Arsenic compounds in taxidermy collections

Last year we took the Bruker III-V+ on the road to analyze a taxidermy collection for the presence (and hopefully absence) of arsenic. Arsenic compounds are used as pigments, pesticides, and insecticides, so their presence in cultural heritage materials is not uncommon. Due to the high toxicity of arsenic it is often necessary to test collections to verify that they can be safely handled. The use of portable XRF units such as the Bruker II-V+ in the identification of heavy metal compounds in museum collections began in the late 1990s, and is now the standard method for such analyses. Arsenic has been used in the preparation of natural history specimens going back to the early 1800s and was used even as late as the 1980s, so its presence is possible in nearly every collection. One of the most common uses of arsenic in taxidermy preparation was arsenical soap composed of the white powder arsenic trioxide mixed with soap, subcarbonate of potash, camphor, and alcohol. The resulting paste was applied to the inside of the animal and bird skins, and readily comes through the skins to the surface where it can be transferred to human hands or contaminate other objects it contacts.

The analysis can be done literally handheld and once the parameters are established analysis can proceed at a rate of about 1 object every 5 minutes. The XRF spectra shown on the right shows a definite peak for arsenic found on a dusky footed wood rat. The presence of other compounds containing mercury or lead can also be important in some collections and can be detected at the same time.

For further information, see chapter 9 "Handheld XRF use in the identification of heavy metal pesticides in ethnographic collections" in Shugar, Aaron N. and Jennifer L. Mass. 2012. Handheld XRF for art and archaeology.

The Traditional Cyanotype

The cyanotype process was invented by Sir John Herschel in 1842.  Herschel's process became the first successful non-silver printing method and the first photographic process to be used for scientific illustration. In publications – such as Anna Atkins’ British Algae: Cyanotype Impressions – cyanotypes easily recorded photograms of botanical specimens.  The intensity of the blue color in cyanotypes, however, is not ideal for certain subjects such as portraiture or landscape.  Thus, the process never became popular for pictorial purposes.

Traditionally, a cyanotype is created by coating paper with a sensitizer composed of ferric ammonium citrate and potassium ferricyanide exposing the sensitized paper, and processing the resulting print.  The process is non-toxic, simple and cheap.  Once the sensitized paper is contact printed with a negative (about thirty minutes), the paper is placed in a tray of water to process the final image.  The water washes away the ferricyanide and “soluble” Prussian blue; the “insoluble” Prussian blue left behind forms the image.  Although cyanotypes are best known for their traditional blue color, prints have been toned to gray, reddish brown, black, violet, and green.  The Prussian blue color, however, remains more permanent over time than any known toning method.

The three prints of Marjorie demonstrate a significant decrease in iron (Fe) concentration when comparing areas of high density to areas of low density.  This is consistent with the presence of Prussian blue Fe₄[Fe(CN)₆]₃.  The calcium (Ca) peaks remain unchanged because the calcium is most likely found in the paper support.

These prints of Marjorie are still in very good condition.  Cyanotypes are, however, subject to fade over time if exposed to light or alkaline substances for extended periods.  In order to maintain the image integrity, a cyanotype can be moved into the dark.  When removed from the light, the air will oxidize the white ferrous ferrocyanide to blue ferric ferrocyanide.  This oxidation process allows the image to regain density.  Cyanotypes are the only photographic process capable of oxidizing in this way.  Unlike many other photographic processes, cyanotypes are also not susceptible to acid degradation from the image support.  As a result, cyanotypes have been printed on a variety of non-traditional materials such as wood, ceramics, and textiles.

Cyanotypes started to be commercially produced by 1872 and then primarily became used for making copies of technical drawings.  The mass production of these drawings became known as “blueprints.”  Eventually, after the invention of paper photocopying, the use of cyanotypes in the commercial sector ceased.

An improvement to Herschel's original process was introduced by chemist Dr. Mike Ware in the late 20th century.  Ware's process is more sensitive to light, has a wider density range, and less time consuming.  Since the three cyanotypes analyzed here were all prepared using the traditional method, this brief description of the new method will suffice.

References:

Anna Atkins. N/A. The J. Paul Getty Trust. 4 January 2013 <http://www.getty.edu/art/gettyguide/artMakerDetails?maker=1542&page=1>.

Kennel, Sarah, Diane Waggoner, and Alice Carver-Kubik. In the Darkroom: An Illustrated Guide to Photographic Processes before the Digital Age. Washington: National Gallery of Art, 2009.

Farber, Richard. Historic Photographic Processes: A Guide to Creating Handmade Photographic Images. New York: Allsworth Press, 1998.

The New Cyanotype Process. 2004. Mike Ware. 4 January 2013 <http://www.mikeware.co.uk/mikeware/New_Cyanotype_Process.html>.

The Traditional Cyanotype Process. 2004. Mike Ware. 4 January 2013 <http://www.mikeware.co.uk/mikeware/Traditional_Cyanotype.html>.

Ware, Mike. Cyanotype: The history, science, and art of photographic printing in Prussian blue. London: Science Museum, 1999.

Webb, Randall and Martin Reed. Alternative Photographic Processes: A Working Guide for Image Makers. Rochester: Silver Pixel Press, 1999.