# How to assay gold?



## catfish (Jun 17, 2007)

Assaying

I would like to try to assay some of my gold. I have a couple of questions first before I take off on this adventure.

Do I need a special cupel or melting dish just for assay work? 

Do I need to prepare the cupel before I melt the silver/gold wrapped in thin lead sheet? I guess that I am concerned about the cupel being able to absorb the lead if I prepared for normal gold melting.

Can I just take a sample of my one (1) ounce gold nugget (drilling residue) from about four different places, and mix with solid silver.

My understanding is, mix about 25% gold to about 75% silver and wrap in very thin sheet of lead. Question is this correct?

It is also my understanding that I need at least a 250mg (25%) sample of gold and 750mg (75%) of silver. Is this correct?

It is also my understanding that I would weigh the gold on my gold scales and record before melting. Then after melting the gold/silver sample in lead, then I would take a hammer or something and flatten the gold/silver nugget to e very thin plate of metal and then twist it in to a spiral, then place it in Nitric acid to dissolve the silver. After the silver is dissolved, then take the remaining gold, weigh and record the resultant weight. Then take the original gold sample weight and divide it in to the resultant gold weight. That would indicate the percent (%) pureness of the gold sample before I started. 

My last question is, my gold scales only has a +/- of .1 gram scale. Can I increase the sample size to increase the accuracy of my tests?

If not, what kind of scales would I need to be able to make a fairly good assay.

Thanks in advance,

Catfish


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## aflacglobal (Jun 17, 2007)

A brief description of how fire assay works:
The sample of gold is weighed very precisely and the amount is recorded. The sample is
wrapped in assay lead foil along with a quantity of pure silver. This wrapped ball is
placed in the furnace in a cupel (a special kind of disposable crucible). All the nonprecious
metals are absorbed by the hot cupel. The precious metal forms a button within
the cupel. The cupel is removed from the furnace, from the cupel and then brushed to
remove any lingering bits of cupel. It is hammered flat, rolled thin and then heated in a
porcelain crucible containing a weak nitric acid solution. The acid removes the silver,
which is poured off and the silver recovered from solution. The gold is then rinsed in
distilled water to remove any residual acid and then dried. The sample of gold is now at
least 99.999% pure. The sample is then weighed again. The original weight of the impure
sample is divided into the weight of the now pure sample. The result is the assay.

Ralph


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## goldsilverpro (Jun 17, 2007)

Your scale is not accurate enough to use for fire assay.

It is only accurate to +/- 0.1 grams. That's the same as 100mg. The sample size is 250mg (.25 grams). Therefore, your possible error is 100/250 or, +/- 40%. If you started with super pure 100% gold, you could get an answer of somewhere between 60% and 140%. With a 2 digit scale (+/- .01 grams), the potential error is +/- 4%. With a 3 digit scale, the error could be +/- .4%. A 4 digit scale (+/- .0001 grams) would be a error of +/- .04%.

Fire assaying doesn't allow for very large samples. You could probably run as much as a .5 gram sample but, that wouldn't help much.

Assume that your gold sample is actually 3 nines - 99.90% pure. Using these 4 different scales, here's the possible ranges of answers you could get. 
+/- .1 gms - from 59.9% to 139.9% pure
+/- .01 gms - from 95.9% to 103.9% pure
+/- .001 gms - from 99.5% to 100.3% pure
+/- .0001 gms - from 99.86% to 99.94% pure

As you can see, the only scale that is even somewhat reliable is a +/- .0001 gram balance. It will weigh a hair. Also notice that even a scale of this accuracy is worthless when trying to distinguish between a purity of, say, 99.95% and 99.99%. And, fire assay is the most accurate method on the planet. To analyze very high purity gold, the only way is to analyze all of the impurities, add all their percentages together, and subtract from 100%.

Another typical example. If you're fire assaying 2 grams of pins, worth $50 per pound, the final bead weight would be about .0105 grams. This can't be weighed with a 1 or 2 digit scale. A 3 digit scale can be 10% in error. A 4 digit scale is only 1% in error.

These 4 digit, +/- .0001 gram scales are called "analytical balances." The new digital ones start at about $1200. There are lots of old used mechanical ones out there for several hundred dollars but, you don't know if they are reliable. It doesn't take much to screw one up.


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## catfish (Jun 17, 2007)

Cris:

I find your post very informative and also I don’t guess I will be doing any assaying of gold soon. Ha-ha. 

Seriously, I don’t think it would be practical for me to even contemplate assaying the small amount of gold that I currently have. I would like to know what the purity is though. I think I will send it to Dallas, to the folks that I sell karat gold and refined gold too and get them to assay it. I talked to them a couple of weeks ago and explained how I had took extra pains in cleaning the gold (Harold’s cleaning method), and they told me it should be very pure after all the special cleaning steps that it went through. 

They do fire assaying of all their gold they produce, so I guess from what I understand is the most accurate test for the money. I believe you said earlier in one of your posts about fire assaying had an accuracy of .02%.

After learning the cost of the proper scales to make the weight measurement, this is a no-brainer. 

Thanks for all the good info. This is another classic example of the good information available through this forum from the experts, such as your self and the many others.

At least I gained some knowledge from the posts.

Thanks for your help.

Catfish


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## Harold_V (Jun 17, 2007)

If it's any help, I have an analytical balance, with decent weights. It's capable of weighing in tenths of a grain, but lacks the necessary precision to function as an assay balance. The old beam type assay balances are unique to assaying, and are not found elsewhere as far as I know. Chris (GSP) has given you some excellent information and advice. 

Harold


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## Steven Crim (Jul 27, 2007)

Just finished C.M. Hoke last weekend and I will be re-reading it again this weekend. I was just wondering how accurate a digital scale newbies should invest in starting out (.1 or .01 or .001 grams)? I will not be doing assays but will be making Solution A, plus Standard Gold, and Plat solutions etc. I will also want to weigh my gold buttons after refining and weigh again after re-refining to see the difference. Money is tight but I understand I need a scale accurate enough to do the job right.

Thanks in advance for everyones help!  Steven


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## Noxx (Jul 27, 2007)

You can get a .01 scale for about 20$, max capacity 100 grams. This is good for normal refining usage but maybe not for assaying.


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## aflacglobal (Jul 27, 2007)

Noxx said:


> You can get a .01 scale for about 20$, max capacity 100 grams. This is good for normal refining usage but maybe not for assaying.




I got one weekend before last at harbor freight, they have a retail outlet here so you can just walk in and out. $ 15.00


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## Steven Crim (Aug 4, 2007)

On the ARMADA web-site (scalesolution.com) I saw a number of pocket digital scales ie......... 50/.01gm for $16 and 10/.001gm for $18 (shipping is included). I thought they would be costing much much more... unless these are factory seconds or these things are easy to mass produce. Does anyone have a idea how accurate pocket digital scales are...if it claims to be 10/.001 will it really be accurate to .001? 

Thanks in advance!  Steven


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## aflacglobal (Aug 4, 2007)

Does anyone have a idea how accurate pocket digital scales are...if it claims to be 10/.001 will it really be accurate to .001

Considering that your product will be weighted again by your buyer, and that is what you will get paid on. You just need to be close enought that you know what you have for yourself. I wouldn't even argure with someone over .001 grams.


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