From:
http://www.brooks.af.mil/dis/DMNOTES/amalgam.pdf
DENTAL AMALGAM
Historical Composition
G.V. Black believed that amalgam should consist of:
67% silver, 27% tin, 5% copper, and 1% zinc
Low-Copper (Traditional, Conventional) Amalgam
Composition:
Silver 60%
Tin 29%
Copper 6%
Zinc 2%
General Setting Reaction:
Ag Sn + Hg ---- Ag Sn + Ag Hg + Sn Hg 3 3 2 3 8
silver-tin mercury silver-tin silver-mercury tin-mercury
gamma gamma gamma 1 gamma 2
When low-copper amalgam is triturated, mercury diffuses into
the silver-tin particles and silver and tin dissolve, to a
very limited extent, into the mercury. As this occurs, the
particles become smaller. Because the solubility of both
silver and tin in mercury is limited (0.035 and 0.6 weight
percent respectively) and because silver is much less soluble
in mercury than is tin, silver precipitates out first as
silver-mercury (gamma 1) followed by tin in the form of tinmercury
(gamma 2).
The set amalgam consists of core gamma particles surrounded by
a matrix of gamma 1 and gamma 2.
High-Copper (New-Generation, Gamma-2 Free) Amalgam
Composition:
Silver 40% to 70%
Tin 12% to 30%
Copper 12% to 30%
Indium 0% to 4%
Zinc 0% to 1%
Palladium 0.5%
High-copper amalgam was developed in 1962 by the addition of
silver-copper eutectic particles to traditional silver-tin
lathe-cut particles in an attempt to dispersion strengthen or
dispersion harden the alloy. Although clinical tests showed 1
that these new alloys had better physical properties, the
improvement was not due to dispersion hardening (because the
silver-copper eutectic particles were too large and too far
apart to impede dislocation movement) but rather were the
result of formation of Cu Sn , the eta phase. The fact that 6 5
tin had a greater affinity for copper than for mercury meant
that the gamma-2 phase was reduced or eliminated. This
resulted in the dramatic improvement in physical properties.
It is important to note that high-copper alloys must contain
at least 12% copper to eliminate the gamma-2 phase. Compared
to their low-copper amalgam counterparts, high-copper alloys
exhibit the following physical properties: greater strength,
less tarnish and corrosion, and less creep. Overall, they are
also less sensitive to handling variables and produce better
long-term clinical results.
Purposes of Constituents in Amalgam
Silver -- increases strength and expansion
Tin -- decrease strength and expansion and lengthens the
setting time.
Copper -- increases strength, reduces tarnish and
corrosion, and reduces creep and, therefore, marginal
deterioration. Copper accomplishes these effects by tying up
tin, preventing the formation of gamma 2, the weakest, most
tarnish- and corrosion-prone phase, and the phase with the
highest creep values. In addition, it reduces creep by tying
up tin and forming copper-tin (Cu Sn ), the eta phase, whose 6 5
crystals interlock to prevent slippage and dislocations at the
grain boundaries of gamma-1 particles which is a major cause
of creep in amalgam. Is added at the expense of the silver.
Copper is insoluble in mercury.
Zinc -- is added for the benefit of the manufacturer
because it prevents oxidation of the other metals in the alloy
during the manufacturing process; in so doing, it keeps the
alloy from turning dark. Zinc accomplishes this by combining
readily with oxygen to form zinc oxide. An alloy with 0.01%
zinc is "zinc-free" while an alloy with 0.01% zinc is "zinccontaining".
If a low-copper, zinc-containing alloy is
moisture contaminated, it will result in surface blistering,
internal corrosion, and a delayed expansion of up to 4% by
volume beginning 3 to 5 days after the contamination and
continuing for up to six months. This can lead to a reduction
in strength of up to 24%. Although, moisture contamination 2
of zinc-containing high-copper amalgams has not been shown to
cause delayed expansion, moisture contamination of all types 3,4
of amalgam should be avoided because it can cause a reduction
in strength. Research has found that high-copper amalgam
alloys that contain zinc in a 1% concentration exhibit lower
rates of margin fracture than do zinc-free alloys. This is 5
believed to be due to zinc's behavior as a sacrificial anode
which delays corrosion of tin in the Cu Sn phase.
--
Don Thompson
Ex ROMAD
"Des Bromilow" wrote in message
...
OK, I've tried googling, and I still don't have an answer...
HOW does the amalgam process work?
I know it invovles mercury (although some don't..) and it basically
allows metal to be molded into your teeth.. essentially making the metal
act like epoxy resin (ie fluid for a "setting up" period, then hard
forevermore.).
Can someone please explain the theory about how it works?
Thanks,
Des
Brisbane
OZ