A metal amalgam is formed by mixing small pieces of solid metal
together with a liquid metal and allowing the mixture to harden.
Alloys are formed differently.
An alloy is formed by raising each of the constituent metals to be
alloyed to a temperature above its melting point, mixing the metal
constiuents thoroughly whilst they are all in their molten states, and
allowing the mixture to solidify by cooling at a controlled rate.
The two types of metallic material formed by these two different
processes are fundamentally different in their internal structure. To
illustrate this point the microstructure of the type of amalgam
commonly used in restorative dentistry can be seen at:
The molten metal component used in the formation of this material
(which would normally be mixed and hardened at room temperature) is of
course mercury. However, as can be seen from the diagram, a
significant proportion of the volume of the resulting material
consists of the "unreacted" cores of the solid metal pieces which do
not have any mercury content at all. And as a result, this material
has a level of inhomogeneity which is not displayed by true alloys.
So, because of its internal structure, an amalgam may not be
accurately described as "an alloy". And it is the inhomogeneous nature
of its internal structure which essentially defines it as an
The following describes a metal amalgam which is formed using gallium
as the liquid metal component rather than mercury:
"Hermetic Sealing of Electronic Hardware Using Gallium Amalgams
In this proposed invention, the gallium amalgam is mixed in a ball
mill, but the mixing action is minimized to limit the abrasion of the
metal particles so as not to have the liquid gallium wet the metal
particles. Since the metal particles are not wetted by the liquid
gallium, they will not dissolve in the liquid gallium. The pot life is
thus much extended. The liquid gallium, laden with metal particles, is
placed on the surfaces to be joined. The faying surfaces are pressed
together and then vibrated with respect to each other. The compressive
stresses and relative motion between the surfaces cause the metal
particles to abrade against each other and the faying surfaces. The
particles and the faying surfaces become clean and thus wettable by
the liquid gallium. The amalgamation process begins solidifying the
amalgam and joining the two surfaces. When the amalgamation process is
complete, the amalgam becomes solid and holds the faying surfaces
The amalgam can be a ternary alloy (sic) having 5-35% Cu and 15-30% Ni
in liquid gallium. The nickel and copper particle size has to be less
than about 0.05 mm so that the faying surfaces are not kept
excessively apart by the particles. The relative motion between the
faying surfaces to initiate the amalgamation action can be achieved
This amalgam contains no mercury at all.
However, it is still accurately described as an amalgam because of the
way in which it is formed, i.e. by mixing bits of solid metal with
liquid gallium at a temperature which is well below the melting points
of the solid metal components; and also because of its resulting
internal structure, which must display the same kind of material
inhomogeneity as displayed by mercury amalgams but not by alloys.
So, to mention just the two examples established here, there are
"mercury amalgams" where the liquid metal component used to effect the
amalgamation process is mercury, and there are "gallium amalgams"
where the corresponding liquid metal agent is gallium. (Gallium melts
at about 30 degree celsius - just above room temperature). Varying the
metals and proportions used for the solid components in the mix allows
for variation in the final physical properties of these two different
types of amalgam. However, neither type is accurately described as an
I sympathise with any material scientist reading this who might be
wondering, "So what's the big deal? This stuff is all pretty obvious."
Well, judging from previous experience I would suggest that some of
the facts stated above may be contradictory to what many dentists are
taught to believe in dental schools.
This is because it appears that some dental schools have attempted to
establish their own alternative definitions for the terminology used
in describing mixtures of metals and, in particular, amalgams.
My experience is that dentists are often inclined to insist that an
amalgam is defined simply as an "alloy" of any metal, or metals, with
mercury. As we have seen, this is not true. An amalgam doesn't have to
be made with mercury. Other liquid metals, such as liquid gallium, can
be used to effect an "amalgamation" process.
However, this misconception perpetrated by some dental schools has
given rise to a false deduction that amalgams are only distinguished
by variations in the solid metals mixed with the liquid mercury; so
for example mixing liquid mercury with copper produces a "copper
amalgam", or mixing it with silver produces a "silver amalgam", etc.
And a seemingly viable but nevertheless spurious consequence of this
logic is that anyone who uses the term "mercury amalgam" must be
implying that an amalgam can be formed by mixing liquid mercury with
liquid mercury - which of course it can't.
As a result, if a person happens to use the term "mercury amalgam" in
referring (quite logically) to that class of amalgams in which the
amalgamating agent is liquid mercury, then it is possible that they
may be met with a quite irrationally condescending or even abusive
response from a dentist who has failed to recognised that it is in
fact he (or she) himself who has been misled by a false rationale into
believing that mercury amalgams "don't exist".
Is there any evidence for this?
Yes there is - the following is part of a newsgroup response on this
topic from dentist Steven Bornfield:
"Mercury amalgam is a pleonasm, Keith. You do know what a pleonasm
is, dontcha Keith? It's like saying AC current."
(sci.med.dentistry 18 June 2005)
Well Steven, you were wrong.
The term "mercury amalgam" is not a pleonasm.
On its own the word "amalgam" is not sufficient to distinguish between
"mercury amalgams" and "gallium amalgams".
So I would like to propose that all dentists agree that the term
"mercury amalgam" is a perfectly correct and logical one to use for
describing that class of metal amalgam which utilises liquid mercury
as the amalgamating agent.
Any dissenters out there?
Keith P Walsh
- posted 14 years ago