how to fire assay - gold

[darkelf2x1]

1. smelt the product first to get rid of nonmetals ( save the slag though because you might be able to recover gold nuggets from it )
2. drill into several areas of the recovered metal and weigh the sampled metal and record the weight
3. wrap sample in lead foil, along with pure silver
4. heat in a cupel at 2000 degrees f - the base metals should be absorbed into the cupel
5. after removing the cupel from the oven, kiln, other, brush the sample to remove bits of the cupel
6. hammer (or roll) the sample into a thin sheet and place in a container of nitric acid (i personally prefer to use sulfuric acid)... remember to save the acid so you can recover the silver
7. wash the sample in water (distilled preferably) and after drying weigh the sample to determine the gold conent of parent product ( divide the original sample weight by the new sample weight)

- - - - -

dont ask me why you add lead or silver - thats just the way its been told to me...

could be for similar reasons as to why the granddaughter cuts off the ends of the bread because the grand mothers breadbox was too small

 

[goldsilverpro]

Here's a pretty good outline of the method, along with some photos.

http://www.nbmg.unr.edu/comstockscience/fireassaying1.shtml

The silver is added so that, after cupelling, there are 3 parts of silver to each part of gold - 3 to 1 ratio (called inquartation). The mint method uses a ratio of 2 to 1. Then, it's put into hot nitric, which dissolves the silver and base metals, leaving the nearly pure gold behind (called parting). The gold is then fired and weighed.

In the crucible fusion part of fire assaying, lead, in the form of litharge, is added to the melt, along with the sample. The lead formed picks up all of the precious metals, only. The base metals remain in the slag. While molten, everything is poured into a small cone shaped mold. The lead plus PM's sink to the bottom. When the slag solidifies, the mold is dumped and the glassy slag is broken away from the PM containing lead. The lead is then placed in a preheated cupel and is heated until all of the lead has soaked in to the cupel material. None of the PM's go into the cupel material but, remain alone as a bead (called a prill) with all contained PM's alloyed together. The bead is weighed and the PM's are separated chemically.

For karat gold, the crucible fusion is not needed. A .25 gram sample and any added silver is wrapped in lead foil and cupelled as above. If both silver and gold answers are needed, no extra silver is added. The sample alone is put in foil and cupelled and weighed. The bead contains all of the silver and gold in the original sample. Then, the bead is wrapped in foil with extra silver, re-cupelled, parted, and weighed, as above. If only a gold answer is needed, the extra silver is added during the first cupellation. The second cupellation isn't necessary.

 

[lazersteve]

Great link, very informative.

Steve

 

[Anonymous]

7. wash the sample in water (distilled preferably) and after drying weigh the sample to determine the gold conent of parent product ( divide the original sample weight by the new sample weight)

************************************************************
Hey there, Darkelf,
I probably wouldn't have responded at all if it hadn't been for the major error in this last statement.

The remaining assay bead weight is divided by the original weight of the drill filings. That is directly opposite of what you typed above.

a man named Sue

 

[Anonymous]

Here's a pretty good outline of the method, along with some photos.

http://www.nbmg.unr.edu/comstockscience/fireassaying1.shtml

*****************
Interesting URL you posted.

a man named Sue

 

[AgAuPtRh]

Sue,

If a sample -- treated as above reveals the presence of Nickel -- which in the fusion process could carry gold in to the cupel -- thereby giving a false read on the amount of gold --

1. Would it help to change the flux in the initial phase?

2. Or -- would it help to scorify to get rid of the nickel before the cuppellation process?

3. Or -- would it be proper to do both in order to get an accurate result?

I've been reading E A Smiths The Sampling and Assay of Precious Metals --

From what I can gather the fusion process properly fluxed should allow for the precious metals to be drawn down in to the lead with the base metals left above. They go on to suggest that that when metals that don't dissolve well in litharge are present they should be scorified before cupellation.

I've not had any real experience in these processes but am interested in learning what I can. More book time than bench time.

Thanks

Steve K

 

[Anonymous]

Hey there, Steve K,

Good response. Good to see you reading some of the classics.

1. Would it help to change the flux in the initial phase?

By all means, YES, it helps somewhat to increase the oxidizer in the initial flux as well as add sufficient silica to oxidize much of the nickel and or other base metals in the initial fusion phase.

2. Or -- would it help to scorify to get rid of the nickel before the cuppellation process?

Again, YES. Scorification is essential when Ni and many other metal contaminants report to the lead prill from the initial fusion.

3. Or -- would it be proper to do both in order to get an accurate result?

Yes, it would be indeed prudent to do both in order to achieve a more accurate cupellation fire assay result. However, the ring in the cupel will be the ultimate arbiter in this situation. And if the ring appears, you have the inquart protocol upon which to fall back on.

The reference from E.W. Smith is correct even if somewhat overly simplistic.

If all the sample components were known exactly prior to assay, it would be easy to flux for them exactly.

To put it another way, it would not be necessary to fire assay at all, if all metallic components were known prior to the final cupellation result.

It is far easier to correct for metal contamination uncovered at the cupellation step via the telltale ring, than to spend hours and money identifying each component quantitatively prior to fire assay.

As Chris and Harold like to point out, once again, it comes down to time and overhead costs vs. results.

Sue

 

[Anonymous]

edit: pulled teaching pix of 3 cupels showing nickel contamination which pulled gold values into cupel and ruined assay.

a man named Sue

 

[goldsilverpro]

I was reading my assay books last night - I have Bugbee, Beamish, and Shepard/Dietrich - about nickel and copper. They all say about the same thing - higher litharge, mostly, can solve the problems. They also use a little silica which I normally use in my basic flux mix, anyway.

I never really had much problems with nickel. I usually use a lot of litharge and, nickel plating is very thin and, therefore, it doesn't weigh much. Kovar, common in gold plated electronics, is another story - it runs 17% Co and 29% Ni. I may have had problems and didn't know it, although I keep a lookout for the cupel colors.

Copper is the one that has given me fits. I usually go the sulfur routine on high copper but have found that an allowance of 2 grams, as written in the books, doesn't really work for me. On anything greater than about 1 gram, I usually get a coppery bead and have to toss it and start over.

Do you figure out flux for each assay or, do you have a basic mix? If so, what do you use? How do you deal with hi nickel and, especially hi copper?

 

[masarh]

can any body share the "The Sampling and Assay of the Precious Metals By Ernest Alfred Smith"

 

[goldsilverpro]

qst gave the link on the other thread
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

At the upper right, click Download. This will give you the entire book as a pdf.