Dent Mater 9:74-78, March, 1993 Early strength and adaptability of amalgam in relation to coherence time D.C. Watts, E.C. Combe Biomaterials Science Unit, Department of Restorative Dentistry, University of Manchester Dental School, Manchester, England, UK Abstract. The minimum mixing time required to give a coherent mix of a dental amalgam ("coherence time") was determined for eleven representative materials using two different amalgama- tors. The effect of mixing these amalgams at various multiples of their coherence time on early compressive strength and adaptability was determined. It was found that the two amal- gamators gave different coherence time values for the alloys, but that the strength data for any given alloy was similar for a given multiple of coherence time. Longer mixing tended to yield amalgams of greater early strength, but reduced the adaptabil- ity of the mixed material. INTRODUCTION The amount of mixing used for the preparation of dental amalgam samples has a significant influence on the values obtained from laboratory tests (Osborne et al., 1977; Rehberg and Gramberg, 1979; Darvell, 1981; Brockhurst and Culnane, 1987). These differences may also be reflected in clinical performance. In particular, electrically-powered mechanical amalgamators exhibit a significant range of mixing power, such that low-powered devices require a longer amalgamation period. Many alloys are provided with instructions that permit some latitude in mixing conditions. One attempt to rationalize the effects of different mixing conditions is by use of the so-called coherence time (Darvell, 1980; Brockhurst and Culnane, 1987). This is an amalgama- tor-dependent parameter that specifies the minimum period required to produce a single unified or coherent mass, from commencement of mixing of the original mercury and alloy powder. Small cracks or a dry looking surface are not deemed to detract from coherence. Use of this parameter leads to the expec- tation that the effects of prolonged mixing periods may be ex- pressed as a function of multiples of the coherence time, in a reasonably amalgamator-independent manner. Two parameters which have given rise to some concern amongst standardization bodies are the early 1 h compressive strength and the adaptability of amalgam. Previous work by Brockhurst and Culnane (1987) has shown that early strength may increase with mixing time. Hence, prolonged mixing may ensure that a dental amalgam achieves the strength mini- mum required by national or international standards. How- ever, prolonged mixing may reduce the lateral flow character- istics of the amalgam and hence give rise to poor adaptability of the material to the dental cavity. This may be a major factor leading to secondary caries (MjSr and Smith, 1985). The objectives of this investigation were to determine the multiple coherence-time dependence of: first, early strength for a representative range of eleven low- and high-copper amalgams, whenever possible using both high- and low-power amalgamators; and, second, adaptation of three representative alloys. MATERIALS AND METHODS Eleven different alloys were investigated in this study, as listed in Table 1. Amalgam mixes were prepared for a range of mixing times, and the minimum time, to the nearest second, to produce a coherent mix with alloy/amalgamator combina- tion was recorded. Three repetitions were made for each mixing time and the resultant coherence times were reproduc- ible in each case to the nearest second. Two amalgamators were used: a high-energy type (Silamat, Ivoclar, Schaan, Liechtenstein) and a lower-energy device (Dentomat 2, Degussa AG, Frankfurt, Germany). Absolute values for the amalgama- tor energies are unavailable, but the present results indicate a mean performance ratio of 3.3 with the wide range of amalgams tested. Test compressive specimens of amalgam were prepared by standard techniques, as described in ISO standard 1559 (1986), except that the mixing time was varied systematically as integer multiples (n = 1 - 10) of the coherence time, [n.CT]. Compressive strength was determined using a calibrated Universal Testing Machine (Model T50 BS, RDP-Howden, Leamington-Spa, UK), at a cross head speed of 0.5 mm/min. Five replicate specimens were studied of each experimental condition, and means and standard deviations were calculated. Data were analyzed by ANOVA and the Student-Newman-Keuls [SNK] multiple comparison test. Amalgam specimens for measurement of adaptability were fabricated using lower-powered amalgamator trituration for different multiples of the previously determined coherence time. Five repetitions were measured for each condition. The stainless steel die for the condensation test is shown schematically in Fig 1. The triturated amalgam mass was emptied into a 4 mm deep cavity with a 2 x 3 mm rectangular cross-section by means of a hand condenser. At the base, the mold was extended laterally to a 7 x 2 mm cross-section with a height of 1 mm. During filling of the mold with amalgam, two small spacers, 2 x 2 x 1 mm in size, were temporarily placed into the sides of the bottom cavity. These spacers were removed with the aid of projecting flanges before the adaptation test 74 Watts & Combe~Coherence time of amalgam