Tin-Mercury Amalgam Mirrors

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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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==Materials and technology==
===History ===
===Materials===
===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. 
===Documentation ===
===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). 
===Interventive treatments===
====Cleaning====
====Stabilization====
====Structural treatments====
====Aesthetic reintegration ====
====Surface treatments====
====Other treatments====
==Health & Safety==
Go to Health & Safety Main Page
<Br>
----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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