An amalgam is an alloy of mercury with another metal. Almost all metals can form amalgams with mercury, sometimes easily but sometimes with difficulty. Notable exceptions are some transition metals like iron and platinum. That is the reason mercury is stored in iron containers. Amalgams find extensive use in almost all branches of material science, analytical chemistry, metallurgy and in dentistry (silver amalgam is used in dental fills). The high affinity of copper towards mercury by amalgam formation is exploited in a probe for detection of very small quantities of mercury salts in a sample. Amalgams are generally prepared by addition of the metal to mercury, or using mercuric salts through a reduction reaction that generates to metallic mercury in situ which reacts quickly with the other metal to produce the desired amalgam. Though preparation of some amalgams are difficult and need specialized techniques. Some of the amalgams have definite composition , e.g. Hg2Na.