XRF or fire assay - does it matter?

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fourninesAU

Member
Joined
Jul 24, 2011
Messages
14
Hello,

I am not a refiner, just a scrap gold buyer, so please forgive my ignorance. Thanks for any replies. :)

I have been using Republic Metals Corp in Miami and been very pleased with their service but want to give NTR Metals a try. I spoke with a rep there who told me that they will melt my material then do XRF analysis.

I do not understand how this is different than what Republic does, where they melt the lot then x-ray the sample to determine the fineness and pay me based on that. Yet he described their process as fire assay, whereas NTR is not doing a fire assay (yet they are still melting the lot into a single bar, then x-raying). I don't understand the difference, and the implications of XRF vs fire assay (which I have heard is more accurate).

I am bringing in a small test lot of karated gold which should melt out to around 10ozt fine. I developed a spreadsheet that calculates average fineness after I put in the weights of everything 9K-24K, then determines plumb yield (which I know jewelry is almost never plumb). So far my results with Republic have been that my lots have melted to around 2% less than plumb. Could I expect similar results with XRF?

Thanks again.
 
If you are now getting 2% (1/2K) under plumb, and the refining charges are comparable, I would probably stick with your present refiner. Although you never know, 1/2K under is a realistic number, based on my experience. Also, I feel that fire assay is more accurate than XRF. However, no matter what the numbers and the charges are, the bottom line is all important. You may get a better bottom line from someone else but, you never know until you try it, which is the scary part.
 
Thanks. Actually my yield with Republic is typically less than 1k under plumb. Typically it's about 2% or 1/2k.

I'm just trying to understand the difference in Republic's process (melt the lot into a bar then x-ray the sample) versus NTR's (melt into a bar then XRF?).


Also, neither refiner charges any assay fees. Republic is currently paying 98.5% of yield with no silver recovery, NTR is 98% with no silver but they agreed to do 98.5% if I brought in 1000dwt of scrap.
 
There is a world of difference between XRF and Fire assay.

In a nutshell, XRF is a non-destructive way of determining the concentration of precious metals in your sample. It uses X-ray radiation to excite the atoms in the sample, and then a detector and computer to analyze the energy released by those atoms. This can be done in about 2 minutes.

A Fire assay actually involves refining a small sample of the material to determine the precious metals content. This process takes several hours to complete.

The standard in the refining world is to use a Fire assay - and has been that way forever. There is almost nothing as accurate as a fire assay for regular karat gold material.

Let me ask you this, in your spreadsheet, how much do you take into account for under karat material?

Do you go to Republic's facility and watch the melt and take a sample? Are you planning on doing that with NTR?

The advice you will hear on the forum is for you to physically watch the melt, take a sample, and have a third party fire assay done. That is the only way you will know if the XRF results are accurate.
 
Thanks. When I go to Republic, I do typically watch the melt and x-ray of the sample. But the melt only takes a few minutes (I'm in and out with payment in < 1 hour). So I guess they were not actually doing a fire assay all this time, and the process is actually the same as NTR?! I was under the impression that I was getting a fire assay.

I've never questioned the process because my results have been so good.



As for my spreadsheet, it calculates average fineness and plumb yield based on the weights of each karat that I input. So the expected yield assumes that all my 10K is exactly .417, my 14K is exactly .585, etc. This is just a reference point, as I know that in the real world very little jewelry is plumb, plus I will have solder/steel clasp springs/etc.

I then enter the actual yield (weight of the final bar multiplied by the fineness) and it divides the two, giving me the difference in percentage between plumb and actual. This is the difference I am referring to that is staying within 2%. This is with me removing virtually all stone weight before melt.
 
With regard to the accuracy of XRF vs fire assay, is the difference large enough that it could matter over a small sample size (like one of my typical lots which would melt down to about 15ozt fine gold), or is it negligible in such sizes?
 
If you want, insist on a fire assay and read up on the process here. It's basically how they do it. Fire assay is still the "gold" standard insofar as gold assaying goes, in fact, 'tis the ASTM method of choice for karat gold.

Republic is a good company.

Bear in mind, that XRF is only as good as its calibration standards and who's running the instrument. Republic uses a MUCH MUCH nicer XRF than NTR to buy with, given that they use a top of the line instrument.
Accuracy is what the operator makes it with XRF....much easier to skew that than a fire assay.
 
Would it be helpful to bring a 24K gold coin and make them show me an XRF result before letting them proceed with the jewelry?


Also, over a small sample size (like 20ozt of karated gold), could operator error make anything other than a negligible difference?
 
fourninesAU said:
Would it be helpful to bring a 24K gold coin and make them show me an XRF result before letting them proceed with the jewelry?


Also, over a small sample size (like 20ozt of karated gold), could operator error make anything other than a negligible difference?

For various technical reasons that are above my understanding, the XRF curve can very accurate with high purity gold, but still be off with mid-range material between 30-70% gold.

You should actually get a 14k standard and have them test that first. Of course, you don't tell them what it is before the test - then you will know how accurate their machine is.

Assume that the XRF is off by 1/2 %. On 20 troy oz of karat that is about 0.1 troy oz of fine gold, or about $160. Some XRF machines I've seen have been off between 1 and 2%. Take a sample, pay for a fire assay - it shouldn't cost you more than $40.

Is $160 negligible to you? It all depends on your perspective I suppose.

I worked for Republic for a while, and have known the owner and many of the people there for years. I agree with everything Lou said.

I don't know NTR aside from what others in the industry have told me....and I've heard a lot.

If you have a system that works, and you are happy with the results, don't change anything.
 
Lou:

"Republic uses a MUCH MUCH nicer XRF than NTR to buy with"

Do you know what machine Republic uses and what machine NTR uses?

Thanks,

Mike
 
No, but I'm presuming it's like what Metalor uses or Heraeus--I'll call up Republic and ask tomorrow.
 
I am a construction inspector. There are several ways to test the density of soil to insure the foundation is stable. One way is to use a density gauge that uses gamma and neutron radiation. If the gauge is not calibrated correctly there are gross errors in the readings. Just saying that "sloppy" quality control procedures can effect results...
 
Not to many months ago, I started on GRF with a very similar type question. I am on my third XRF machine with my fourth one coming soon. I keep buying and upgrading to have the latest and greatest plus adding an alloy package to machine for other metals. One of your first problems is the homologous melt as gold is going to stratify into separate levels. I have taken several 600 grams samples, multi melted in an induction melter, and cut them in quarters with a band saw. I have then run XRF readings on each of the sections to get a good reading of the 600 grams. Yes, I have run pin samples, drillings, end mill cuttings, drilled holes all the way thru, and tried the different methods of cold ingots, hot ingots, letting it sit in the ingot, and dropping it in pickle when the metal is still moving. Bottom line is gold is not an even homogenous melted ingot and the readings can vary an easy 2% purity depending on were you take a reading on the bar. In watching several different "Refiners" melt and pull pin samples tend to give the lowest possible readings. I cash out 2-3 times every week. My payout vary to dead on to about $100 difference on $10-12,000 average payouts. Sometimes it's a little more and sometimes it's a little less but overall it's very close to what I figure it. Different machines are going to give you slight differences. Not all NTR's have the same XRF machine. NTR takes a 90-120 second NTR report usually on one sample of a pin. I take NTR a clean triangle shaped ingot were I have taken 6 XRF readings and I average them. That's my number and I must say, I get it 90% of the time. If I have a big difference, I take it and remelt it and retest. I double check the SG against the XRF. Got to say, I am not always 100% right and have to reboot and double check my XRF machine before. Overall my system works and I got a few smaller friends who send me their metal to cash them out.

I must say that I find that my local NTR guys are honest people. However, having a better XRF and taking the time to test and calculate the amount before I cash out is the key. Like Ronal Reagen said: Trust but verify!

Dan Dement
 
This is based on what Matt (Fournines) showed me first hand at the refininery. I had asked Matt to XRF a necklace I had so that I could get an idea of the metals it contained. I wanted to make a matching pendant for the chain and needed to know the composition of the chain to do so. The chain was hallmarked 10k. We took 3 readings and all three readings were different. If I remember correctly one was slightly under 10k one was almost dead on and the other was slightly above 10k. So in the simplest terms, karated gold is best fire assayed. But since fire assay can only take you so far XRF is best for already fine gold.
Edited
 
If you think the percentage of gold returned to you on XRF based settlements are always consistent and accurate, reconsider. XRF can get you well into the ballpark for gold and silver percentages however, the percentages reported are completely suspect to inaccuracies. If you are refining a large gold or silver lot ballpark isn’t good enough, these inaccuracies can add up to cash going into the refiners pocket instead of yours.

An XRF analysis can be performed in minutes by anyone who can pull the trigger on the XRF unit and read numbers on a screen. Imagine a fast food worker testing your precious metals, even less skill is required for basic operation of a handheld XRF unit. This is a case where faster is not better. An XRF unit like most electronics today can be hacked or tweaked so that the calibration favors the business and not the customer. Accidentally or by design a 1 to 5 percent error can make a big difference on your “so called” 98% final settlement.

Many gold buyers and precious metal blogs will attempt to convince you that an XRF analysis is just as accurate as a fire assay, biased manufacturers of XRF units will also make this case. Inquiring minds who take time to research this thoroughly will find it isn’t true. For gold buyers, it’s in their best interest to tell you this because a fire assay requires overhead they can’t afford. A fire assay takes hours to complete, requires a lab, requires specialized scales, requires an assay furnace and requires a highly trained professional to perform.

Ever wonder why so many gold buying businesses were popping up during the last extended bull market for precious metals? In three letters the answer is XRF. This technology provided a way for people with little to no experience in purchasing precious metals to quickly enter into the business. It also allowed larger refineries such a NTR to setup remote buying locations with very little overhead. For testing scrap jewelry, coins and other forms of precious metals, XRF is a great way to weed out the fakes or anything that doesn’t seem legit. It’s also a great way of testing without damaging the sample. However for testing a melted homogeneous bar, it will never be as accurate or consistent as a fire assay.

At our refinery we utilize both a handheld and a more powerful tabletop XRF unit however, we do not use these for settlements on refining lots. We perform a fire assay on all refining lots. Fire Assay is the gold standard for professionals in the precious metals industry, for hundreds of years this method has been trusted for determining precious metal content. Our history and integrity as professionals in the Jewelry/Precious Metals industry demand that we provide this level of accuracy for our customers.

The moral of this story… Not all settlement percentages are equal. When you consider the pros and cons for each method of determining precious metal content, a fire assay performed by a reputable refinery is clearly the most accurate and consistent way to ensure you are getting the full value for your precious metal scrap.
 
Depends on what you're doing. If you're buying, an XRF is fine. When selling, I want fire assay.

If buying karat gold, there's nothing wrong with a touchstone.When you get good at it, it's faster than an XRF. It will zero you in on karat gold well within the accuracy you need for profitable buying. Sure cheaper than XRF.
 
I get quite a lot of alloy in that some one has used an XRF to find value.
normally it is chasing Tungsten and is a complete waste of time and acid.
I would not pay out on the word of an XRF. If some one found a particular alloy that confused a certain set it would leave people using that model open to abuse until they worked out the vulnerability.
It would be a heart stopper seeing the low returns that week.
 
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