Assaying Karat GOLD 8K > 18K and FINE GOLD

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masarh

Member
Joined
Aug 15, 2007
Messages
12
Location
KOSOVA
i am writing this procedure as i was told but i need some explanation [if any]

ASSAYING KARAT GOLD
1.drill sample from bullion,
2.make three sub-sample of gold 0.25000 gramin in weight
3.add 0.32000gram pure silver
4.foil in lead
5.put in cupell, and cupell in furnance at 960grade Celcius which forms a button contaning only Gold and Silver
6.roll the button to for a sheet,
6.boil with nitric acid in 32 baume 1 time for 20minutes
7.boil with nitric acid in 49 baume 2 times for 20minutes
8.wash with distilled water, and let dry
9.measure the left over of the gold and divide /primary sample
[for example leftover=0.15203 divide by 0.25000 =0.608‰

that means 608/1000 pureness


ASSAYING FINE GOLD
1.drill sample from bullion,
2.make three sub-sample of gold 0.25000 gramin in weight
3.add 0.45000gram pure silver and 0.05000gram Copper
4.foil in lead
5.put in cupell, and cupell in furnance at 960grade Celcius which forms a button contaning only Gold and Silver
6.roll the button to for a sheet,
6.boil with nitric acid in 32 baume 1 time for 20minutes
7.boil with nitric acid in 49 baume 3 times for 20minutes
8.wash with distilled water, and let dry
9.measure the left over of the gold and divide /primary sample
[for example leftover=0.24239 divide by 0.25000 =0.96956‰

that means 969/1000 pureness

why is that i use for assay of karat gold 0.25gram Gold and 0.32gr silver
while for FINE GOLD 0.25gram gold + 0.32 silver + 0.05 Copper
 
It will work. But I'm not sure about the accuracy though.

Why do you wrap your metals in lead foil ? Also, you should mention to use pure metals to inquart.
 
The procedures given are excellent, although very exacting, fire assay procedures. They seem to follow the "Mint" method, which requires a lot of expensive equipment. For example, whoever wrote them had access to a 5 decimal place scale, which is quite expensive. A 4 decimal place scale should be adequate for decent routine results. Here are my comments of the various steps as to how I would do it, with lesser equipment.

ASSAYING KARAT GOLD
1.drill sample from bullion,
Unless the bar was cooled immediately after pouring, there can be stratification and, therefore, differences in the compostition of the drillings. It is best, in my opinion, to sample in the well-stirred molten stage - stir with a carbon rod. This can be done by pin sampling (a small sealed glass tube containing a vacuum is quickly dipped into the molten metal - the tip of the glass melts and the gold is sucked into the tube - the glass is then broken away from the gold pin) or by ladling out a sample and pouring it into water (shotting).

2.make three sub-sample of gold 0.25000 gramin in weight
The more samples you run, the more samples you can average, and the better the final results. For routine work, 2 samples should be adequate. It is not necessary and is difficult to weigh exactly .2500 gram samples. Just weigh close to .25 grams and record the exact weight - for example, .2483 grams. Use this figure in the calculation in step 9.

3.add 0.32000gram pure silver
In the "Mint" method, they use Platinum crucibles to part (dissolve) the silver from the gold. They also use a rolling mill (expensive) to roll out the Au/Ag bead into a long strip (step 6). They then shape the strip into a spiral. They only use a 2.2 to 1, silver to gold, ratio to prevent the gold from breaking up. Since I have no rolling mill, I flatten the bead with a clean hammer and anvil and end up with a thicker bead. Therefore, I use a higher silver ratio to insure that I dissolve all the silver. For a .25 gram sample, I use about .8 grams of silver - about 4 or 5 times more silver than gold. With this ratio, the gold will break up. You have to be very careful not to lose any of the gold specks.

4.foil in lead
Use about 10 grams of .005", silver-free, lead foil. Wrap the foil securely around the metals and form it into a package so it's about 1/2" square, or less.

5.put in cupell, and cupell in furnance at 960grade Celcius which forms a button contaning only Gold and Silver
Preheat the cupel at this temperature for about 20 minutes, or until it's a uniform color (no dark center), before adding the foil. A very slight amount of air is allowed to enter the furnace, in order to oxidize the lead. When all the lead and base metals are absorbed into the cupel, you are left with essentially a gold/silver alloy. Note: A full page could be written on the cupellation procedure.

6.roll the button to for a sheet,
Like I said, I flatten it with a hammer and anvil

6.boil with nitric acid in 32 baume 1 time for 20minutes
Starting with a weaker acid, the bead will break up less. I use #1 Coors porcelain crucibles on a hot plate. I usually use 7/1, distilled H2O/nitric, as a first acid. I use reagent grade nitric. Since my gold breaks up, I don't boil the acid. I heat it to near boiling until I see no more reaction. Carefully pour off the acid, without losing any black gold particles. This is easily done by swirling the crucible, to collect the gold into a pile, and then slowly pour out the acid down a glass rod.

7.boil with nitric acid in 49 baume 2 times for 20minutes
My second acid is 2 to 3, distilled water/nitric. Don't boil.

8.wash with distilled water, and let dry
I do 3, quite hot, water rinses. After decanting the final rinse, make sure all the gold is together, at the bottom edge of the crucible. Dry at very low heat on the hot plate. If it is too hot, it will spit gold out of the crucible. When COMPLETELY DRY, anneal the gold by holding the crucible over a bunsen burner. The gold will turn from a dark color to bright yellow. If the gold wasn't dry, it will spit out of the crucible. Set the crucible on a dry insulator (like a brick) to cool. If you place it on a metal surface, the crucible will likely break.

9.measure the left over of the gold and divide /primary sample
[for example leftover=0.15203 divide by 0.25000 =0.608‰
that means 608/1000 pureness
When cool, dump the gold onto a tared weighing paper or boat on the scale, weigh it to 4 decimal places, and calculate the results. Make sure you get any stray gold specks.

I use the same procedure for pure gold, although the "Mint" procedure given would give more accuracy on the final digit. In this case, triplicate samples (or, more) would be best. For the pure gold, they used .45 grams of silver, still maintaining the 2.25 to 1 ratio.

why is that i use for assay of karat gold 0.25gram Gold and 0.32gr silver
while for FINE GOLD 0.25gram gold + 0.32 silver + 0.05 Copper
I don't know why they add the copper, although I have seen it in the "Mint" procedures. They surely have a reason for it. I don't use it

Hope this helps.
 
on step 6) when rolling, i have rolling mill since my primary business is jewellery production, but are there any spec. on how thin/thick should the sheet be?
since i am suspecting that MAYBE, the sheet is to thick and the Acids can not ETCH [remove the silver]??

on steps 6 and 7 when "Heating" /"Boiling" in Acid
also i am suspecting that MAYBE, the 'faltened sheet' is not heated enought and i have to wait to boil?

how true it is that when heated with SECOND ACID, * 2 times, will remove/disolve all the silver left over.
 
why is that i use for assay of karat gold 0.25gram Gold and 0.32gr silver
while for FINE GOLD 0.25gram gold + 0.32 silver + 0.05 Copper

I don't know why they add the copper, although I have seen it in the "Mint" procedures. They surely have a reason for it. I don't use it
could it be that because of the low content of non-PM they add copper, so it can react with LEAD better and it smoke all the impurities and the lead during the cuppelation?
 
Masarh,

In my books, I have several descriptions of the Mint method. All are somewhat different and only one mentions the thickness of the gold after rolling. T.K. Rose, in his 1937, 7th edition of "The Metallurgy of Gold," says the thickness should be 0.25 mm. I would think that if it were too thin, the gold would break up. If it were too thick, all of the silver would not dissolve when using a low Ag/Au ratio.

The bead is first flattened a bit, with a hammer, so the rolls will grab it. It is rolled into a long strip and then shaped into a cornet (a spiral) by hand. The portions of the spiral should not touch each other. The cornet is annealed at a dull red heat and then allowed to cool before parting. The rolls must be very clean with no oil.

I would first try the lower Ag/Au ratio of about 2.0 to 2.2, to prevent the gold from breaking up. Since you are rolling the bead, this ratio should work. To prove your process, assay some known 999.9 Fine gold. A Canadian Maple Leaf is a good source.

If the gold is about 70-80%, or better, slight amounts of silver will be retained in the gold. This is called a "surcharge". If the gold is of less purity, there will be a slight loss. I would suggest running 2 samples plus a "proof" sample, all run side by side.

For karat gold. To make the proof sample, weigh some fine gold and add some copper. Use enough of each to approximate the composition (and quantities of metals) of the unknown samples that you are running. Then, correct the assay based on what happened to the proof

For refined gold, just use the proof gold alone.

None of the references I have add copper to a pure gold sample. I am thinking that the guy added copper to discolor (green) the cupel. I think that the copper would be the last thing to be oxidized and absorbed into the cupel. If so, the green color would be an indication that all of the lead has been absorbed into the cupel. I may be completely wrong about this.
 

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The small amount of copper remains with the gold and silver in the button and aids in reducing the tendency of spitting and sprouting.

http://books.google.com/books?id=i_JMAAAAMAAJ&pg=PA267&lpg=PA267&dq=copper+in+mint+fire+assay&source=web&ots=fIrn5c6uqk&sig=DoDncrIdHESogL53NtOmoitLsDY&hl=en&sa=X&oi=book_result&resnum=5&ct=result
 
Very good, qst. I stand corrected.

I can't imagine any spitting to occur at a 2/1 ratio, which I have never used, but perhaps it would. Spitting/sprouting can also be avoided by cooling the bead very slowly. I used to cover the cupel, very carefully, with a hot empty cupel (upside down), before removing it from the furnace, in order to cool the bead slowly. Even with pure silver, I rarely got sprouting. This is difficult with bone ash cupels because they tend to crumble. Although the bone ash cupels are the best, especially for silver, I usually used DFC composite cupels (bone ash + Portland cement, I think) due to their durability. For precision, I used bone ash.
 
This is from an early text (1913) written by the son of a Royal Mint Assayer and the technique may only be part of the handed down tradition rather than a having a firm basis in science. Later text may have dropped the use of copper for that reason.

It was also mentioned it aided malleability for rolling.

I have read in a 1909 text that spitting is blamed on a contaminate in the cupel itself that reacts with the litharge. Calcium carbonate if I remember correctly?

This may be an old debate we have stumbled on based on where you learned your assaying from?
 
I downloaded that book, awhile back, but haven't read it. I think I should.

Pure silver bars spit like crazy, especially if you melt them too hot.

In the late 60s, I went to work, in L.A., for Sel-Rex, a big scrap refiner and the largest supplier of gold plating systems in the world. At the time, they were located only on the East coast. Soon, they built a large refinery in L.A. and I was in on the ground floor, as their Senior Chemist.

My boss was friends with the owner of a large, very famous fire assay lab in S.F. that had been around since the gold rush. It is no longer there and I don't remember the name. Anyway, I spent a week there in training and that's how I got the basics. Then, I got copies of Bugbee, and Shepherd and Dietrich, and Beamish, and Fulton and Sharwood , and started learning.

At the L.A. refinery, we had 2 large DFC gas assay furnaces and we usually ran from 72 to 144 assays per day. I trained an employee to do the assaying and I just filled in for him when he was on vacation.

Since then, I have run fire assays for 40 years, in almost every place I worked or owned - at least 10,000 assays. Never as a business - only for our own deals.

Once you learn it, it's like falling off a log.

My experience is about 95% in scrap, with only about 5% in ores.

If you know exactly what a batch of material runs, it's hard to lose money on it. Of course, the other side of the coin is the art/science of sampling. A good assay on a non-representative sample is less than worthless. Good sampling is much more difficult than good assaying.

Where did you pick up your assaying savvy, gst? It's obvious that you have a lot of knowledge.
 
I learned from a 1909 copy of Fulton.

Mostly just as a hobby before I knew anything of making my own chemicals. I had always thought the chemicals were unobtainable without a business. I had used the fire assay techniques for a crude refining of silver contacts and some gold plated stuff. Using a propane torch and kiln bricks. I would guess I have run hundreds of assays none technical or professionally.

I learned a lot making my own cupels from roasted bone meal and larger ones from straight Portland cement.

The Portland cement cupels do sometimes spit with only the melting of lead. I got in the habit of starting with the lead, the cupels that spit would be roasted until they calmed down then I could add the silver.

I have experienced the pure silver sprouting long antlers spitting off sizable pieces, and sometimes the whole bead would crystallize entirely looking much like a head of cauliflower. I never did figure that one out but I wish I would have saved one to see what you thought of it.
 
goldsilverpro said:
If you know exactly what a batch of material runs, it's hard to lose money on it. Of course, the other side of the coin is the art/science of sampling. A good assay on a non-representative sample is less than worthless. Good sampling is much more difficult than good assaying.


This is a take home message and clearly he who said it knows what analysis is all about. Proper acquisition and handling of a sample is imperative or an assay isn't worth the paper it is printed on let alone the big money that probably rides on it. This is true for almost every type of test--an XRF on an improperly prepared sample may also be worthless (this is a notorious example).


When one starts dealing with UHP metals and doing analysis on them, even very small things start to accrue and throw off the results. A case example is rolling out a sample--your sample will pick up metal off the rollers, or perhaps even grease. Even handling a sample with your bare hands can lead to measurement errors (fingerprints have mass). Go even further and for more precision and temperature, humidity, and pressure fluctuations will affect your balance's performance. Most of the new Cahn or Sartorius microanalytical balances have multiprocessors that correct for the such variables through sophisticated algorithms.

Analytical chemistry can be taken as far as one wishes to take it, it's all what you want to settle for accuracy and precision-wise.


I love seeing this type of discussion on the forum. I think it is very important that members here understand that there's no magic to an assay. Knowing what happens to your sample when sent out for an assay is key to understanding what your assay means and how to value it.

Everyone willing, I think we should make various threads about assay methodologies--one dedicated to traditional fire assays, one to alternative fire assay, and perhaps one directed towards instrumentation. I'd love to discuss XRF, ICP, EDX, and AA at length and how they pertain to the precious metals if there is enough interest.

I've some time off this week obviously, so I'm probably going to make some value posts soon :). Next I think will be proper sample preparation.

Merry Christmas all,

Lou
 
Well Lou you have talked to me before about operator error with XRF. If my vote counts for anything I would love to hear you outline proper procedure for this. Although the cost of such equipment is beyond the means of the majority of this forum many accept XRF as the holy grail of testing. If they understand the process thoroughly they will be in a position to judge if the operator giving results truly knows how to use his equipment.

There are those out there that have the money for this equipment, but have no clue.
 
Noxx said:
It will work. But I'm not sure about the accuracy though.

Why do you wrap your metals in lead foil ? Also, you should mention to use pure metals to inquart.

why should there be used PURE metals, when they will dissolve and/or 'smoke'?


lead collects all other non precious metals, while being heated in cuppellation furnance, thats why you need lead foil.
 
qst42know said:
It was also mentioned it aided malleability for rolling.

this couldnt be true, since the easiest and the best gold that melts is that of high gold content like 90% or more.
 
This could be true.

Assays are performed when the total alloy is questionable. If the starting gold had traces of Antimony or Arsenic they could be very brittle. Traces of some metals will pass the fire portion of the assay and must be removed in the parting step.

I could not find a specific reference to support this but copper may help keep your rolled cornet in one piece for parting. When your sample tears in rolling there is always a possibility small crumbs could be lost spoiling the accuracy of the assay.
 
This text points to other elements that add brittleness to PMs. One of particular note is lead where 1 part in 500,000 can have a noticable effect.

http://books.google.com/books?id=2SpVAAAAMAAJ&pg=PA66&lpg=PA66&dq=brittle+alloys+of+gold+and+silver&source=web&ots=UPLgvztUbl&sig=8Q19PU5hCOJt5ZuBBpwQs8hToGA&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA66,M1
 
why should there be used PURE metals, when they will dissolve and/or 'smoke'?

lead collects all other non precious metals, while being heated in cuppellation furnance, thats why you need lead foil.

this couldnt be true, since the easiest and the best gold that melts is that of high gold content like 90% or more.

You are right. Superpure metals are not necessary for inquartation. However, the silver used should be of decent quality and you should avoid such contaminates as tin, since it would only complicate the process.

The lead foil alloys the silver and gold and helps to drag the base metals into the cupel. To prepare for parting, some people alloy the Au and Ag with a torch, skipping the cupellation altogether. This is a poor practice. There can be losses and the base metals are still in the alloy, some of which can be retained in the final gold.

Concerning the increased malleability, qst was quoting the book that he gave a link for. Your argument is with Ernest Alfred Smith and not with qst.

Lou,

Although the handling of samples, after you get them, is extremely important, I was talking about the OBTAINING of the samples, in the first place. In other words, if a guy wants to sell you a pile of mixed scrap, how do you pull representative samples from that pile that will allow you to determine the value of the entire pile. You can guess and bid low but, even then, you can be bitten. The reason I said that a good assay on a bad sample is less than worthless, is because a high sample will lose money and a low sample can cause you to miss the deal. Each deal is different. Good sampling requires knowledge, ingenuity, and some math - although not absolutely required, a little knowledge of statistics can be very helpful.

Here's a few situations that I have found myself in:

(1) A company I worked for generated 50 drums of silver bearing slag every few months. They shipped it to be refined. The refiner processed a 5% sample, taken here and there, and gave my boss the results. He had suspected of being cheated for some time. To keep the refiner honest, I was given the chore of sampling the material and then determining the value. The questions I had to ask were: How do I pull the samples? How many samples (and their size) do I pull to get decent statistical results? How do I process the samples?

(2) A guy brings in a 100 oz bar of mixed karat gold that he had melted. The bar has some slag on it and there are a few magnetic objects embedded in it. We want to buy it and he is willing to let us sample it in any way that we want. He wants 95% of the gold value, which we agree to. However, unless we do things right, 95% is pretty tight. How do we sample the bar, in order to determine the value with less than a 1/4% error?

(3) A guy brings us 6 drums of mixed gold pins. They have been out in the rain and the drums contain water. There are leaves and other trash in the drums. He is willing to hang around until we get numbers. How do we sample the material?

(4) We went to a Ma Bell auction viewing somewhere in OR or WA. The lot we were interested in was 750,000# of mixed electronic scrap. In general, it looked pretty good. It was winter and the stuff had been sitting out in the weather. They allowed the bidders to pull samples as they required. How should we sample the material? No matter how we did it, it would only be an half-assed approximation. After we ran the samples, how should we bid the lot? BTW, we didn't get the deal.

(5) A scrapper brings us a pickup load of mixed boards. He agrees to leave them with us until we evaluate them. He says they are worth $10/#, but he's only guessing and is also jacking up the value. How do we evaluate them?

I could go on and on.
 
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