Here in the NMSC archeology lab, we are pretty familiar with corroded ferrous metal. Nearly all iron deteriorates when exposed to the elements upon burial, so we see a lot of rusty nails. We also fairly frequently add the term “patinated” to catalog records when processing old glass bottle fragments. (Perhaps you have read our 2011 post on glass deterioration. If not, check it out! Enemies of the Curator: Glass Deterioration) Recently, we have come across a couple of other material types that lend themselves to severe deterioration over time. Do you have natural rubber or early plastics in your museum collection? Do the images in this post look painfully familiar? Natural rubber and early plastics are subject to inherent and inevitable deterioration, especially in the case of archeologically recovered artifacts that have been subject to the elements for many years. With early identification and intervention, however, it is possible to slow this process and avoid the level of loss pictured here. (Please note: this post does not address cellulose nitrate film, which presents a health hazard and is highly flammable. Please see Conserve O Gram 14/8: Caring for Cellulose Film for more information.)
At the time of excavation, the collection of black dust pictured above left was an intact artifact with diagnostic qualities: a man’s dressing comb with eighteen countable teeth and the printed letters “rubbe” visible on the spine. The comb probably resembled the one at the above right that was pictured in the 1895 Montgomery Ward & Co. catalog.
Native South Americans of the Amazon River basin used natural rubber to waterproof their cloaks and shoes long before Europeans knew about it. Even once Europeans were introduced to rubber, its use did not catch on because of its tendency to harden in winter, soften in summer, and emit an unpleasant smell. These issues were resolved when Charles Goodyear developed the process of vulcanization in 1839. Hardened natural rubber was then used to manufacture buttons, combs, and other items.
Ironically, hard rubber combs were often touted as “unbreakable” in contemporary catalogs and advertisements. So what happened to the “unbreakable” comb whose residue we encountered in the lab? In a nutshell: time, air, and humidity. Natural rubber deteriorates when exposed to oxygen in the air. Fluctuating levels of relative humidity exacerbate this process.
During and after World War I, Germany and the USSR succeeded in developing synthetic rubber. The United States joined them in this endeavor during World War II when it was cut off from its suppliers of natural rubber, including India and the Dutch East Indies. The potential for deterioration in synthetic rubber depends upon its chemical make-up. Some synthetic rubbers, like neoprene, are highly resistant to oxygen, ozone, oil, heat, flame, and tearing, and preserve better than others.
In addition to the rubber dressing comb (or what is left of it), we have also recently processed a collection of celluloid combs which, although not quite reduced to dust, exhibit signs of severe deterioration due to time and inadequate storage conditions. Hard rubber had a plain, dull, black finish and was suited for utilitarian use. Celluloid, developed in the 1860s, imitated beautiful and expensive natural materials like ivory, shell, horn, and tortoise shell, and became a common material for decorative combs.
The combs pictured below are examples of side combs and back combs that were used to sweep a woman’s hair up and off of her neck. As with hard rubber, celluloid was marketed as a solid, lasting material; the Montgomery Ward catalog from 1895 “recommend[s] for durability ornaments described as Celluloid. They are very light and will bend double without breaking.”
Because of its composition, celluloid is inherently subject to continuous deterioration over time. The two major components of celluloid – cellulose nitrate and camphor – are organic in nature and fundamentally unstable. (Cellulose is found in cotton, flax, jute, and wood pulp; camphor is resin from an evergreen tree.) Cellulose nitrate molecules crystallize over time, and camphor evaporates following its expulsion to the surface of the object. The chemical fillers added to celluloid to promote flexibility make it even more unstable by migrating to the surface of the plastic and leaving the material tacky, brittle, warped, and distorted.
As with natural rubber, improper environmental conditions accelerate and intensify the natural deterioration of celluloid. Unstable levels of relative humidity may cause swelling and shrinking in celluloid objects, causing cracking. Pollutants, visible and ultraviolet light, and water are also detrimental to celluloid. Because of the evaporation of and the release of other gases over time, celluloid requires a ventilated environment. Sealed plastic bags only trap these gases and hasten the process of deterioration.
What Can You Do?
Although the decomposition of early rubber and plastic is natural and inevitable, implementing the right storage conditions can slow this process and prevent the loss of historic objects.
- Identify and isolate problem objects. Before visibly deteriorating, plastic emits a vinegar-like odor that signals impending composition. At this point, separate these objects from the rest of your museum collection to prevent cross-contamination by released gases.
- Store plastics in a ventilated space as opposed to sealed bags that trap damaging gases. As an alternative, use gas absorbents like molecular traps or scavengers.
- Consider an oxygen-free environment for natural rubber objects.
- Keep light levels low.
- Keep temperatures below 68 degrees Farenheit and relative humidity below 65%.
- Do not use water to clean rubber or plastic. Use brushes or a vacuum.
- Photograph and document rubber and plastic objects before any potential deterioration compromises their integrity.
- A cool, dry, dark, well-ventilated environment is ideal.
- For more information on how to care for plastic objects in your museum collection, see Conserve O Gram 8/4: Care and Identification of Objects Made from Plastic.
Natural rubber and celluloid objects are important vestiges of late 19th-century American history. These ultimately fragile, organic materials preceded completely synthetic rubber as well as plastics like Bakelite and Catalin that were made wholly from chemicals and do not decompose as readily. Celluloid objects, produced to imitate natural materials like shell, horn, and ivory, are often delicate and beautiful and reflect the stylistic tastes of the era in which they were made. Although deterioration is inherent and inevitable for these materials, it can be managed through the use of awareness, environmental control, and a watchful eye.
Care and Identification of Objects Made from Plastic. National Park Service Conserve O Gram 8/4, September 2010.
Care of Objects Made from Rubber and Plastic. Canadian Conservation Institute, CCI Notes 15/1. (Accessed online, http://www.cci-icc.gc.ca/publications/notes/15-1_e.pdf)
Catalog No. 57, Montgomery Ward &Co., 1895.
Encyclopedia Britannica online (www.britannica.com)
Reilly, Julie A. Celluloid Objects: Their Chemistry and Preservation. Journal of the American Institute for Conservation, 1991, Volume 30, Number 2, pp. 145-162. (Accessed online, JAIC online, http://cool.conservation-us.org/jaic/articles/jaic30-02-003.html)
Robinson, Julie Pelletier. Bakelite and Celluloid – the Differences. www.celluloidforever.co/
Rubber in Chemistry Encyclopedia online (www.chemistryexplained.com)
Schwartz, Lynell. Purse Masterpieces. Paducah, KY: Collector Books, 2004.
Sears Roebuck & Co. Catalogue, 1897.
Sears Roebuck & Co. Catalogue, 1902.
Van Patten, Joan and Elmer and Peggy Williams. Celluloid Treasures of the Victorian Era. Paducah, KY: Collector Books, 1999.
Wisniewski, Debra J. Antique and Collectible Buttons. Paducah, KY: Collector Books, 1997.
Wisniewski, Debra J. Antique and Collectible Buttons Volume II. Paducah, KY: Collector Books, 2002.