Gamma 2 phase amalgams
After widespread adoption and wildly varying standards, the multitude of formulas for making amalgams were standardized into the gamma-2-phase amalgam formula in 1895.
The gamma-2-phase amalgams contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:[citation needed] [3]
* > 65% silver (Ag)
* < 29% tin (Sn)
* < 6% copper (Cu)
* < 2% zinc (Zn)
* < 3% mercury (Hg)
The resulting amalgam is composed of the gamma phase (the silver-tin eutectic Ag3Sn), which reacts with mercury, yielding the gamma-1 phase (Ag2Hg3) and gamma-2 phase (Sn7-8Hg). The gamma phase is prone to corrosion and its mechanical strength is low. The alloy tends to undergo crevice corrosion and form local galvanic cells, due to the potential difference between the gamma-1 and gamma-2 phases.
Around 1970, the ingredients changed to the new non-gamma-2 form, with lower manufacturing cost, greater mechanical strength, and better corrosion resistance. The reduced-gamma-2 amalgams (sometimes referred to as "high-copper" amalgams [44]) contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:[citation needed]
* > 40% silver (Ag)
* < 32% tin (Sn)
* < 30% copper (Cu)
* < 2% zinc (Zn)
* < 3% mercury (Hg)
The amalgam alloy is strengthened by presence of Ag-Cu particles. The gamma-2 phase reacts with the Ag-Cu particles to form eta phase Cu6Sn5 and gamma-1 phase.
The possible difference in toxicology between the two has not been studied conclusively.
