Tin-Mercury Amalgam Mirrors

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Contributors: Kerith Koss Schrager
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Copyright: 2011. The Objects Group Wiki pages are a publication of the Objects Specialty Group of the American Institute for Conservation of Historic and Artistic Works. The Objects Group Wiki pages are published for the members of the Objects Specialty Group. Publication does not endorse or recommend any treatments, methods, or techniques described herein.

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THIS ENTRY IS A DRAFT

Materials and technology

Technology

The use of tin-mercury amalgam was the primary method for producing glass mirrors from the 16th to the early 20th century. In this process tin foil and liquid mercury were applied to glass; the resulting two-phase amalgam consisted of tin-mercury crystals surrounded by a mercury-rich liquid phase.

Identification

Distinguishing mercury-containing mirrors from silvered mirrors can be difficult. If the mirror was produced before the early 20th century, it is likely a mercury amalgam mirror. Silvering, the other predominant historic mirroring technique, involves the deposition of silver on the glass. While silvering was developed in the mid-19th century, it did not completely replace the amalgam process until the 20th century. Unfortunately, most mercury mirrors are only identified once mercury droplets are discovered along the bottom of the frame or on floors and baseboards. Occasionally, beads of liquid mercury are also visible from the front of the mirror, which can be observed migrating under the glass.

There are several additional ways to identify an amalgam mirror if liquid mercury is not visible. Scientific testing, such as X-ray Fluorescence (XRF), can easily confirm the presence of mercury. However, if these methods are not accessible, mercury and silver mirrors can be distinguished by subtle differences in color and reflectivity. Mercury mirrors reflect less light and have a bluish appearance, while silver mirrors look more yellow. This effect can be enhanced by placing a thin piece of paper over the mirror, which will appear paler and brighter over the silver mirror (Hadsund 1993).

Deterioration

The condition of mercury mirrors can be deceptive; they may seem to be in good condition even if they are actively deteriorating and releasing mercury. Look for surfaces that exhibit small points of light, which create a glittering, rather than reflective, surface. The lower section of the mirror may be more heavily deteriorated with numerous small holes. These holes may only be visible when the object is backlit and are rarely visible during normal use. Corrosion begins as small dark patches that create a dark and cloudy appearance. More severe corrosion manifests as grey layers or as grey, yellow-brown, and/or white concentric bands resulting from oxidation of the tin.

Conservation and care

This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.

• To find a conservator, please visit AIC's Find a Conservator page.

• To learn how you can care for your personal heritage, please visit AIC's Resource Center.

Preventive conservation

The degradation of the amalgam can be retarded by taking preventative steps, such as maintaining low and constant temperatures and relative humidity. In addition, mirrors should be displayed and stored in their original orientation; changing the orientation (e.g. laying a wall mirror on its back) will alter the equilibrium and promote further deterioration. Mirrors should also be checked regularly to ensure they are free of particulates such as dust and spider webs that can retain moisture. Preventative measures can also be employed to limit mercury exposure. Mirrors containing mercury should be clearly labeled. Consider replacing severely degraded mirrors with stable modern mirrors, if possible. Covering the back of the mirror serves to both protect the amalgam and contain mercury; however, mirrors should not be completely sealed to prevent build up of mercury vapor and because slow evaporation of the mercury is necessary for curing the amalgam. To protect the back of the mirror while allowing for air circulation, Hadsund recommends sealing the junction between the glass and the frame with soft felt, attaching a piece of paper or closely woven textile across the back of the frame, and reattaching the original mirror backing (Hadsund 1993: 14). Another method of containment employed by some museums involves partially or completely lining the backs of mirrors with Mylar, which allows for continuous monitoring. For temporary storage, the lower halves of mirrors can be bagged with polyethylene to prevent leakage and vapor build-up (Swan 2010). Experimental methods that use backing layers that can absorb mercury vapor are also currently being investigated (Torge et al. 2010).

Health & Safety

Go to Health & Safety Main Page

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Disclaimer: Some of the information included in this section may be out of date, particularly with regard to toxicological data and regulatory standards. Also, because new information on safety issues is continually published, resources outside of AIC should be consulted for more specific information.

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Improper handling of historic mercury amalgam mirrors presents a potential risk for elemental mercury exposure. The tin-mercury amalgam from which these mirror were fabricated is inherently unstable and releases mercury liquid and vapor as it deteriorates. As a result, the mirrors and frames, as well as storage, work, and exhibition spaces can easily become contaminated, placing anyone who interacts with these objects at risk for mercury exposure.

Mercury emissions result from several processes: oxidation of the amalgam, evaporation of the liquid phase, and migration of liquid mercury to the bottom edge of the mirror due to gravity.

Mercury exposure from amalgam mirrors can be mitigated by identifying mirrors containing mercury; responsible exhibition, storage, and handling; following safety protocols when cleaning up mercury spills; and proper recycling and disposal of mercury-contaminated products.

While the degradation of tin-mercury amalgam is inevitable and cannot be reversed, taking these preventative measures and safety precautions allows for the safe exhibition, storage, and handling of these historic objects.

Visual characteristics may help in identification (see above), however, it is safest to assume that mercury is present in any mirror produced prior to the latter half of the 20th century, unless otherwise documented or proven by scientific methods, and therefore should be handled appropriately.

Handling

Handle mirrors that contain or may contain mercury with extreme caution. Always wear disposable Personal Protective Equipment (PPE) including rubber, nitrile or latex gloves as well as protective clothing and eyewear. Work in well-ventilated, cool areas. If workspaces cannot be properly ventilated, only use respirators with cartridges that are approved for use with mercury vapor (these will have a special “end of service life indicator” to warn the user of potential mercury vapor breakthrough). Work surfaces should be covered with a disposable material. Examine storage and exhibition areas for liquid mercury, which can collect on floors and be easily distributed by foot traffic. Regularly train staff on proper handling and spill response measures (see next section).

The Occupational Safety and Health Administration (OSHA) guidelines limit mercury vapor exposure to 0.1 mg/M3 of air. However, this level does not reflect current toxicological literature on mercury health effects. The more current and conservative occupational exposure level is 0.025 mg/ M3 as an 8-hour time-weighted average, established by the American Conference of Governmental Industrial Hygienists (ACGIH 2012). Although studies measuring mercury vapor emissions in gallery spaces (Hadsund 1993; Swan 2010) and lab conditions (Torge et al. 2010) have demonstrated that the amount of mercury vapor released from undisturbed historic mirrors is below recommended safety guidelines, take precaution when moving, handling, or dismantling mirrors. Warm or poorly ventilated indoor spaces and agitation of liquid mercury increases the risk of exposure to mercury vapor.

Mercury vapor levels can be measured by using special passive dosimeter badges (laboratory analysis often included) or with indicator/detection cards that can be purchased through laboratory safety suppliers. More expensive analytical instruments are also available; however it may be more appropriate to contract with an environmental monitoring company to monitor mercury emissions. One source is the American Industrial Hygiene Association’s Consultant Listing. Since mercury liquid and vapor control is a major issue in most municipalities, especially in school systems, monitoring and guidance may also be available through local public health departments upon request. A valuable and free resource for small to medium-sized businesses is the OSHA On-Site Consultation Service, offering advice (separate from enforcement) through offices in every state.

Clean-up and Disposal of Elemental Mercury

Small liquid mercury spills can be safely handled using appropriate precautions. Ventilate contaminated areas and wear protective equipment. Never use a broom or vacuum to collect liquid mercury, unless it is a specially designed mercury recovery vacuum. Commercial mercury spill kits, sponges and powders that sequester and contain the liquid should be used for collection. Sprays, powders and papers are also produced for controlling mercury vapor levels. If spill kits are not available, gently collect beads of mercury into sealed containers using disposable materials. Never pour liquid mercury down the drain. Dispose of clothing and any absorbent materials that have come in contact with mercury and do not launder contaminated material in a washing machine. Be particularly aware of tracking liquid mercury on shoes. All contaminated items should be placed in sealed containers, clearly labeled and disposed of according to state, local, and institutional regulations.

Many civic and government agencies offer collection and exchange programs for mercury and mercury-containing devices as part of an ongoing awareness to provide proper disposal for hazardous materials. For information about these programs, contact local officials to find out when and where a collection will be held. Resources such as Earth911.com can provide information about local collection programs.

Health Risks of Elemental Mercury Exposure

Elemental mercury primarily causes health effects when it is inhaled as a vapor. After exposure to liquid mercury, less than 1% of the total amount is absorbed through ingestion or dermal contact, while 80% of inhaled mercury vapor is absorbed by the respiratory tract and retained in the kidneys and brain (WHO 2000). Symptoms of high levels of mercury exposure can occur within hours and include respiratory distress, tremors, emotional changes, insomnia, neuromuscular changes, headaches, disturbances in sensations, nausea, vomiting, diarrhea, and changes in cognitive function. Chronic exposure may result in more severe kidney, respiratory, and cognitive effects. Individuals concerned about their exposure to mercury should consult their physician within three days of exposure for testing and treatment.

Mercury Supplies

Mercury specific detection and spill products are available through most laboratory supply companies such as Grainger Industrial Supply and Fisher Scientific. Customer service representatives will be able to assist with finding the appropriate supplies for your needs. Some of the most useful products are:

Mercury vapor badges and cards

Passive dosimeter badges contain adsorbent cartridges that are sent to laboratories to quantify exposure levels, while vapor indicator cards and badges contain special papers that change color in the presence of mercury.

Mercury indicator powder

Indicator powder changes color in the presence of mercury and can identify mercury residues.

Mercury spill kits

Kits contain everything needed to clean up small spills including personal protective equipment (PPE), cleaning supplies, and collection containers.

Mercury recovery vacuum

Vacuums safely collect mercury, mercury vapors, and mercury-contaminated particulates with specially adapted filters and collection containers.

Mercury vapor respirator cartridges

Cartridges will have a special “end of service life indicator” to warn the user of potential mercury vapor breakthrough.

Mercury amalgamation powder

Adsorbent powder converts elemental mercury into an amalgam, preventing mercury vapor emissions. Powder can be safely collected into appropriate disposable containers.

Mercury amalgamation sponge

Adsorbent sponges collect and convert elemental mercury into an amalgam, preventing mercury vapor emissions.

Mercury containers/jars

Polyethylene containers that use sponges to collect and contain liquid mercury. Some containers only collect the liquid mercury and others use adsorbent sponges to collect the mercury as an amalgam.

Mercury vapor powders and sprays

Powdered sorbents, such as iodized activated carbon, and commercial sprays reduce and suppress mercury vapors.

Additional Reading

Agency for Toxic Substances and Disease Registry (ATSDR), Centers for Disease Control. 2010. “Mercury and Your Health.”

American Conference of Governmental Industrial Hygienists (ACGIH). 2012. Threshold Limits Values and Biological Exposure Indices. Cincinnati, OH.

Earth911. 2012.

Environmental Protection Agency (EPA). 2012. “Mercury Releases and Spills.”

Environmental Protection Agency (EPA). 2012.“Elemental Mercury Effects.”

Hadsund, P. 1993. “The Tin-Mercury Mirror: Its Manufacturing Technique and Deterioration Process.” Studies in Conservation, 38(1): 3-16.

Koss Schrager, K. 2013. "Tin-Mercury Amalgam Mirrors." AIC News, 38(1): 12-16.

Occupational Safety and Health Administration (OSHA), United States Department of Labor.“Guideline for Mercury Vapor.”

Payne de Chavez, K. 2010. “Historic Mercury Amalgam Mirrors: History, Safety and Preservation.” Art Conservation, Spring 2010: 23-26.

Podsiki, C. 2008. “Heavy Metals, their Salts, and Other Compounds: A Quick Reference Guide from AIC and the Health & Safety Committee.” AIC News, November 2008: Special Insert.

Swan, C. 2010. “Mercury: the Problem with 18th-century Looking Glasses.” In C. Hawks, et. al. (eds), Health & Safety for Museum Professionals. New York: Society for the Preservation of Natural History Collections, 516.

Torge, M., et. al. 2010. “Mercury Emissions from Historical Tin Amalgam Mirrors.” In H. Roemich (ed), Glass & Ceramics Conservation 2010, Papers presented at the Interim Meeting of the ICOM-CC Glass & Ceramics Working Group, October 3-6, 2010, Corning, New York: ICOM Committee for Conservation in association with the Corning Museum of Glass, 156-163.

World Health Organization (WHO). 2000. “Chapter 6.9: Mercury.” In Air Quality Guidelines - Second Edition, Copenhagen, Denmark: WHO Regional Office for Europe.

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