An amalgam is an alloy created by mixing mercury with one
or several other metals. Typically amalgams are soft when first mixed, and then they
harden into a durable solid material.
Silver based amalgams
have been used as dental fillings for cavities since the 1830's. The initial
malleability of the amalgam allows it to be packed tightly into the cavity, where it
hardens without either expanding or contracting, forming a hard, permanent
filling.
Amalgams are also used in gold mining. Mercury is
washed over sand or crushed rock where any small particles of gold will bond to it. The
amalgam thus created is recovered and the gold is extracted. While this technique is
effective, it is also a major source of pollution from mining and gold sluicing
operations.
A third use is detection of mercury salts in
water. Amalgam probes are made by applying a nitric acid solution to copper foil, and
then immersing the foil in a water sample. A positive test will show a spot of silvery
appearing mercury amalgam forming on the foil.
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