Color by the 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


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).


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.


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.


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.

Rows of fur samples.  AMNH/F. Ritchie

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.


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


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


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

 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.


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.


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.

workspace_primate copy_1

Surveying Historic Taxidermy Part 3: Results

Our condition survey of historic mammalian taxidermy in the American Museum of Natural History’s Department of Mammalogy (see previous post), supplied an understanding of the most common condition issues affecting them, and clearly displayed their probable causes.

Cracks, Splits, and Dust: Responses to Environment

649_3_detail of crack_split on tail

Split in the hide at the base of a specimen’s tail.  AMNH/F. Ritchie

It is not unusual to find cracks and splits in historic taxidermy mounts. The organic materials comprising taxidermy (hide, skin, horn, teeth, manikin materials, etc.) will expand and contract as a response to changes in relative humidity and temperature in the environment. This is similar to the way one’s hair increases in volume during more humid days, but is flat on dry ones. If there are small tears or cracks in the hide, they may open up during these fluctuations and become bigger. Much of the historic taxidermy in the Museum is more than 100 years old, meaning it was acquired well before the invention of modern environmental control systems in use today. We were not surprised to discover cracks and splits in hides, teeth, and other organic components.

Akeley elephants_dust slide_1

Dust accumulation on a glass slide that was placed in a museum public hall for one year.  AMNH/F. Ritchie

Dust may seem innocuous, but it is a serious concern for taxidermy. “Dust” can be anything from lint or dried skin cells to coal dust or other sources of air pollution. These small particles can be abrasive, oily, and/or hygroscopic, meaning that they attract moisture that creates localized varying microenvironments on the surface. Furthermore, accumulated dust detracts from the perceived vitality of a specimen and alters its apparent color; even when the dust itself presents only a minor risk, the aesthetic considerations of display may require investing resources in cleaning methods that could introduce more significant risks like hair breakage, slippage, staining, or disruption of previous recoloring treatments. Most of the specimens surveyed were stored in enclosed storage cabinets or covered by protective plastic sheeting. Mounts in open storage, however, are particularly vulnerable to dust accumulation.


Breaks and Loss: Responses to Handling

As mounted skins age, they often become brittle and more sensitive to damage by handling. During the survey the conservators noted broken limbs, detached pieces, and other signs of damage that may have occurred as these objects were handled for various purposes over their long lives. Some taxidermy may have suffered these damages even before entering into the collections. Taxidermy is also vulnerable to damages due to handling during the exhibit installation and de-installation processes, during movement of the collections, and during research. For these reasons, our collections staff follows detailed guidelines that are specifically intended to mitigate these risks.

650_2_detail of broken foot

Detached toes that may have occurred during the de-installation of this specimen. Detached pieces are stored in the same location as the specimen until they can be repaired/conserved.  AMNH/F. Ritchie

Fading: Responses to Light Exposure

Discoloration and fading in fur is minimized by dark storage. Some taxidermy specimens surveyed were previously on display at the Museum and now exhibit light-induced discoloration and fading, not unlike that seen in the Bernard Family Hall of North American Mammals before its recent renovation. Comparing these discolored examples to the unfaded study skins in adjacent storage can be very useful in determining the degree of fade or discoloration. Where they can be used appropriately, re-coloring techniques have the potential to restore the naturalistic appearance of faded specimens and extend the possibilities for their use in dioramas or other exhibits.

fading example

Specimens of the same species – on the left, the taxidermy examples that were on display for a number of years and exhibit fading of the fur from light exposure. The unexposed unfaded (darker) study skins line the right side of the drawer.  AMNH/F. Ritchie

Other Condition Issues

Several other types of damage were reflected in our survey. These include loss of hair due to old, (currently inactive) pest activity; chemical deterioration of materials used in manikin construction or finishing work, such as rusting metal ear-liners or flaking paint; and structural issues in the base, such as loose attachment of the taxidermy mount and cracks in wood or plaster.


Squirrel specimen that has lost the nut previously displayed in its mouth. Note also the area of loss of foliage on the display base (yellow area).  AMNH/F. Ritchie

Future Work


Project Conservator treating loose hide on a moose mount.  AMNH/J. Sybalsky

As the project continues, we will be working to stabilize and restore some of the specimens evaluated in our survey. Examples and case studies derived from these treatments will be shared in the various informational and training resources under development.




bat drawer

Surveying Historic Taxidermy Part 2: Fun Finds

At its outset, execution of our inventory and condition survey of taxidermy mounts in storage in the American Museum of Natural History’s Department of Mammalogy (see previous post) required clarification of what exactly can and cannot be considered “taxidermy.”

What exactly is taxidermy?

The word taxidermy is derived from the Greek words “taxis” meaning arrangement, and “derma” meaning skin. Strictly speaking, a specimen must have preserved skin that is arranged in a lifelike form to be considered taxidermy. Taxidermists achieve this using different materials and methods, but in our survey we considered a specimen to be “taxidermy” if it had an articulated pose and glass eyes (indicating that it was meant to be exhibited). This criteria discounted study skins (preserved specimens with stuffing, but without an articulated pose or eyes), skin rugs (preserved hides with glass eyes and reconstructed head, but without an articulated pose), and mummies (specimens that may appear articulated, but lack internal armature or glass eyes).

hutia mummy

Hutia “mummy” that appears to be in a lifelike pose, but further inspection reveals that there is no internal armature and no glass eyes. This specimen therefore was not considered taxidermy.  AMNH/F. Ritchie

rat drawer2_1

Rodent drawer of study skins that have glass eyes, but not articulated poses, and therefore are not considered taxidermy.  AMNH/F. Ritchie

The bat collection proved to be the trickiest to classify because a majority of specimens were mounted onto external glass panels. They were not fully articulated internally to form an accurate lifelike pose. It is difficult to pose the thin skin of bat wings, especially of smaller specimens, because, having qualities similar to parchment, it deforms and tears easily.


Bat specimen mounted to an external thick glass plate. Note the glass eyes and articulated mouth.  AMNH/F. Ritchie

The glass plates provided a way to support the wings while on display.

810_3_detail of back of glass to show paint

The verso of a bat specimen mounted to an external glass plate.  AMNH/F. Ritchie

Many of the bats encountered in the survey had glass eyes and an articulated mouth, a metal wire armature in their wings, and were previously exhibited. For these reasons, we decided they were akin to other mammal mounts and included them in our survey. Half-mounts (also known as shoulder or trophy mounts) were also considered taxidermy, even though the whole animal isn’t represented because the preserved hide is still arranged to mimic a living pose.


Taxidermy Materials and Methods

In order to accurately identify the technology and materials used to create the mounts and to appropriately describe the damages we observed, we researched historical taxidermy practices. The choices the taxidermist makes can have an important impact on the condition of the object.

743_2_detail of split ear with earliner

Splitting skin around a rigid ear liner.  AMNH/F. Ritchie

If the internal manikin is made of excelsior or “wood wool” (slivers of wood, a common material from the late 19th and early 20th century), it will move in response to fluctuations in environmental conditions just as the mounted hide around it does. This movement can eventually cause tension or tears, and loosen the hide from the manikin. Conversely, if the manikin is too rigid, the hide may shrink over time and split open around the internal support.




Small copper-alloy pins added to hold fingers in place can react with the skin to form a waxy-green corrosion product called copper stearate. The corrosion can stain surrounding skin and hair, and can be difficult to remove.


Waxy green (most likely copper (II) stearate) corrosion on the pins that hold small fingers into place on a display branch. Red arrows indicate areas of corrosion.  AMNH/F. Ritchie

Some glass eyes can also exhibit an inherent deterioration known as “glass disease.” The cloudy appearance or even crizzling (fine cracking) occurs because of a breakdown of the chemical composition of the glass, often exacerbated by contact with skin. Once the disease begins it can only be slowed, not stopped.

735_3_detail of crack in eye

A crack in the glass eye of a specimen.  AMNH/F. Ritchie

860_3_detail of cloudy eyes

White accretions covering the glass eyes of a specimen, possibly glass disease.  AMNH/F. Ritchie

A future blog post will discuss taxidermy methods in more detail. In the meantime, check out the book Windows on Nature, written by longtime Museum exhibition project manager Stephen Quinn.

Here is a selection of some of the most interesting taxidermy specimens that we came across during our survey.

One of the oldest specimens that we assessed was an agouti that was collected in 1843, before the Museum was founded.


Image  AMNH/F. Ritchie

The largest mount was an elephant seal that is so large it must be stored in the Museum’s special large species room.


Image  AMNH/F. Ritchie

The smallest taxidermy specimen was a harvest mouse.


Image  AMNH/F. Ritchie

The most unexpected specimen (for a North American conservator) was a platypus.


Image  AMNH/F. Ritchie

The exceptionally skilled execution of historical taxidermy techniques is exemplified by some of the small mammals, like squirrels, that were mounted in dynamic positions. This specimen was acquired through one of the founding collections (Verreaux).


Image  AMNH/F. Ritchie


Surveying Historic Taxidermy Part 1: Goals and Parameters

Alongside the lightfastness testing described earlier in this blog, we are developing tools to support the efforts of other individuals and institutions seeking to preserve collections of historic mammalian taxidermy. To do this, we needed to deepen our understanding of the historic and modern materials and techniques used in creating these objects, the common condition issues affecting them, and methods of remediation, both historic and modern.

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Taxidermy viscacha specimen from the collection during condition surveying. (c) AMNH/F. Ritchie

Working toward these aims, we conducted an inventory and condition survey of taxidermy in storage in the Department of Mammalogy of the American Museum of Natural History. This survey was intended to accomplish the goals set out above with the added benefit of providing the department with a searchable, data driven inventory of the entire mammalian taxidermy collection. This kind of inventory can serve as a basis for planning and decisions related to collection management and storage, loans, exhibits, and associated conservation needs.

The Department of Mammalogy is one of four departments in the Museum’s Division of Vertebrate Zoology Division. The department’s collection comprises more than 420,000 specimens from around the world, although only a tiny fraction of those in storage are taxidermy mounts. This fraction still represents about half of the mammal taxidermy at the Museum, with the other half on permanent display. These numbers should not be surprising: museum-quality taxidermy is costly to produce and limited in its scientific uses compared to materials such as study skins or skeletons. Instead, taxidermy is valued primarily for display, so it has been produced in relatively small numbers for specific exhibits over the years. Thus, the percentage of specimens on display versus in storage is much higher for taxidermy than it is for other materials that are more often used in scientific research. Among the Museum’s mammal taxidermy holdings are numerous examples from the founding collections that were acquired in 1869 and are now approaching 150 years old. How are these specimens holding up after so many years?


Project intern Caitlin Richeson examining taxidermy fruit bats in collection storage. (c) AMNH/F. Ritchie

Over a period of four months we spent an average of two to three days per week surveying. We worked around visiting researchers and staff using temporary photography and examination stations in each room. Each specimen took five to 10 minutes to assess, depending on its complexity and accessibility. After opening every storage cabinet and pulling out every drawer to ensure that no specimen was overlooked, we assessed approximately 635 individual mounts in 30 mammalogy-collection storage spaces.


Project conservator Fran Ritchie examining a specimen at a temporary surveying station in collection storage. (c) AMNH/K. McCauley

Using a custom-built database, we tailored our survey parameters to record identifying information for each specimen, an assessment of its condition, and recommendations for treatment. If desired, the data collected can be exported in CSV and PDF file formats and then imported or attached to records in other existing databases, such as the EMu database system used by the Museum’s Division of Vertebrate Zoology.

Data gathered for each specimen included ‘identifying information’ such as:

  • Specimen Description – Basic taxonomic and locality information, as well as notes about special historical, scientific, or ecological significance
  • Current Storage Location – Building, floor, room, cabinet number(s), and cabinet label(s)
  • Transcriptions – Data from all labels and inscriptions, including taxonomy, catalog and other numbers, and other scientific or historical details
  • Digital Photograph(s) – An overall identifying photograph as well as details of specific condition or preparation issues, when appropriate

Survey database example entry (not actual specimen in the collection).

We evaluated the condition of each specimen, looking closely at the following elements:

  • Internal armature
  • Skin/hide
  • Fur/hair
  • Antlers/horns/hooves/nails/claws/teeth
  • Eyes
  • Finishing materials (for sculpting lips, nose, etc.)
  • Base
  • Specimen label

Summer intern Kelly McCauley using the survey database to examine a specimen in collection storage. Note the grey photography paper used to photograph each specimen. (c) AMNH/F. Ritchie

Each specimen was given an overall condition summary, identifying it as Excellent, Good, Fair, or Poor, and further noting whether it is Stable or Unstable, based on the likelihood of existing damage worsening if left untreated.

In the final section of our survey, we recorded the nature and extent of any conservation treatment that would be required to make the specimen stable or suitable for exhibit, such as skin repairs, reconstruction, general grooming, dry cleaning, etc.

Together, all of this documentation will be used to guide decisions about how best to manage, store, and exhibit historic mammal taxidermy at the Museum, while offering supporting resources for the preservation of similar collections at other museums or sites.

Our next post will reveal some of the unique examples that we discovered during the survey.

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It’s All In the Preparation: Part 2

With our sample set chosen, our substrate selected, and our sample mounts determined, we next needed to devise a process for applying the dyes. Since the ultimate treatment would likely involve airbrushing, we chose to spray deposit the dyes onto the quartz plates. For this we had to consider and anticipate the different requirements and challenges that might arise in the application, including:

  1. What is the best technique for mixing the dye solutions?
  2. What delivery system gives us even coverage and the most control?
  3. How much dye should be applied and how can we measure that?
  4. What shape should the dye film deposit be to allow repeatable color measurements?
  5. How should we mask the substrate to achieve the desired sample shape?
  6. How do we economize and reuse the substrates?

1.  Mixing: Our mixing procedure reflects the method reported by Ciba-Geigy (the manufacturer of Orasol® dyes before its acquisition by BASF in 2008) in the literature describing their product testing. [Ciba_orasol_brochure] Where Ciba’s protocol is unclear, we have standardized our method based on our experience of what works well.

The powdered dye is measured and then incorporated into a volume of solvent to create a 1% (weight to volume) solution. The solution is stirred using a magnetic stirrer for approximately one hour. After mixing, the solution rests for approximately 24 hours. During this period, undissolved dye particles settle, and afterwards the solution can be decanted into a new glass bottle.

Sample Preparation

Former project intern Ersang Ma decanting newly mixed dye into the spray bottle. AMNH/B. Hunan

We have observed that when left in storage, the dye solutions appear to have a shelf life. In some cases we have seen changes in the color of the solution and the formation of crystalline deposits. We haven’t looked into what these changes might mean for the behavior of the solution. We have just taken them as indications that something has changed, and in response, we prepare fresh dye solutions for each round of sample production.

2.  Delivery System: We elected to apply the dye to the quartz plates using an airbrush. Airbrushing is, in general, the delivery method that permits the greatest range of expressive use of the dyes in taxidermy restoration, where controlling color value, gradation, and blending is critical to achieving a successful result. For the same reasons, it is easier to control the amount of dye deposited onto the plate using an airbrush than by brush, roller, dropper, or dipping techniques. The spray application facilitates putting down a thin, even layer, which can be gradually built up to the desired level.

during spraying

Former project intern Ersang Ma spraying dye onto a masked quartz plate. AMNH/F. Ritchie

Our dye solutions are applied to quartz plates using an Iwata Eclipse HP-BCS airbrush at 20 psi of air pressure from a compressor. Of course, having elected to use the airbrush, our samples are always prepared in a fume hood while the researcher is wearing appropriate personal protective equipment.

3.  Controlling Dye Application: It is important to control the dye application because the color produced by a medium-value application is inherently more light-sensitive than a heavy or light application of the same color. The sensitivity of the medium-value color derives from its reflectance spectrum having values that are not pinned at the low values (for very dark colors) or high values (for very light). It has also been found that for a given amount of dye loss, reflectance values around 30-40% show the largest increase. So when we spray our dye solutions on the quartz plates, we are attempting to produce samples whose reflectance spectra have minima that fall in that range. Doing so also avoids pinning large parts of the spectrum at the very high or low reflectance levels. (Specifics of reflectance spectra will be described in an upcoming post.)

However, controlling the amount of dye deposited onto the plate, even in approximate terms, presents significant challenges. Theoretically it would be possible to accurately measure the weight of the dye deposited, but adjusting one’s delivery to repeatedly match this measurement is fanciful. A more realistic approach to dealing with this uncertainty is to aim for the most sensitive application in which color change can be accurately measured and described; that is, the lightest application that is not so light as to make impossible the observation and measurement of fading.

Experiments conducted for the paint industry have demonstrated that such samples have a minimum reflectance of 30-40%; that is, in the spectrum of light reflected from the sample surface, the amount of light reflected at the most readily absorbed wavelength is 30-40%. When this condition is met, the entire spectrum will fall between the extremes of light and dark, so that there is room for it to gradually rise or fall as the colorant fades or darkens.

relfectance percentages

Mockup demonstrating how much dye corresponds to different reflectance values. AMNH/F. Ritchie

Since it can be difficult to visually gauge whether a particular dye sample on a clear plate meets these criteria, we created a set of small mockups with white backgrounds that reflect a range of minimum reflectance values (14% – 76%). These give us a sense of what each of those dye loadings look like. To arrive more precisely at the 30-40% minimum reflectance value in our samples, in the course of spraying, each sample is placed on a white membrane filter backing and measured using the spectrophotometer. More dye is added, if needed, until the desired loading is achieved.

color reading during spraying

Current project intern Caitlin Richeson measuring the percent reflectance on a freshly-sprayed quartz plate dye sample using a handheld spectrophotometer. AMNH/F. Ritchie

4.  Sample Shape: As our last post mentioned, our experimental procedure calls for taking periodic color measurements from each replicate throughout the duration of the exposure. At each time index, the values obtained from three locations on the plate are measured and averaged, to produce an overall measurement for the sample. In order to accurately calculate color change (which we will discuss in depth in an upcoming post), it is important that these three locations are reproducible, i.e., they do not change from measurement to measurement.

jig positions

Jig used to hold the quartz plate dye samples during color measurements. The samples are placed in each of the three positions for the three readings obtained using a spectrophotometer. AMNH/F. Ritchie

This requirement informed the design of a small jig with a wide T-shaped window into which the quartz plate can be inserted in any of three positions: left, right, and bottom. The spectrophotometer is placed in a fixed position atop the window. As each set of measurements is taken, the plate is moved through the three arms of the T. As long as the plate is inserted into the jig in the same orientation each time, the three measurement locations are constant. To ensure proper orientation of the quartz plates for each round of measurement, the dyes are applied to form an inverted T-shaped deposit on the plate. This irregular shape ensures the plate is properly orientated to provide the three viable measurement locations. If the plate were positioned in the jig in an incorrect orientation, at least one measurement would include an undyed area on the plate, giving the researcher an immediate cue that she has made a positioning error.

5.  Masking: A metal mask easily clips on to the plate to expose the same T-shape sample area in each application. After application, the metal mask is removed and cleaned before applying the next dye solution.

assembling samples

Assembling the bare quartz plate, metal mask, and backing material (to help see how much dye has deposited onto the clear plate) before spraying with dye. AMNH/F. Ritchie

6.  Reusing substrates: Since our quartz plates are in limited supply, their use does not end after a test cycle. After final color measurements and final photography, the plates are cleaned so that they can be reused for the next test round. Our cleaning procedure is intended to ensure that the plates are free of contaminants prior to the application of dye. Although our substrate is chemically inert, contaminants or dye degradation products can interfere with the results. Through testing and troubleshooting, a streamlined process is now in place:

Quartz Plate Cleaning Procedure:

  1. The quartz plates are first wiped with a solvent to remove grease and/or remaining dyes left on the surface from the last completed test cycle. The solvent we have chosen to use for this process is acetone, as it is effective on grease and all of the dyes.
  2. Solvent cleaning alone is not sufficient to fully remove the dye and all residues from the plates. The initial solvent cleaning step can redeposit dye in the roughened edges of the plate, where it becomes stubbornly embedded. Furthermore, we found that many dyes as they age create a thin, transparent, insoluble film on the plate – a “ghost” pattern on the surface, which accepts new dye solution differently than the virgin quartz surface. To remove these residual deposits and “ghosts,” we next polish the plates with Bueller MetaDi Monocrystalline Diamond Polish, 1 μm. The monocrystalline diamond particles are single grain particles with sharp edges. These particles are suspended in a fluid, which we can spray onto the plates and gently buff the surface of the quartz using cotton pads. The polish removes the residual dye and insoluble deposits on the plates.
  3. Next, the quartz plates are transferred to a bath of detergent and water. The detergent, Sparkleen, (Sodium Carbonate 10 to 25%, Sodium Dodecylbenzenesulfonate 1 to 10%, non-ionic detergent 1 to 10%) is a conventional laboratory glassware detergent that aids in the removal of both organic and inorganic deposits.
  4. Finally, after the quartz plates have been rinsed of the detergent, they are allowed to dry and then receive a final wipe with acetone. This final round of solvent cleaning is intended to ensure that no contaminants are left on the plates from handling. Once the plates are clean, they are ready to be sprayed with dye once again.

The preparation of the quartz plates is both time-consuming and stringent, but is an essential part of the experimental procedure.

A subset of 9 of the Orasol dye colors, dissolved in propylene glycol monomethyl ether (PGME) and airbrushed to the quartz plate substrate. AMNH/F. Ritchie