Not sure what I'm doing wrong

R

Rookie

New Member
#1
I've just recently started trying to perform fire assays and comparing my results to known results from professional assays. I'm using pin samples drawn from molten scrap gold. I weigh out my sample, inquart it with silver, wrap it in lead foil and toss it into a cupel and bake at 850 C. After about 15-20 minutes I pull it from the furnace, roll the bead flat, roll it into a cornet and place the cornet into the Nitric Acid.From everything I've read, this is pretty straight forward. Now, when I put the sample in the acid I'm noticing it turn bluish-green, which as I understand it, means there's copper in my sample. How is this? Isn't it supposed to be only gold, silver and the platinum groups, or am I wrong? My final results have been coming in a bit heavier than the pros and the final bead will sometimes have a copper color and not the bright yellow of 24k gold. Am I screwing something up here? I know there is a lot I need to learn, and most of what I've picked up has come from what I could piece together here and there, so I could really use the advice. Does anyone have a good recommendation for something to read or watch that can help fill in the blanks that I am running into?

Thanks for any help.
 
fireguy

fireguy

Supermoderator
#2
Rookie said:
I've just recently started trying to perform fire assays and comparing my results to known results from professional assays. I'm using pin samples drawn from molten scrap gold. I weigh out my sample, inquart it with silver, wrap it in lead foil and toss it into a cupel and bake at 850 C. After about 15-20 minutes I pull it from the furnace, roll the bead flat, roll it into a cornet and place the cornet into the Nitric Acid.From everything I've read, this is pretty straight forward. Now, when I put the sample in the acid I'm noticing it turn bluish-green, which as I understand it, means there's copper in my sample. How is this? Isn't it supposed to be only gold, silver and the platinum groups, or am I wrong? My final results have been coming in a bit heavier than the pros and the final bead will sometimes have a copper color and not the bright yellow of 24k gold. Am I screwing something up here? I know there is a lot I need to learn, and most of what I've picked up has come from what I could piece together here and there, so I could really use the advice. Does anyone have a good recommendation for something to read or watch that can help fill in the blanks that I am running into?

Thanks for any help.
Hi Rookie:
Bullion analysis is quite an art- especially if you are going to do high accuracy assays to compare with refiners and potentially go to umpire.
How much lead are you wrapping your bullion in, and how much bullion are you using.
Is the bullion primarily gold or silver? Are you running a preliminary analysis to determine the approximate composition so you know how much inquart silver to add?
High accuracy bullion analysis will also require the addition of a small quantity of high purity copper and require silver loss correction.
Do you have proper venting of the furnace so fumes get out and air gets in? Are you sure the cupellation has "finished"? Did you see the "blick"?

You might consider Volume 2 of the SMA Methods Manual. It includes a section on bullion analysis:
http://www.lmine.com/product/9051x.html
 
R

Rookie

New Member
#3
Thanks for the reply. I'll fill in as much as I can right now, with more answers to come...and likely more questions too.

I don't have the weights of the lead I am using with me right now. That's all at the office. I am currently using lead foil that is 3" wide which I usually cut into 1.5" x 1.5" squares and wrap my samples up in that. I had actually also quartered those thinking that perhaps I had too much lead. Now I am wondering if I don't have enough lead to pull away the other base metals during cupellation.

The bullion is primarily gold. Responses from the other assayers usually put it at around 55% gold and 7% silver. The pin samples I begin with are about 1.5g which I then separate into three sort of equal parts. I take the weight of the separated sample, multiply it by .553 (the average Au typically found) and then multiply that by 4 to get the amount of silver to add. Probably too much silver, I'm sure but what I had read is not enough silver meant separation in Nitric would be difficult.

Did not know about adding copper. So that's something new to me.

As far as ventilation, there is a top vent on the furnace, but as I've been reading more, I should have the door ajar to let more air in. And seeing the "blick"...nope...I'm usually involved in a dozen things at the time I'm running these so I toss the cupels in and give it a bit then come back. I've never read an expected time for them to be done, and even though I've read about the "blick" I don't think I've ever witnessed it.

To fill you in, I work for a very small company that buys the scrap gold from the jewelry stores and other dealers in the area and takes the larger batch to the refiners. Typically we run a 200ozt bar about once a week and we send a separate pin sample to a third party assayer to keep the refiner honest. We're looking to reduce that by performing our own assays, and in cases of variance then using the other assayer as an umpire. I also understand that I'll soon be learning how to perform Aqua Regia refining in the year to come so that we'll be sending down already refined gold and getting a higher percentage of spot. The boss likes to make sure I don't get bored. :)

So, basically I'm squeezing learning how to do assays in between hammering out the handles on knives, sorting the silver we've received and testing the gold that's come in. I did just order a couple of books strictly on assaying that I saw mentioned on this forum, and I'm hoping that they'll fill in some of the blanks. But I'll be honest, I feel like I'm walking around a furnished room with the lights out, and my shins are getting pretty bruised.
 
fireguy

fireguy

Supermoderator
#4
"Now I am wondering if I don't have enough lead to pull away the other base metals during cupellation."
This sounds possible. I would suggest trying about 4x as much lead, and make sure your unused cupel weighs twice as much as the lead button.

The bullion is primarily gold. Responses from the other assayers usually put it at around 55% gold and 7% silver
This leaves about 40% "something else" that has to be drawn into the cupel. If you assume it is copper, the cupel should turn green or black depending on how much is present. The more base metal present, the more lead you will have to add (and the larger cupel you will have to use).

As far as ventilation, there is a top vent on the furnace, but as I've been reading more, I should have the door ajar to let more air in.
You need to have "enough" air coming into the furnace to oxidize the lead, but not so much that the temperature drops. You may want to run a "fence row" of blank cupels in front of your samples so they are not blasted by cold air coming into the furnace.

I've never read an expected time for them to be done, and even though I've read about the "blick" I don't think I've ever witnessed it.
There really isn't an expected time. You should see the lead turn bright red. Keep the door closed until this happens. That's called "driving". The button size should start getting smaller, but the button will stay bright red. After it's finished, it will turn dark. A couple of second later it will "blick", flash as the last of the lead is oxidized. You may or may not catch this. You should then remove the buttons as soon as possible. If you see "sprouting", this is the silver oxidizing and it is caused by cooling too fast. You may have to place the cupels at the front of the furnace with the door open to cool more slowly. Sprouting is bad as it causes losses. If your results are coming out high, this would normally indicate retained base metals or silver. To correct for this you need to follow some of the more obscure correction techniques discussed in the SMA Methods Manual recommended below. Getting a pretty close answer is not that hard. Getting the absolute answer, that can go to umpire and win, is much harder.
 
K

KRowe

New Member
#5
It could also be that your are not rolling out your silver/gold bead thin enough before disolving away the silver. If it is not thin enough you will not be able to disolve away all of the silver, therefore reslulting in a higher Au gravimtric assay.

KR
 
W

wotavidone

New Member
#6
New member here reading old threads for fun.
Sprouting can be caused by excess copper increasing the solubility of oxygen in molten silver. If you suspect the gold is very impure, scorify with pure lead first. That'll help with the 40% of "other" - assuming you aren't assaying 20 or 30g at a time, in which case if the other was all copper, the scorifying wouldn't handle it reliably.
 
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