Mercury is a liquid at room temperature and is able to form a ‘workable' mass when mixed with the alloy. This behaviour renders the material suitable for use in dentistry. Dental amalgam has been used for many years with a large measure of success. For many years it was the most widely used of all filling materials. For various reasons, including the development of viable alternatives based upon resins and ceramics and perceptions of a dubious and frequently questioned level of safety, its popularity has declined.

Composition

Mercury used in dental amalgam is purified by distillation. This ensures the elimination of impurities which would adversely affect the setting characteristics and physical properties of the set amalgam.

For spherical alloys the method of manufacture dictates that each small sphere is like an individual ingot. Thus homogenization is normally carried out for the reasons outlined above. Many alloy powders are formulated by mixing particles of varying size or even shape in order to increase the packing efficiency of the alloy and reduce the amount of mercury required to produce a workable mix.

Clinical handling notes for dental amalgam

Cavity design:Many designs of cavity have been used for amalgam restorations, starting with modification of Black's design for cavities for gold restorations. Over the years the cavity design has been refined to minimize destruction of sound tooth tissue and to give an appropriate form to the restoration to ensure that the physical properties of the material are optimized in the end product. Amalgam has no intrinsic ability to bond to enamel and dentine, hence cavities have to be used which are undercut, i.e., the cavity is wider within the structure of the tooth than at its surface, in order that the material should be mechanically retained. At all times the cavity should be no wider than is compatible with removal of caries from the dentine, removal of any unsupported enamel and adequate access to pack the amalgam into the cavity.

It is important when rebuilding the proximal surfaces of any tooth to restore its contact relationship with any adjacent tooth. Obviously the use of a matrix may compromise this objective as the thickness of the matrix is interposed between the filling material and the tooth. This problem is surmounted when using amalgam in two ways. First, having adapted the matrix to the tooth it is burnished outward to try to achieve a contact with the adjacent tooth. Second, a wooden or metal wedge should be inserted between the teeth if possible. This has a dual benefit in that it helps to maintain adaptation of the band to the tooth surface cervically and it separates the teeth slightly. Once the wedge is in place the matrix can be loosened slightly to facilitate burnishing against the adjacent tooth.