red, brown, and black colors that are representative of the species.
  • There are several areas of fur loss located on the mount. The first is a a substantial loss of fur on the chin and underside of the snout. This species is identifiable by its characteristic “beard”, and thus the fur loss in this area detracts from the accurate representation of the species. There are also small losses associated with the detached PL hand and at the coccyx.
  • There are also beige accretions on the fur, primarily located along the seam at the center of the torso, and an overall layer of moderate dust accumulation on the fur.

Display Base (branch):

  • The specimen is well-secured to the tree branch habitat base, which appears to be in good condition. When examined in ultraviolet light, the base fluoresced a milky yellow/green color, and appears to have been coated.

After completing this type of photographic and written documentation, the conservator then submits a proposed treatment to the Collections Manager and/or Curator for approval. Follow along with the progress of this treatment in the next post.

 

 

Color by Numbers, Part 1: Spectrophotometry vs. Colorimetry

In our lightfastness testing of metal-complex solvent dye solutions, we see some samples that fade a lot and are very easy to qualify as “fast-fading” dyes. (See our previous post for more information on metal-complex solvent dyes and why we’re using them; as well as previous posts that detail our sample set and sample preparation) By visually comparing the test samples to the controls we can see those differences, even after as little as 24 hours in our exposure test-chamber. However, we need to quantify those changes in order to describe them in a more objective way than can be done based on human sight alone. Doing this requires some means for reproducibly recording color and calculating change. But how do we do that?

Answer: we use an instrument called a spectrophotometer to make color measurements, and then use principles of colorimetry to interpret them.

Supplier/Color/Solvent/Substrate: Kremer/Blue GN/acetone

Sample of Kremer Blue GN (CIGN Solvent Blue 67) in acetone, after testing. The A and B test plates (left and middle) were exposed to the accelerated aging chamber, while test plate C (right) was not. There is very little color fade detectable by the naked eye. AMNH/F. Ritchie

Supplier/Color/Solvent/Substrate: Kremer/Yellow 4GN/acetone

Sample of Kremer Yellow 4GN (CIGN Solvent Yellow 146) in acetone, after testing. The A and B test plates (left and middle) were exposed to the accelerated aging chamber, while test plate C (right) was not. The light fading was extreme enough to be detectable by the naked eye. AMNH/F. Ritchie

Background: A Note on the Difference Between Color and Colorants

Before describing the workings of a spectrophotometer and the principles of colorimetry, a refresher on the two different types of color theories – the way humans see color – is necessary.

1.  Subtractive color theory

subtractive color

An illustration of subtractive color theory where colorants (in this example, the pigment from oil pastels) are mixed and absorb light from certain regions of the visible spectrum, thereby subtracting them from the reflectance spectrum that is perceived by the human eye. [Photo source]

This is the type of color-mixing we are taught in grade school: whereby colorants are mixed and then absorb light from certain regions of the visible spectrum, thereby subtracting them from the reflectance spectrum that is perceived by the human eye. Under this theory the primary colors are red, blue, and yellow, and secondary colors are orange, green, and violet. For example, a blue colorant (which absorbs light in the red, orange, and yellow parts of the spectrum) mixed with a yellow colorant (which absorbs light in the red, blue, and violet parts of the spectrum) will produce green (the remaining color in the reflectance spectrum). In modern color printing, cyan, magenta, and yellow are the usual primaries. Black is produced when all colors are present because all wavelengths are absorbed.

2.  Additive color theory

RGB_illumination

An illustration of additive color theory where different light sources are shown onto a wall. Where the lights overlap, another color is perceived because visible wavelengths in the spectrum of light are mixed (or added) together, received by the eye, and interpreted by the brain as color. The white light in the center is the mixture of all three primary additive colors, mixing together to encompass the entire visible spectrum. [Photo source]

This is the type of color perception that we encounter on computer- and television monitors when projected light is combined to make different colors. In additive color theory, visible wavelengths in the spectrum of light are mixed and/or added together, received by the eye, and interpreted by the brain as color. The additive primary colors are red, green, and blue, roughly corresponding to cones in the eye that are sensitive to long, medium, and short wavelengths, respectively. Secondary colors are yellow, cyan, and magenta. For example, a mixture of equal parts red (~650 nm) and green (~510 nm) light stimulates the cones of the eye in a manner that is similar to pure spectral yellow (~570 nm), so the eye does not detect the difference between the two conditions. White light is created when red, green, and blue are all present because they encompass the entire visible spectrum.

Spectrophotometry: Making Color Measurements

Spectrophotometry is a “noninvasive technique that measures the amount of light reflected or transmitted by a material at individual wavelengths of the spectrum” (Johnston-Feller, 2001: p. 1). A spectrophotometer is the instrument that’s used to measure light reflected, or transmitted, by a material. To do this, it relies on a light source, a monochromator that isolates and directs the wavelengths, and a photodetector that detects reflected or transmitted radiation. The measurement is unit-less, or may be given in percent absorbance or transmittance. The reflectance or transmission spectrum of the material describes how a spectrum of light incident on it will be modified upon reflection from, or transmission through, it. That modified spectrum is what we observe and interpret as the material’s color under that particular light.

For this project, we purchased an X-Rite Ci62 handheld spectrophotometer with a tungsten light source, a grating monochromator, and a photodetector consisting of blue-enhanced silicon photodiodes. With this instrument we measure successive reflectance spectra from each sample at intervals during our light exposures. When a sample is measured, the amount of light reflected at each wavelength in the visible spectrum is recorded and plotted along a curve that describes the reflectance of that dye sample. By comparing successive measurements, we can see the spectral-curve change as the dye fades or discolors over time.

MSC solvent red 122 in ethyl acetate

Spectral curves of sample Solvent Red 122 during an accelerated aging test cycle. The initial color reading/spectral curve is in red (the lowest line). During each color reading, the curve flattened as the color faded and there was less to measure. The final measurement is represented by the top line (green).

jig

Top: Spectrophotometer in the jig without quartz plate dye sample. Bottom: Quartz plate dye sample in position for color measurement when spectrophotometer. AMNH/F. Ritchie

Then, from each reflectance spectrum we can calculate parameters that uniquely describe the color we will observe under a particular light and the changes in that perceived color from our exposure test. That translation of light spectrum into color description is done with the tools of colorimetry, the scientific description of color.

Colorimetry: Interpreting Color

Colorimetry is a branch of color science that studies precisely what we need for our research: it is the quantification of color, or, as Ruth Johnston-Feller describes, a way to “describe a color unambiguously and uniquely to distinguish it from all other colors” (Ibid.; p. 15). Most colorimetric systems characterize color in three dimensions because they describe human visual perception, which is a function of the three types of photoreceptors in our eyes. A variety of these systems exist, but most of the descriptors for perceived color are derived from the CIE color system administered by the International Commission on Illumination since 1931, which is itself based on additive color mixture and the visual response to various wavelengths of light that reach our eyes.

CIE 1931 RGB and CIE 1931 XYZ Color Spaces

The 1931 RGB and 1931 XYZ color spaces created by the CIE were the first numerical models linking the wavelength regions of the visible spectrum to human color vision. The CIE system defines a series of standard illuminants (sources) and a standard observer (mathematical representations of average human color vision), since the perception of color requires both a light source to illuminate a material and the human visual response to the light reflected (or transmitted) by that subject. A standard illuminant is characterized in terms of the spectral power distribution that distinguishes a specific light source, including the one most widely used: 6500K daylight (abbreviated D65). A standard observer is defined by a set of three mathematical color-matching functions that describe the average spectral sensitivity of the human eye at each wavelength in the visible spectrum, as derived from experimentally determined response curves for a particular field of view (such as 10 degrees, a wide field suited for viewing rough textured surfaces like fur samples). These color-matching functions are dependent on the selection of a set of three additive primary colors (for example, monochromatic red, green, and blue wavelengths, as in CIE RGB, described below) that together with a reference white point (such as D65) define a unique color space containing the gamut of all possible combinations of those primary color components. The functions are used to convert the reflectance spectrum measured with the spectrophotometer into tristimulus values, which are the relative amounts of each primary color needed to match the color sample measured.

For example, the CIE RGB color space is defined by red (700 nm), green (546.1 nm), and blue (435.8 nm) primary colors, and contains the gamut of colors that can be produced from RGB tristimulus values. A mathematically related color space, CIE XYZ, is derived from CIE RGB by means of a simple conversion, but it is useful because it makes for easier calculations than the RGB space. [For further reading on CIE XYZ, check out other on-line resources.]

CIE 1976 (L*a*b*) Color Space

There have been many developments in color science since 1931, in attempts to describe perceived color in a more straightforward and intuitive way, leading to the development of Lab color space by Richard S. Hunter, in 1945, and later the CIE 1976 (L*a*b*) color space. CIE L*a*b* color space is a mathematical derivative of the CIE XYZ color space, in which the L* parameter captures lightness, and the a* and b* parameters capture the other color dimensions. L* is proportionally related to tristimulus Y, the luminance, with darkest black at 0, and brightest white at 100. Red/green colors are represented along the a* axis, with neutral gray at 0, green at the negative values, and red at positive values. The yellow/blue colors are represented along the b* axis, with neutral gray at 0, blue at negative values, and yellow at positive values.

L*a*b* color is widely used in many industries because it offers a number of advantages over previous RGB and XYZ systems. It more closely approximates the nonlinear response of human vision, and includes a much wider color gamut than its predecessors. And it is more perceptually uniform, meaning that uniform changes in color values produce uniform changes in perceived color. Because of these capabilities, we are using L*a*b* values to describe the appearance of dye samples in our project.

Related to the L*a*b* color system is the color space CIE L*c*h°. In this system the a* and b* parameters are replaced by parameters describing chroma (or relative saturation) and hue. Unlike the L*a*b* system, which uses three-dimensional xyz coordinates to locate a point in color space, L*c*h° uses a polar coordinate system, with h° denoting a hue angle, and c* a radial distance from a central point. This color space is conceptually related to the familiar Munsell notation of color in terms of its hue, value, and chroma. It is also the color description necessary for calculating color differences with certain formulas.

RGB_color_wheel_10

Additive color theory illustration depicting how variations of color are achieved by the different combinations of red, green, and blue light (rgb values). [Photo source]

Putting It All Together

Like many spectrophotometers available today, the X-Rite Ci62 that we are using for this project has integrated software that can quickly execute the complex mathematical calculations needed to translate the measured reflectance spectrum into various colorimetric systems. From each reflectance spectrum, it computes and outputs tristimulus XYZ, L*a*b*, and L*c*h° values based on the standard illuminant (D65) and standard observer (10°) that we have selected. We then use those values to consider the overall amount of perceived color change, as well as changes in individual parameters, such as lightness.

We will go into greater depth about how we are interpreting our color measurements and tracking color changes in the up-coming Part 2 of this blog post.

References/Resources

Billmeyer, Jr., Fred W., and Max Saltzman. 1981. Principles of Color Technology. New York: John Wiley & Sons, Inc.

Christie, R.M. 2001. Colour Chemistry. UK: Royal Society of Chemistry.

Johnston-Feller, Ruth. 2001. Tools for Conservation: Color Science in the Examination of Museum Objects, Nondestructive Procedures. Los Angeles: The Getty Conservation Institute. Available online.

Whitmore, Paul M., Editor. 2002. Contributions to Conservation Science: A Collection of Robert Feller’s Published Studies on Artists’ Paints, Paper, and Varnishes. Pittsburgh, PA: Carnegie Mellon University Press.

Dyed Fur Samples: Part 2

Fur-Mounting System

We next needed a method of mounting the deer and fox furs so that they could be dyed and exposed in the test chamber. [See Part 1 of this series for an explanation on why we chose deer and fox furs.] Acquiring a series of meaningful color measurements from dyed fur demands a sample design that is more sophisticated than dye on quartz plate. The successful mounting system would secure a group of hairs at both ends, creating a flat sheet onto which the dye can be airbrushed. It must completely immobilize the hairs against handling, measurement, and the high rate of airflow inside the test chamber. Any loss or displacement of a dyed hair from the sample surface could be reflected as lost color in our measurements, indistinguishable from the dye fading that we are principally interested in.

In addition, the mounting system must:

  • Be compatible with deer and fox hairs of average length
  • Consistently distribute hairs across the sample to create a relatively planar surface
  • Avoid hair breakage during sample construction, accelerated aging, or measurement
  • Be made from materials that would not slump, melt, migrate, off-gas, or otherwise degrade during accelerated aging
  • Produce samples of consistent size and shape that can be used with a jig to ensure measurements are repeatable
  • Produce samples that can be handled without accidental disruption of hairs
  • Produce samples that can be secured inside the test chamber
  • Be simple and relatively quick to execute
Title/Description of Object:

Early attempts at mounting hair for research samples that were unsuccessful.  AMNH/J. Sybalsky

Our initial experiments cast a wide net. We looked at various types of clips, from binder and bulldog clips to staples and barrettes. These attachments were quick and simple to use, lacked components that would be likely to degrade, and could be adjusted to accommodate any fiber length. But we found that they produced samples that were bulky, insufficiently secure, incompatible with the measurement aperture of our spectrophotometer, or could not easily be standardized for use with a simple jig.

We explored other non-adhesive methods, employing flat materials that can be crimped or heat-sealed to trap hairs at both ends. We laid hairs on top of strips cut from nonwoven polyester and Mylar, folded the ends of each strip over the hairs, and sealed them in place. While these mounts created visually attractive, planar samples that could be made to fit into preexisting sample holders in our test chamber, the heat seal did not pinch the hairs well enough to fully immobilize them. A similar design using strips of aluminum sheet with crimped ends was more secure, with improved tightness and rigidity. They were, however, laborious to execute and would be challenging to standardize. A further adaptation using pieces of aluminum vapor-barrier tape in place of crimping made sample construction much easier, but introduced an organic adhesive. (See images above.)

Next, we looked into designs based on the concept of an embroidery hoop: we laid hairs across the open end of a cylindrical piece of steel hardware, then secured them in place using fasteners that could be wrapped and tightened around the cylinder. Stainless steel zip ties and hose clamps both held well in general, but fibers were loose around the locking mechanism in both cases. Execution was fussy, leaving fibers inconsistently distributed. (See images above.)

Fruitful discussions with several colleagues pointed us toward polyethylene sample holders designed for X-ray fluorescence (XRF) analysis of powdered samples. These holders usually consist of several separate components: a cylindrical tube or “cell,” a collar that snaps over the top of the cell, and a cap that covers the bottom end.

To mount the fur onto an XRF sample cup, a small piece is cut from the hide. The hair on the hide is brushed using an eyebrow comb to align the hairs parallel to one another. One end of the cylindrical cell is lined with Teflon sheet to act as a stable, white backing material during color measurements in case any gaps are left between hairs. The aligned hairs are carefully placed on top of the cell and immobilized by attaching the snap-on collar in a fashion similar to an embroidery hoop. Longer hairs extending from the side of the sample cup are trimmed using a scalpel. Any hairs standing proud of the sample surface and those not trapped by the sample ring are also trimmed away using scissors and tweezers.

1_1

XRF sample cup with Teflon sheet applied to the opening of the cell; the snap-on collar before applying.  AMNH/F. Ritchie

3_1

Applying combed fox hair to the Teflon-covered end of the XRF sample cup. The snap-on collar is added after positioning the hair.  AMNH/F. Ritchie

4_1

Completed fur cup samples  AMNH/F. Ritchie

We found this method of sample construction to be labor-intensive, but it allowed us to make standardized samples more successfully than any other. Nevertheless, before committing ourselves to one method over the others, a trial run in the test chamber was in order. We subjected our two favorite mounting systems, the XRF sample cups and the aluminum sheet and barrier tape, to 600 hours of accelerated aging at 0.35 W/m2, 55% RH, 63ºC black panel temperature, and 48ºC chamber air temperature. Both systems held up to the environment and blowers well. There was no evidence of creep or adhesive migration in the aluminum mounts. In both systems, a few underfur fibers were stirred up above the sample surface by airflow inside the chamber, demonstrating the importance of early removal of fibers inclined to dislodge. This can be accomplished through application of gentle friction or canned air to the sample followed by cutting or tweezing away any loose hairs.

Because of the importance of producing samples of consistent size and shape, we ultimately adopted the mounting method based on the XRF sample cup. A selection of XRF sample cups with different dimensions and features was subsequently tested with our furs. Some were more compatible with the length of our fibers than others, and smaller cups gave us greater control over the distribution of fibers. Among the examples we tested, we felt that the Chemplex SpectroCertified Quality XRF Sample Cup No 3115 worked best.

In total, 300 sample cups were constructed (150 with fox, 150 with deer) and will be used for testing. Future blog posts will describe the testing methods and results.

Caitlin making fur cup

Project intern Caitlin Richeson preparing fur cup samples using fox hair.  AMNH/F. Ritchie

Dyed Fur Samples: Part 1

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.

Fur Selection

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.

arctic fox skin

Arctic fox skin used for research samples.  AMNH/F. Ritchie

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.

deer hide

White-tailed deer hide used for research samples.  AMNH/F. Ritchie

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.

Tanning

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.

leathers

Scraps of leathers prepared using different techniques and finishes. The type of tan can affect the condition of a taxidermy mount overtime.  AMNH/F. Ritchie

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
deer hide cross section

Cross section of the white-tailed deer hide used for the research samples.  AMNH/F. Ritchie

The upcoming Part 2 post will describe how we are mounting the furs to run as research samples.