A Lesson On Amalgams

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:
http://book.boot.users.btopenworld.com/setting.htm
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 "amalgam".
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 together.
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 by..."
See:
http://www.priorartdatabase.com/IPCOM/000137544 /
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 alloy.
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
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wrote:

SNIP
Keith:
Since you cross-posted your drivel to sci.materials I feel obligated to disagree and reply briefly to your definitions as being metallurgical nonsense.
The 1948 edition of the Metals Handbook published by the American Society for Metals defines an alloy simply as: "a substance having metallic properties and being composed of two or more chemical elements of which at least one is a metal".
The Metals Handbook, Desk Edition (2nd, 1998) has the same definition but also adds a second one of "to make or melt an alloy.
The 1948 edition of the Metals Handbook, defines an amalgam simply as: "a mercury-base alloy". The Metals Handbook, Desk Edition (2nd, 1998) provides a more precise definition of an amalgam as: "a dental alloy produced by combining mercury with alloy particles of silver, tin, copper, and sometimes zinc".
An amalgam is an alloy, and an amalgam implies mercury.
Pittsburgh Pete
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On 26 Aug, 04:09, snipped-for-privacy@hotmail.com wrote:

"Amalgam" has only come to imply "mercury" through familiarity with mercury amalgams.
But to invoke from this the "law" that amalgams are only formed with mercury as the amalgamating agent is false.
The citation at:
http://www.priorartdatabase.com/IPCOM/000137544 /
- describes how liquid gallium may be used to amalgamate solid pieces of copper and nickel. The author correctly describes the resulting solid as an "amalgam". And there is no mercury in it.
And if you go again to:
http://book.boot.users.btopenworld.com/setting.htm
- you will see the microstructure of a typical dental amalgam where liquid mercury has been used to amalgamate bits of a solid metal alloy.
Much of the volume of the amalgam is made up of the cores of the original solid alloy pieces which are have not mixed with any mercury at all. These are held together by a solid matrix which has been newly formed by the mixing of the liquid mercury with only the outer surfaces of the original solid alloy pieces.
Clearly it is at best misleading (and more likely just plain wrong) to call this material "an alloy". It might be more accurately described as a mixture of alloys, but a mixture which displays a much greater degree of material inhomogeneity than any true alloy.
This explains the essential difference between an alloy and an amalgam.
Any more dissenters out there?
Keith P Walsh
PS, If you go to:
http://www.elektrotechnik.hs-magdeburg.de/Mitarbeiter/hinken/news/N6.htm
- you will see a graphical representation of the thermoelectric current and associated electromagnetic field which are generated whenever an element of one electrical conductor is completely encased within another and subjected to a thermal gradient (figure d is particularly instructive).
As we have seen, a typical dental amalgam may be accurately described as an inhomogeneous mixture of dissimilar metals in which a great many elements of one electrically conductive material are all completely enclosed within a matrix of a dissimilar electrically conductive material.
One might expect therefore that when a metal amalgam dental filling is subjected to a temperature gradient thermoelectric eddy currents would also circulate around the inclusions of dissimilar metals within the material, and that the associated magnetic fields would be generated.
You wouldn't expect the same behavior from an alloy.
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wrote:

Keith:
It is well known that liquid gallium can be used to produce mediocre dental alloys. Three years ago I replied to a post on sci.materials inquiring " Can Gallium form amalgam-type alloys?"
I replied:
Try searching Google under gallium amalgam (actually gallium AND amalgam). You will find this four- year old British Dental Journal article [S. M. Dunne and R. Abraham, "Dental post-operative sensitivity associated with a gallium based restorative material", British Dental Journal, Vol. 189, No. 6, (September 23, 2000)] about some commercial dental products: http://www.nature.com/bdj/journal/v189/n6/pdf/4800755a.pdf
Note that in the references to this article the gallium restorative materials are referred to as gallium alloys, NOT as gallium amalgams
Pittsburgh Pete
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On Aug 26, 9:02 pm, snipped-for-privacy@hotmail.com wrote:

Pittsburgh Pete,
I am already aware that attempts have been made to formulate dental amalgams using gallium as the liquid metal amalgamating agent rather than mercury.
I am also aware that the resulting materials are being referred to within the dental profession as gallium "alloys".
However, it seems likely that the adoption of the term "alloy", rather than "amalgam", to describe the material in which liquid gallium is the amalgamating agent arises from the assertion also prevalent within the dental profession that the definition of an amalgam is "an alloy of mercury with one or more other metals".
And it remains my contention that this definition of an amalgam is inaccurate on two counts. Firstly that owing to the process by which it is formed and the resulting degree of inhomogeneity in its internal structure, an amalgam may not be accurately described as an alloy. And secondly, it is possible to form an amalgam without any mercury in it - i.e. by using some other liquid metal as the amalgamating agent (such as gallium).
I believe that the proof of this contention lies in the veracity of the following statement; particularly with regard to the use of the word amalgam, which corresponds most accurately with its true meaning in the context of mixtures of metals:
"There are mercury amalgams in which the liquid metal used to amalgamate solid pieces of other metals is mercury, and there are gallium amalgams in which the liquid metal used to amalgamate solid pieces of other metals is gallium."
And in fact I would argue that any mixture of metals formed by mixing only one liquid metal with other metals in their solid state would be a more accurate definition of an amalgam - because that's what you'd get, an amalgam, not an alloy. Alloys are formed by raising ALL of the constituent metals to a temperature above their melting points on being mixed.
I keep asking if there are any more dissenters out there.
So far I've had just you and the foul-mouthed Jim Beam.
Are there any endorsers?
By the way, if you go to:
http://www.elektrotechnik.hs-magdeburg.de/Mitarbeiter/hinken/news/N6.htm
- you will see a graphical representation of the thermoelectric current and associated electromagnetic field which are generated whenever an element of one lectrical conductor is completely encased within another and subjected to a thermal gradient (figure d is particularly instructive).
As we have seen, a typical dental amalgam may be accurately described as an inhomogeneous mixture of dissimilar metals in which a great many elements of one electrically conductive material are all completely enclosed within a matrix of another, dissimilar, electrically conductive material.
One might expect therefore that when a metal amalgam dental filling is subjected to a temperature gradient thermoelectric eddy currents would also circulate around the inclusions of dissimilar metals within the material, and that the associated electromagnetic fields would be generated.
One might also expect that mercury amalgams and gallium amalgams would share this type of thermoelectric behavior.
But not alloys.
Keith P Walsh
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They are not "unreacted" (dereactu), but nonreacted.
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The term "unreacted alloy" is taken directly from Richard van Noort's book.
He's a Senior Lecturer in the Department of Restorative Dentistry at Sheffield University in the UK.
I wouldn't say that this necessarily makes him infallible in his usage of words, but if you think he's made a mistake then maybe you should tell him. (Say hello from me.)
In his defence I'd guess that Richard doesn't intend to imply that the "unreacted" pieces of alloy have at some stage "reacted", and then by some mysterious process "unreacted" themselves. That would be silly (in a similar way to which the habit of inferring that anyone who uses the term "mercury amalgam" must believe that an amalgam can be formed by mixing liquid mercury with liquid mercury is also silly).
I'm sure he just means that at no time during the amalgamation process, or afterwards, does any mercury act upon, join with, or invade this portion of the silver-tin alloy.
Which reminds me, I still think that it is inaccurate to call this type of amalgam an alloy (in spite of what Pittsburgh Pete says), and I will explain my argument again in a little more detail.
Richard van Noort's book shows a cross-section of the microstructure of a typical dental amalgam (one made with mercury as the liquid metal amalgamating agent).
See:
http://book.boot.users.btopenworld.com/setting.htm
If you zoom in on just one of the unreacted alloy pieces, or "cores of gamma", as described in the text then what you are looking at is the representation of a volume of material which is "an alloy". At the scale of this diagram it would not be possible to show the relative arrangements of the atoms of silver and tin which make up this "alloy". This is because it has been mixed in the alloying process to a high degree of material homogeneity. It has no mercury in it.
If you then zoom back out you can appreciate that the amalgam is made up of a large number of these "bits" of "an alloy" all held together in a solid matrix which has a significantly different material composition (there's lots of mercury in it). So it doesn't make sense to describe this material as "an alloy". It is "bits of an alloy" held together in a dissimilar solid metal matrix. (And even the matrix itself has dissimilar components which give it degree of material inhomogeneity much greater than that of the "unreacted" silver-tin alloy.)
So it isn't "an alloy". It's much more accurate to call it an inhomogeneous mixture of different mixtures of metals. An amalgam.
Point labored.
I think that what we actually call these materials is of secondary importance to our ability to distinguish them by their physical properties and physical behaviors.
Twice now in this thread I have argued that one should expect the thermoelectric properties of what I call amalgams (inhomogeneous mixtures of dissimilar metals formed by binding together bits of solid metal with a liquid metal amalgamating agent) to be significantly different from those of what I call true alloys, because of their greater degree of inhomogeneity.
So far no one has put up any argument against this point. (Not even Pittsburgh Pete.)
I believe that in the absence of any experimental evidence to the contrary, it may be proposed that because of their greater inhomogeneity amalgams should be expected to generate greater thermoelectric potentials than less inhomogeneous alloys. And if this is correct then it may also be the case that an amalgam is more able to dissipate electrical energy to its surroundings as a result of its thermoelectric behavior than an alloy.
One thing's for certain. The only way to find out is by experiment.
Does anyone know if experimental procedures to measure the thermoelectric properties of a typical dental amalgam have ever been carried out?
Keith P Walsh
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The 15 I've checked with state that they aren't aware of anyone who has carried this out. Since we have a holiday weekend coming up maybe I'll have time to check with the remaining 6.6 billion people left to ask. Patience man, patience.
carabelli
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Know for a fact that I haven't...
    So that's one less to check with.
Perhaps this will save you some time.
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Keith P Walsh wrote: <snip>

Keith, Have you ever heard the expression "An hour in the library is worth a month in the laboratory"? It's usually sound advice for anyone who wants to find the answer to a question. But, you have been researching this subject for over ten years! Its time for you leave the "library" and get into the lab.
So, borrow some equipment, purchase some amalgam material, fabricate some samples, and run some tests!
We look forward to seeing your results.
--

Paul D Oosterhout
I work for SAIC (but I don't speak for SAIC)
  Click to see the full signature.
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The [descriptive] dictionary definitions don't reflect the word's meaning <http://dictionary.com/browse/amalgam but its use. There is no "mercury" or "hudrargurs" in "amalgam".
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