Question on SEM-EDX methodology...

MrEureka · February 20, 2009 6:09AM

If a report shows a coin at 50% gold and 50% silver, does that mean that 50% of the coin is gold by weight, or is it 50% of the volume?

FYI, I'm trying to double check the lab results via specific gravity, and I don't think I can do that without an answer to the above question.

Longacre · February 20, 2009 6:15AM

I am certainly no scientist (though sometimes I play one on TV), but wouldn't 50% of the volume of a static weight be the same as 50% of the weight?

I look forward to the answer of someone who knows what they're talking about, though!

MrEureka · February 20, 2009 6:16AM

The specific gravity of each metal is as follows:

Silver 10.46

Gold 19.29

So what would the specific gravity of a coin be if the SEM-EDX report called it 50% gold and 50% copper?

Longacre · February 20, 2009 6:25AM

<< The specific gravity of each metal is as follows:

Silver 10.46

Gold 19.29

So what would the specific gravity of a coin be if the SEM-EDX report called it 50% gold and 50% copper? >>

You're out of my league now, man.

bumanchu · February 20, 2009 6:42AM

Simple solution: contact the people that made the analysis and ask them.

If they can't tell you, then you cannot rely on their report anyway.

adamlaneus · February 20, 2009 6:50AM

If they do not specify, then i'd guess it is a "mole percentage".

Also, this is a surface analysis, not a bulk analysis. Hopefully they weren't looking at a silver crystal within a gold coin whilst making this measurement.

coindeuce · February 20, 2009 8:43AM

<< The specific gravity of each metal is as follows:

Silver 10.46

Gold 19.29

So what would the specific gravity of a coin be if the SEM-EDX report called it 50% gold and 50% copper? >>

Could the SG of copper be more relevant to your above question?

PerryHall · February 20, 2009 8:50AM

It's by weight. A 12K coin is half gold by weight and half alloy by weight.

coindeuce · February 20, 2009 9:14AM

P.H - correct me if my physics are wrong. A 12K gold coin that is gold/silver alloy would consist of 1.84 cc of pure silver for every cc of pure gold.

TomB · February 20, 2009 9:57AM

If you have a spectrum with the analytical report then I would assume, without seeing the information, that the results are in weight percent.

zeebob · February 20, 2009 9:58AM

SEM/EDX Scanning Electron Microscope/Energy Dispersive Using X-Ray (Analysis)

What the heck does SEM/EDX have to do with specific gravity?

coindeuce · February 20, 2009 10:05AM

<< SEM/EDX Scanning Electron Microscope/Energy Dispersive Using X-Ray (Analysis)

What the heck does SEM/EDX have to do with specific gravity? >>

I believe it is possible to calculate the specific gravity of an alloy if the percentage by weight of each elemental component is known. Assuming that the SEM/EDX analysis is reported by weight percentage, the SG can be calculated and compared to an actual SG test of the coin.

adamlaneus · February 20, 2009 10:06AM

The X-Ray analysis will give you a number that indicates how much of each element is present. Unfortunately, those numbers, which in raw form are simply counts of xrays of particular energies, can be normalized for weight, volume, molarity. It's sort of important to know up front what the numbers represent.

Then, once you have an assumed ratio of one metal to another, you have an estimate of the alloy you have.

Each alloy has a particular specific density. I assume that the original poster wants to cross-check the measurements to assure the presence of a particular alloy.

jfoot13 · February 20, 2009 10:18AM

<< 

<< The specific gravity of each metal is as follows:

Silver 10.46

Gold 19.29

So what would the specific gravity of a coin be if the SEM-EDX report called it 50% gold and 50% copper? >>

You're out of my league now, man.

>>

next time you're globe trotting stay at a holiday inn express...

IGWT · February 20, 2009 10:44AM

Percentages of composition are almost certainly by mass. The relative proportions of mass -- not of space -- are those that matter most.

Longacre · February 20, 2009 11:07AM

<< Percentages of composition are almost certainly by mass. The relative proportions of mass -- not of space -- are those that matter most. >>

Are you sure you're a lawyer, or did you stay at a Holiday Inn last night?

IGWT · February 20, 2009 11:17AM

When was the last time anyone bought precious metal based on volume instead of weight (or mass, assuming we're only concerned about mass on earth)? I'll take a cubic centimeter of gold. No, no . . . make that a fluid ounce.

CaptHenway · February 20, 2009 3:21PM

<< It's by weight. A 12K coin is half gold by weight and half alloy by weight. >>

If you take one troy ounce of pure copper and one troy ounce of pure gold and melt them together, the resulting alloy is 50% gold by weight and roughly 30% gold by volume, since the ounce of copper takes up a greater volume than the ounce of gold.

Andy was asking if the SEM-EDX test would show it as 50% gold or 30% gold.

TD

Cam40 · February 20, 2009 4:01PM

just dont tell me pie are squared
theys round.anyone can see that!

BlindedByEgo · February 20, 2009 5:32PM

<< The specific gravity of each metal is as follows:

Silver 10.46

Gold 19.29

So what would the specific gravity of a coin be if the SEM-EDX report called it 50% gold and 50% copper? >>

I'm still stuck on whether the question refers to Gold-Silver or Gold-Copper.

Were there Goloid patterns struck on a 50-50 AU/AG?

GoldenEyeNumismatics · February 20, 2009 5:34PM

I should have paid more attention in chemistry class...

PerryHall · February 20, 2009 7:12PM

<< P.H - correct me if my physics are wrong. A 12K gold coin that is gold/silver alloy would consist of 1.84 cc of pure silver for every cc of pure gold. >>

Yup since gold is much more dense than silver.

sinin1 · February 20, 2009 8:17PM

I don't have my calculator handy but this link explains how ANDY

how to calculate specific gravity of alloys

You may need to contact a metalurgist to find out if the metal lattice/structure of a struck coin changes making the entire density higher
ex, is a solid poured gold bar the same density as a solid gold struck coin and does this relationship change with alloys

Sunnywood · February 20, 2009 8:45PM

Alloy constituents are always measured by mass. There is no ambiguity in this. The correct answer is mass. Normally it is given by percentage. Example: an alloy of 50% gold / 50% copper contains 50% gold by mass, and therefore 50% gold by weight. Volume is absolutely not relevant to this calculation.

Some other relevant terms:

density = mass per unit volume
specific gravity = ratio of density of a solid or liquid to that of water
karat = 1/24 purity by mass (usually applied to gold in an alloy)

weight = the gravitational force acting upon a mass
specific weight = weight per unit volume

For our purposes, unless you are going to the moon, or Mars, we are dealing with the earth's gravitational field, and therefore measurements and comparisons based on weight and mass are proportional and interchangeable. Specific gravity and specific weight are also related, but only on a macro level - not at the atomic level.

Thus our alloy of 50% gold and 50% copper contains 50% gold by mass, and by weight.
That makes it a "12 karat gold" alloy.
It's specific gravity may be calculated as .50 (s.g. of gold) + .50 (s.g. of copper).
It's density may be calculated as .50 (density of gold) + .50 (density of copper)

The specific weight of the alloy is its weight per unit volume, but that measure does not apply to its alloying constituents (gold and copper). It is meaningless to talk of the volumes occupied individually by the constituents within an alloy, due to the way the atoms are interspersed within the lattice structure of the alloy.

Gold alloys measured in "karats" are usually alloys of gold and copper. However, in commercial jewelry usage, one finds that the alloying elements include not only copper, but often additional components of tin, zinc, and silver. These impurities not only reduce the cost of production, but also the melting point.

In general, the density of solid metal cannot be increased under pressure, e.g. when striking a coin. The density of gold in a solid poured bar equals that in a struck coin. The same cannot be said of carbon, for example, which offers multiple possible lattice structures, and thus solid graphite may be densified into diamond under sufficient pressure.

I hope this clarifies the answer to Andy's question.

Best,
Sunnywood

(20+ years' background in a business based on metallurgical technologies)

BAJJERFAN · February 20, 2009 10:56PM

<< just dont tell me pie are squared
theys round.anyone can see that!

>>

yep, cornbread are square.

BAJJERFAN · February 20, 2009 11:33PM

<< If a report shows a coin at 50% gold and 50% silver, does that mean that 50% of the coin is gold by weight, or is it 50% of the volume?

FYI, I'm trying to double check the lab results via specific gravity, and I don't think I can do that without an answer to the above question. >>

It would normally be by weight as in 50:50 wt/wt. The specific gravity would have to be determined by experiment.

If both metals completely alloy at that composition the answer would be 13.597 grams per cubic centimeter. The value obtained by taking the average of the 2 would be 14.905 gm/cc but mixing exactly one volume of A with exactly one volume of B does not always produce exactly two volumes of AB; in fact its probably very rare or dumb luck if that ever happens.

BAJJERFAN · February 20, 2009 11:37PM

<< The specific gravity of each metal is as follows:

Silver 10.46

Gold 19.29

So what would the specific gravity of a coin be if the SEM-EDX report called it 50% gold and 50% copper? >>

13.597 grams per cubic centimeter.

BAJJERFAN · February 20, 2009 11:40PM

<< P.H - correct me if my physics are wrong. A 12K gold coin that is gold/silver alloy would consist of 1.84 cc of pure silver for every cc of pure gold. >>

That is true, but it doesn't answer the question.

jgrinz · February 20, 2009 11:44PM

<< Alloy constituents are always measured by mass. There is no ambiguity in this. The correct answer is mass. Normally it is given by percentage. Example: an alloy of 50% gold / 50% copper contains 50% gold by mass, and therefore 50% gold by weight. Volume is absolutely not relevant to this calculation.

Some other relevant terms:

density = mass per unit volume
specific gravity = ratio of density of a solid or liquid to that of water
karat = 1/24 purity by mass (usually applied to gold in an alloy)

weight = the gravitational force acting upon a mass
specific weight = weight per unit volume

For our purposes, unless you are going to the moon, or Mars, we are dealing with the earth's gravitational field, and therefore measurements and comparisons based on weight and mass are proportional and interchangeable. Specific gravity and specific weight are also related, but only on a macro level - not at the atomic level.

Thus our alloy of 50% gold and 50% copper contains 50% gold by mass, and by weight.
That makes it a "12 karat gold" alloy.
It's specific gravity may be calculated as .50 (s.g. of gold) + .50 (s.g. of copper).
It's density may be calculated as .50 (density of gold) + .50 (density of copper)

The specific weight of the alloy is its weight per unit volume, but that measure does not apply to its alloying constituents (gold and copper). It is meaningless to talk of the volumes occupied individually by the constituents within an alloy, due to the way the atoms are interspersed within the lattice structure of the alloy.

Gold alloys measured in "karats" are usually alloys of gold and copper. However, in commercial jewelry usage, one finds that the alloying elements include not only copper, but often additional components of tin, zinc, and silver. These impurities not only reduce the cost of production, but also the melting point.

In general, the density of solid metal cannot be increased under pressure, e.g. when striking a coin. The density of gold in a solid poured bar equals that in a struck coin. The same cannot be said of carbon, for example, which offers multiple possible lattice structures, and thus solid graphite may be densified into diamond under sufficient pressure.

I hope this clarifies the answer to Andy's question.

Best,
Sunnywood

(20+ years' background in a business based on metallurgical technologies) >>

Sounds like the expert has spoken ... Impressive read that actually makes sense to a lay person
Good Job

CaptHenway · February 20, 2009 11:44PM

<< The specific gravity of each metal is as follows:

Silver 10.46

Gold 19.29

So what would the specific gravity of a coin be if the SEM-EDX report called it 50% gold and 50% copper? >>

Are you asking for 50/50 gold/silver or 50/50 gold/copper?

TD

BAJJERFAN · February 20, 2009 11:45PM

<< SEM/EDX Scanning Electron Microscope/Energy Dispersive Using X-Ray (Analysis)

What the heck does SEM/EDX have to do with specific gravity? >>

A lot if you think about it. It tells you what the percentage is and for that percentage there should be a unique specific gravity. While you may not have a SEM-EDX device in your garage or basement even your local high school chem lab should have a balance and a graduated cylinder good enough so that you could determine specific gravity for yourself.

BAJJERFAN · February 20, 2009 11:49PM

<< I don't have my calculator handy but this link explains how ANDY

how to calculate specific gravity of alloys

You may need to contact a metalurgist to find out if the metal lattice/structure of a struck coin changes making the entire density higher
ex, is a solid poured gold bar the same density as a solid gold struck coin and does this relationship change with alloys >>

Yup; same thing I found.

CaptHenway · February 20, 2009 11:52PM

<< 

Thus our alloy of 50% gold and 50% copper contains 50% gold by mass, and by weight.
That makes it a "12 karat gold" alloy.
It's specific gravity may be calculated as .50 (s.g. of gold) + .50 (s.g. of copper).
It's density may be calculated as .50 (density of gold) + .50 (density of copper)

>>

FWIW, years ago at Collectors Clearinghouse, Ed Fleischmann taught me how to do specific gravities, and mentioned that for some reason the s.g.'s of gold/copper alloys were always off by a few percent from the (% of gold x density of gold) + (% of copper x density of copper) formula. In 35 years of observation I have confirmed that this is correct. The difference is small, but there.
TD

BAJJERFAN · February 20, 2009 11:53PM

Its always weight weight unless specified otherwise. Mostly because its easier to exactly weigh 100 pounds of gold than it would be to measure exactly 50 gallons of the stuff.

If I asked some to get me exactly 4.362 gallons of water you would probably weigh it out rather than try to measure it any other way/weigh

BAJJERFAN · February 20, 2009 11:58PM

After reading some of the responses, heavy metals aren't the only things that are dense around here.

Sunnywood · February 21, 2009 12:05AM

Bajj & Capt. Henway, you guys are going to force me to be even more technical !!!

Capt. Henway, most likely the reason you are seeing a measurable difference is that you may have been working with commercial or jewelry-grade alloys of gold and copper. As I noted above, these are not pure binary (i.e. two-part) alloys, because they often have non-trivial amounts of tin and zinc present, which are metals of lower density. Therefore, the specific gravity of jewelry-grade 12K gold, for example, is quite measurably different from that of pure binary 50% gold - 50% copper alloy.

Bajj & Capt. Henway, there is also another deviation from the simple linear relationship that I described, which depends upon the alloying proportions. For every binary alloy (i.e. one with two elemental metals as components), there is a "perfect" proportion that results in a "eutectic" alloy. The eutectic proportion is different for each binary system. For example, in the silver-copper system, the eutectic alloy is approx. 72% silver - 28% copper. In the gold-copper system, it is approx. 80% gold - 20% copper. A eutectic (literally "well woven" or "well blended") alloy behaves like a pure metal; it has a melting point rather than a melting range, and it has a perfectly homogeneous lattice structure throughout. As you approach the eutectic point in the proportional composition of the alloy, there is a slight densification that occurs, leading to a slight correction from the simple linear formula that I gave.

Incidentally, alloy percentages are always given by weight, but it is also possible to calculate atomic percentages. In the 80% Au / 20% Cu eutectic, for example, the atomic percentage of gold is approx. 57%. But again, this has nothing to do with volume.

Best,
Sunnywood

BAJJERFAN · February 21, 2009 12:14AM

Well I'm assuming its probably been done long ago but one could make various alloys of gold and silver, experimentally determine the specific gravity of each and graphically plot sp gr vesus percentage of one component. If you get a straight line, then the composition of of any unknown Au/Ag alloy could be determined from sp gr alone.

Question for Sunnywood; do gold and silver alloy in all proportions?

RKKay · February 21, 2009 12:22AM

<< If a report shows a coin at 50% gold and 50% silver, does that mean that 50% of the coin is gold by weight, or is it 50% of the volume?

FYI, I'm trying to double check the lab results via specific gravity, and I don't think I can do that without an answer to the above question. >>

It's by weight per S&N.

Sunnywood · February 21, 2009 12:26AM

Bajj, great question. "Alloying" is generally possible in most two-part systems throughout the range of percentages. But there are different kinds of "alloys." For example, the gold-silver system does not have a eutectic point. The alloys of gold and silver are technically called "solid solutions" i.e. blends rather than true alloys. For this reason, gold and silver are much more commonly alloyed with copper, rather than with each other.

You are also quite right that one could plot specific gravity vs. percentage for every binary alloy system. In general it will adhere rather closely to the simple linear formula that I gave earlier: s.g. (alloy x% A and y%

= x*(s.g. A) + y*(s.g.

. However, there will be deviations based on the peculiarities of each binary system. For example, the systems with eutectic points will have a measurable deviation from the linear as you approach the eutectic point.

OK, I'm going out for the day ... hope I was of some help.

Best,
Sunnywood

tacynnep · February 21, 2009 1:32AM

<< The specific gravity of each metal is as follows:

Silver 10.46

Gold 19.29

So what would the specific gravity of a coin be if the SEM-EDX report called it 50% gold and 50% copper? >>

Let me try this..

Density= Mass/ Volume

If a Volume/Mass weight: Gold 19.29 (19.32) Silver 10.46 (10.49) Copper(8.96)

Example-1: Compare the 1Oz. bullion coin the silver coin is much bigger (Volume) then the gold because the difference in Density.

Example-2: If you have a 1 kg. Silver coin composition .925 silver = 925gm / silver. 75gm / alloy = 1 kg

The answer : Weight.

CaptHenway · February 21, 2009 1:35AM

<< Bajj & Capt. Henway, you guys are going to force me to be even more technical !!!

Capt. Henway, most likely the reason you are seeing a measurable difference is that you may have been working with commercial or jewelry-grade alloys of gold and copper. As I noted above, these are not pure binary (i.e. two-part) alloys, because they often have non-trivial amounts of tin and zinc present, which are metals of lower density. Therefore, the specific gravity of jewelry-grade 12K gold, for example, is quite measurably different from that of pure binary 50% gold - 50% copper alloy.

>>

No, I'm talking major country large gold coins, such as U.S. $20's or Mexican 50 pesos.
Jewelry is always a crapshoot.
TD

MrEureka · February 21, 2009 2:00AM

The responses here are fantastic. Thanks, guys!

One more thing: How good is SEM-EDX at measuring the overall composition of clad and/or plated coins? Does the test measure deep into the coin, or is it just the surface composition that is being measured?

BAJJERFAN · February 21, 2009 3:45AM

<< The responses here are fantastic. Thanks, guys!

One more thing: How good is SEM-EDX at measuring the overall composition of clad and/or plated coins? Does the test measure deep into the coin, or is it just the surface composition that is being measured? >>

I'd guess that it would do better on a uniform sample than on a non-uniform one, so if its just a common clad piece a modern crap, your best bet would be to melt it into a homogeneous pool of metal first.

MrEureka · February 21, 2009 3:51AM

if its just a common clad piece a modern crap, your best bet would be to melt it into a homogeneous pool of metal first.

It would be a good start...

MrObvious · February 21, 2009 4:29AM

Andy -

The answer to your question is that this methodology only evaluates the surface of the coin. How deeply in penetrates, exactly, depends on the energy used. I am not an expert at this, but have hired experts to do this sort of analysis in the past. Presuming the planchet has a uniform composition throughout, the technique should be quite accurate in measuring the percentages of the various elements. If you expect some variation in composition with depth then you should talk to those doing the analysis and try to get an estimate of the power used and how that translates into depth of material analyzed.

My use of this has been in analyzing coatings and specifically the completeness and uniformity of coatings. As such, we tried to analyze only the surface. By setting the power low we could adjust the EDX so that if the substrate under the coating showed up in the analysis, then the coating process had failed.

adamlaneus · February 21, 2009 4:42AM

Shallow surface measurement, not a bulk analysis.

I have some experience writing software for XRay analysis. Unfortunately, that means I have little practical knowledge, but a zillion bizarre details.

RWB · February 21, 2009 8:28AM

This was used a few years ago to test the Smithsonian’s MCMVII EHR and HR $20s. The result was that the EHR pieces were nearly pure gold on the surface, but HR pieces had only slightly elevated surface gold content.

BAJJERFAN · February 21, 2009 8:32AM

<< This was used a few years ago to test the Smithsonian’s MCMVII EHR and HR $20s. The result was that the EHR pieces were nearly pure gold on the surface, but HR pieces had only slightly elevated surface gold content. >>

That would mean then that one of the metals flowed to the surface faster under the striking pressure used.

RWB · February 21, 2009 8:49AM

The EHR pieces were struck 7 times. Between each blow, they were heated then quenched in a weak nitric acid bath. This leached the copper from the surface leaving nearly pure gold. The HR pieces were struck 3 times and only had 2 annealing cycles – evidently not sufficient to significantly deplete copper on the surface.

BAJJERFAN · February 21, 2009 8:51AM

These days they'd just wise up and use pure gold.

perfectstrike · February 21, 2009 3:49PM

Sunnywood, thanks for all the details. Bajjerfan, thanks for pushing the envelope for knowledge. Us poor physics teachers only know enough to blow holes in ceiling tiles, splatter colored water on students with leaf blowers and boil water in test tubes held in our bare uncovered hands

David

Question on SEM-EDX methodology...

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