C08 Discuss and demonstrate how variations in trituration time, mercury/alloy ratio, condensation pressure, porosity, and rate of hardening influence strength properties.
Overtrituration results in an amalgam that may set so fast that the matrix is disturbed and cracked during condensation. Undertrituration results in an amalgam that contains particles that have not been completely wet during trituration. Such regions may form weaknesses if the condensation is not done well.
By following the instructions from the manufacturers, the amalgam powder will be properly wet, and at the same time, the amalgam will not set too fast. This amalgam will behave well in most clinicians hands and result in an amalgam that has the highest chance to have the highest strength.
More mercury will consume more of the dental amalgam alloy particles. Thus, more mercury will result in weaker amalgams. Too little mercury results in an amalgam that does not cohere easily during condensation.
It is in general believed that higher condensation pressure always result in stronger amalgams. This belief is not absolutely correct. For example, during condensation, a very high condensation pressure can cause the amalgam to crack if the surrounding system does not produce a counter pressure during condensation. When such cracks form, and the amalgam is to dry, incomplete wetting will occur, and the strength decreases. In other words, one should use as high condensation pressure as possible without introducing cracks in the final restoration.
Porosities act as stress concentrators and decrease strength
Rate of hardening
Fast setting amalgams often contain less mercury and are in these cases also stronger. However, a fast setting amalgam, used by a slow condensing dentist, can result in a weak amalgam
The figure below shows the one hour compressive strength of a conventional microcut, a spherical conventional (c), a high-copper admixed (a) and a high-copper spherical (s). As seen from this figure, the two high-copper amalgams are the strongest. The higher strength can be related to the Cu6Sn5 phase that stabilizes the gamma-1 phase. Comparing spherical versus lathe-cut or admixed within the conventional or the high-copper groups reveals that the spherical are stronger. This relates to the lower mercury and porosity content of spherical amalgams. The lowest strength value of the microcut amalgam relates to an almost complete depletion of the original dental amalgam alloy particles. The small alloy particles are more or less completely dissolved by the mercury.