wear to the opposing teeth, but they do conduct heat and cold, which can be irritating. there are two categories of gold fillings, cast gold fillings (gold inlays and onlays) made with 14 or 18 kt gold, and gold foil made with pure 24 kt gold that is burnished layer by layer. for years, they have been considered the benchmark of restorative dental materials. recent advances in dental porcelains and consumer focus on aesthetic results have caused demand for gold fillings to drop in favor of advanced composites and porcelain veneers and crowns. gold fillings are sometimes quite expensive; yet, they do last a very long time—which can mean gold restorations are less costly and painful in the long run. it is not uncommon for a gold crown to last 30 years. other historical fillings lead fillings were used in the 18th century, but became unpopular in the 19th century because of their softness. this was before lead poisoning was understood. according to american civil war-era dental handbooks from the mid-19th century, since the early 19th century metallic fillings had been used, made of lead, gold, tin, platinum, silver, aluminum, or amalgam. a pellet was rolled slightly larger than the cavity, condensed into place with instruments, then shaped and polished in the patient's mouth. the filling was usually left "high", with final condensation—"tamping down"—occurring while the patient chewed food. gold foil was the most popular and preferred filling material during the civil war. tin and amalgam were also popular due to lower cost, but were held in lower regard. one survey of dental practices in the mid-19th century catalogued dental fillings found in the remains of seven confederate soldiers from the u.s. civil war; they were made of: gold foil: preferred because of its durability and safety. platinum: was rarely used because it was too hard, inflexible and difficult to form into foil. aluminum: a material which failed because of its lack of malleability but has been added to some amalgams. tin and iron: believed to have been a very popular filling material during the civil war. tin foil was recommended when a cheaper material than gold was requested by the patient, however tin wore down rapidly and even if it could be replaced cheaply and quickly, there was a concern, specifically from harris, that it would oxidise in the mouth and thus cause a recurrence of caries. due to the blackening, tin was only recommended for posterior teeth. thorium: radioactivity was unknown at that time, and the dentist probably thought he was working with tin. lead and tungsten mixture, probably coming from shotgun pellets. lead was rarely used in the 19th century, it is soft and quickly worn down by mastication, and had known harmful health effects. acrylic polymers acrylics are used in the fabrication of dentures, artificial teeth, impression trays, maxillofacial / orthodontic appliances and temporary (provisional) restorations, however they can not be used as tooth filling materials because they can lead to pulpitis and periodontitis as they may generate heat and acids during (setting) curing, and in addition they shrink. failure of dental restorations fillings have a finite lifespan; composites appear to have a higher failure rate than amalgam over five to seven years. how well people keep their teeth clean and avoid cavities is probably a more important factor than the material chosen for the restoration. evaluation and regulation of dental materials the nordic institute of dental materials (niom) evaluates dental materials in the nordic countries. this research and testing institution are accredited to perform several test procedures for dental products. in europe, dental materials are classified as medical devices according to the medical devices directive. in usa, the u.s. food and drug administration is the regulatory body for dental products.