# assay



## PRECIOUS METALS (Apr 22, 2007)

what would be the step to do your own assay on gold


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## darkelf2x1 (Apr 24, 2007)

check out fire assaying


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## AgAuPtRh (Apr 24, 2007)

That's a loaded question there PM.

Could you maybe be a little more clear as to what you want to accomplish?

Are you looking to test your already refined gold to check for karat values? or -- Are you interested in the amount of gold in a certain material you have?

In a general way -- an assay is a test -- used by miners and mining companies to get an estimate of the amount of gold per ton of material. Like quarts rock, clay, black sand deposits etc. There are basically 2 kinds of assay methods -- the wet and the dry. Otherwise known as Chemical assay and Fire assay. 

You can use an assay to figure how much gold per ton -- or how many grams per pound as in electronic scrap. 

If you are looking to test your gold for how pure it is there are some really simple tests talked about in other places in the forum here. Test needles and test acids to find out how pure your refined material is.


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## PRECIOUS METALS (Apr 24, 2007)

im looking to test my already refined gold to check for karat values


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## Noxx (Apr 24, 2007)

Well, acid tests could work. Or Mizar tester if you have more budjet.


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## goldsilverpro (Apr 24, 2007)

PM,

When your gold gets purer and purer, it gets more difficult to get meaningful fire assay numbers. On 99.99% gold, fire assay completely fails. Even though, according to a respectable source on the internet, fire assay is still the most accurate direct method (within .02%), on the planet, for assaying gold. 

For pure gold, the best method is an indirect one. You send the sample to a lab that determines, not the gold itself but, all the percentages of the impurities. These are added up and subtracted from 100% to get the gold percentage. This is expensive and the results are slow in coming.

If your gold is not very pure, fire assay would work but, it would cost around $2000 for a semi-decent setup. This seems a lot but, any other halfway accurate method would probably start at $50,000. The touchstone is cheap but, at the hands of an expert, is only accurate to about 1% - 2%.

The cheapest way is to simply look at your gold button or bar. Starting at what seems to be about 99.9%, squarish crystals start forming on the top surface, along with a squarish hole, usually in the center, which is called a "sink" or a "pipe". This is where gases are expelled when the gold solidifies. As the gold gets purer, the crystals become more defined and visible and the "sink" gets deeper. At very high purity, the crystals seem to crack and separate, a bit, near the sink

The crystal appearance I just discussed is somewhat subjective and I've never seen anything else on it on the web, other than what Harold and I have said. However, any refiner that is capable of producing high purity gold or any buyer worth his salt, has seen it and knows what I'm talking about. 

When I have sold gold with good crystal and sink to a knowledgeable buyer, I have always gotten top dollar. If the crystal and sink aren't evident and, if it's a small lot, the good buyer will just pay a lot less. If it's a large amount, he will want to remelt it and, guess who pays for that. He might also run it through jeweler's rolls to check for brittleness (usually lead) and look for fernlike crystals, which indicates platinum group. If the crystal still isn't evident, after remelting, you receive less money - enough less for the buyer to cover his butt. After all, if it isn't at least 99.95%, somebody will eventually have to re-refine it and this costs money. If the bad signs of impure gold are evident and, it is a large lot, the buyer will want to assay it. You pay for this, also.

Instead of trying to assay your refined gold, I would try to produce at least 99.95% pure. It'll save a lot of hassle.


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## Harold_V (Apr 24, 2007)

goldsilverpro said:


> This is where gases are expelled when the gold solidifies.



Nope! There are no gasses involved. Remember, you're dealing with pure gold, which is inert. What you're observing is the effect of shrinking volume due to temperature change. It's the same thing that makes the center of an ingot lower than the edges. The coefficient of expansion explains it perfectly well. The pool of molten metal is not large enough to fill the remaining area, so the pipe is created. It doesn't occur with unclean gold, due in part to the contaminants freezing the metal before the pipe can develop. Only by heating a button or ingot to control cooling can the pipe be avoided *if the gold is pure*.



> As the gold gets purer, the crystals become more defined and visible and the "sink" gets deeper. At very high purity, the crystals seem to crack and separate, a bit, near the sink



Yep!



> The crystal appearance I just discussed is somewhat subjective and I've never seen anything else on it on the web, other than what Harold and I have said. However, any refiner that is capable of producing high purity gold or any buyer worth his salt, has seen it and knows what I'm talking about.



Fact is, while you can't say, with certainty, that gold is a given fineness by this observation, what you can determine is if it's pure, or not. Any gold that sets with a huge array of tiny frosted crystals on the surface has contamination of some kind. Color, too, is a great indicator, buy you have to be very familiar with the appearance of pure gold before that makes much difference. The slightest amount of silver in gold will shift the color to green. It may be subtle, but it is observable. 

In my experience, the best indicators of unclean gold were the absence of the pipe, a frosted surface, and a hint of oxides when the gold cooled. The gold shot, below, came out bright----and needed no pickling to improve appearance. A sure sign you've achieved fine gold. 



> Instead of trying to assay your refined gold, I would try to produce at least 99.95% pure. It'll save a lot of hassle.



That was my goal. To me, nothing less made sense, but some of these guys are content to do the recovery and sell the spoils. Truth be known, if that's their goal, they'd likely have the same degree of success by stripping, washing, then melting to a button, which would then be sold. Eliminate any attempt at refining beyond recovery. 

It makes no sense to screw around with the spoils if all you do is change the level of contamination. Makes about as much sense as washing only your hands when you need a bath. 

I've encouraged readers to refine properly from the outset, but that may not be what some care to do. No right or wrong, but doing it all the way can lead to some interesting results. Sure did for me.

Harold


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## goldsilverpro (Apr 24, 2007)

Seems like we agreed on these things except for how the "pipe" occurs. I remember reading about this but, I can't find it. The first place I looked was Rose. No luck. Where did you get your info? Hoke?

I imagine that you're right. What you're saying makes sense. I did read that molten gold won't appreciably dissolve oxygen or nitrogen, in Rose. I do know that the reason for the eruptions on silver bars is due to the release of gas. I may have assumed that the gold pipe occurred by the same mechanism.

BTW, I just read that the array of very small crystals is caused by small quantities of copper


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## AgAuPtRh (Apr 24, 2007)

> I've encouraged readers to refine properly from the outset, but that may not be what some care to do. No right or wrong, but doing it all the way can lead to some interesting results. Sure did for me.




My first couple of years in woodworking was learning how to sweep, cut scrap and make deliveries. heh heh !!!!! With an occasional job sanding. I learned to love sand paper. LOL. I appreciate what you are saying here Harold. My personal quest is to do things right -- so I have been reading as much as possible -- gathering the appropriate equipment. This summer I will be building a furnace and making some tools for picking up crucibles and cupels.I've also been doing some small tests here and there just to get used to how acids and metals react. I have used the same cell Steve's tutorial shows and have done some small lots of edge fingers in Nitric acid. Here in a bout a month or so I'll have some major commitments completed and nothing but time to practice learning the Art of Refining. Thanks Harold and GSPro for all you guys contribute here on the forum. I consider this my Refiners University. I get to go to class everyday. LOL 
AND LOVE IT !!!!!!!!!!!


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## Harold_V (Apr 24, 2007)

In my opinion, you're approaching your project from the best possible direction. I really like your comment about doing small tests to become familiar with the reactions. Don't get in a rush during the learning curve. Doing it right, now, will pay huge dividends in the future. 

Regards the learning curve, while it's difficult to lose your materials, it's not impossible. Should you do something stupid------really, really stupid-------you'll minimize your losses that way, and you still learn the essentials. 

Let me give you one tip------one that proved futile when repeated attempts to correct something stupid was not accepted in the spirit it was offered to a friend. Only after the horse got out did he close the gate. 

When I started refining, it was before it was legal to do so. Shortly thereafter it was legalized, and I bumped into a guy at a surplus electrical supply house that was looking at the same things I was interested in buying. We struck up a conversation and I learned that he was pursuing silver, but was also interested in gold. We swapped what little information we each had at our disposal and the guy became a friend. This friend had a habit of filtering his solutions in a sink. I cautioned him time and again that if he should have a crash, he'd lose his values down the drain, but he kept using the sink. 

Got a phone call from him a few years later, asking me if I could advance him a couple ounces of gold, which he needed to replace his customer's gold---------which had gone down the drain. Had he broken his beaker on a counter, he would have recovered a lot of the metal with anything absorbent. Would likely have lost some value, but not all of it. Down the drain? Gone------for ever!

Work, *ALWAYS*, in such a way that you can recover any losses. I was tight enough that I kept a covering of newspaper on my counter. In a few days, when it was tattered and torn, it was replaced with new, and the old got incinerated, with the ashes added to my waste material. You're regularly splashing tiny amounts------which are very recoverable as long as you don't toss them. Anything that touches the refining process should be recycled. I even incinerated my damaged gloves. 

Harold


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## Harold_V (Apr 24, 2007)

goldsilverpro said:


> Seems like we agreed on these things except for how the "pipe" occurs. I remember reading about this but, I can't find it. The first place I looked was Rose. No luck. Where did you get your info? Hoke?



I'm not sure I can provide any cites. Hoke, certainly, disuses the phenomenon, but, like all my books, aside from one or two, my copies are in storage, so I haven't seen them in a few years. As a result, I can't verify anything I've said. 

I offer you the same evidence as you provided for silver. It's known oxygen won't react with silver easily, but is readily absorbed at a rate of 9 times the volume of silver, expelling it upon solidification. It does that only with considerable turbulence, leaving behind clear evidence of having been expelled, assuming you're dealing with pure silver. The shiny, tranquil and smooth surface of pure gold displays none of that. 

Common sense dictates that one should not expect gassing in *pure* gold, if for no other reason, it refuses to interact with elements that would create gassing. The balance is simply common knowledge, at least to those of us that have worked with metals for a life-time. Expansion is something I must deal with on a regular basis as a machinist----and is orderly and logical. The expansion coefficient of metals was determined long ago, and is published in many journals. Mark's Engineer's Handbook, for one. Gold expands @ 7.9" °F x (10-6) per inch, per degree. Considering its melting point of 1945° F, there's considerable change in volume from the molten state to solid. Once expanded, unless it can contract uniformly (chilling from the outside to the inside insures it doesn't), the missing material manifests itself as a void. You can see it happen if you make a close observation. The pool of molten metal is absorbed by the walls, eventually running out of molten metal, leaving the void. But that happens ONLY if the gold is quite pure. If there are contaminants present that preclude zonal cooling, that interferes with the formation of the pipe. The button then shrinks uniformly, top to bottom, leaving a depressed center, but no pipe. 

If, by chance, I've said anything that is not true, I welcome corrections. This is where sue could have proven a valuable asset, had he been willing to discuss issues instead of casting dispersions. 

A comment on gassing. Having refined for jewelers, and working closely with them, one of the things I learned early on is that alloyed gold, when melted too many times, starts gassing. That's caused by oxidation of the base metals. Gold alloy, so contaminated, does not form a pipe, but ends up with pockets of porosity. It's one of the reasons that smart bench men don't use scrap gold for new creations. You often end up carving a wax a second time to replace a casting ruined by oxidized gold. 

Only when readers have achieved a level of high purity will any of this start making sense. You have to see how it looks in order to see the difference. It took me a couple years before I understood it. I was caught up with the idea that it didn't matter, or that, somehow, magically, I was smarter than the book. I wasn't.  

Harold


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