What kind of Crucible for the Lead-Ore-Borax part of a Fire Assay ?

[gunDriller]

http://www.mine-engineer.com/mining/assay2.htm

reference to Fire Assay process for testing for presence of gold in ore samples; will reply to my first post to add pictures etc.

[gunDriller]

http://media1.riogrande.com/Products...rge/704155.jpg

I have one of these crucibles and a kiln that goes up to 2000 F. The graphite crucible in the picture is about 3 inches diameter and about 5 inches high, weighs about 5 pounds.


That web page describes the fire assay process as a 2 step procedure.

As I understand it - first you mix the ground up ore sample with powdered lead and borax.

It doesn't tell me the amounts so I am going to go with about 2 ounces (by volume) or ore, 2 ounces of lead, and 1 ounce of borax. I'll put it in the crucible, heat it up to 2000 F, and let it cool off.

It makes it sound like the kiln should be pre-heated.

I'm wondering if the kiln is not pre-heated, if the molten lead just oozes to the bottom of the crucible, without getting a chance to mix with any gold in the ore sample.

But my kiln is not the greatest, it's not so easy to pre-heat to 2000 F and then open the door and insert the loaded crucible.

Will it still work if you don't pre-heat the kiln ?


When it cools, according to the process description at the link, you end up with a 'thing' with lead at the bottom, and some kind of slag on top. If all has gone well, whatever gold was in the ore sample has ended up in the lead 'button'.


Part 3 of the process - you take the lead and put it in a ceramic crucible called a "cupel." In my case I got some #5 bone ash cupel's from these guys -
http://www.minerox.com/

The lead soaks into the cupel, and if there was any gold in the lead, it is left in the bottom of the cupel as a tiny bead.


Part 2 -
I have a hunch that I'm using far more lead than my #5 cupels can absorb, so I was going to re-melt the lead after Part 1, and then pour it on a smooth ceramic-ish surface (e.g. concrete floor). then cut the resulting lead sheet up into small pieces & weigh them, and divvy them out to the cupels. Then when I do the part 3, I might have 6 cupels each with 10 grams of lead.


DANG I sure would like to open that kiln door after part 2 and find a tiny bead of gold in the middle of the cupel !


Beefsteak, Crazy Chicken, whoever has done the fire assay process, and every body else, I welcome your feedback.

Am I on the right track ?

Are my proportions of lead/ore sample/ borax OK ?


All I have is the graphite crucible, so I was going to use that for Part 1 and Part 2, unless it gets messed up by Part 1. They're only about $30 from Rio Grande -
http://www.riogrande.com/Product/Sal...s/704155?pos=8

A picture of the #5 bone ash cupel - they're only about 1" high and 1" wide -
http://minerox.com/images/products/M%208952full.jpg

[Quixote2]

make your own with bone ash and cement plus water

http://www.google.com/#sclient=psy-a...3&pf=p&pdl=300

http://www.google.com/#hl=en&cp=20&g...=1133&bih=1023

http://goldrefiningforum.com/phpBB3/...hp?f=49&t=9827

http://www.goldrefiningforum.com/php...ortland+cement

http://nevada-outback-gems.com/Prosp...fire_assay.htm

http://www.prospectorsparadise.com/html/assaying.html

[Quixote2]

make your own with bone ash and cement plus water

http://www.google.com/#sclient=psy-a...3&pf=p&pdl=300

http://www.google.com/#hl=en&cp=20&g...=1133&bih=1023

http://goldrefiningforum.com/phpBB3/...hp?f=49&t=9827

http://www.goldrefiningforum.com/php...ortland+cement

http://nevada-outback-gems.com/Prosp...fire_assay.htm

http://www.prospectorsparadise.com/html/assaying.html

http://www.tomslibrary.net/p_wk_fR1.htm

http://www.tomslibrary.net/Link_Wf.htm

[gunDriller]

make your own with bone ash and cement plus water

THANKS !

to make bone ash - can i just take some bone meal (like they sell at garden supply stores) and heat it up in the fireplace to about 1500 F for an hour ?


in the write up at

http://www.mine-engineer.com/mining/assay2.htm

it sounds like this assay technique 'gets' / extracts most of the gold, enough to make it useful as an analytical tool.


so - why not use it as a production process ?

you would end up with a batch of lead-filled used cupels, but i'm pretty sure that the lead can be extracted from the used cupels using a centrifuge or something.


my kiln heated up remarkably fast, got to 1500 F in a half hour, usually it takes longer.

http://oi52.tinypic.com/14lokk1.jpg

ore, lead powder, borax.

the upside down cup is the crucible. it is looking very used.

the little cup is the cupel.


may your cupel runneth over

[Quixote2]

Just buy the bone ash at a local craft/pottery supply store, about $2 a pound.

http://www.axner.com/bone-ash-di-calcium-phosphate.aspx

http://www.google.com/#hl=en&cp=12&g...=1133&bih=1023

You cannot keep using the bone ash cruicble. The lead metal is oxidized to lead oxide and absorbed into the bone crucible pores. The crucibles are sized to absorb only so much material. After use you cannot reclaim the lead from the lead oxide by melting. Throw away the used bone ash crucibles. (Do not tell the EPA that you are pitching them in the muncipal dump.)

[gunDriller]

Well, I managed to get my lead out of the crucible.

And then I poured it onto a steel sheet to make a crude lead sheet.

Then I took some pieces of that and put it in the cupel.

And it got a Spongey Orangey Crust - I'm thinking that's Lead Oxide. From doing a web search, I'd say that is almost definitely lead oxide.

The lead didn't soak into the cupel, as it was supposed to.

Back to the instructions - "So, the cupel's with the lead buttons are again placed in a Assay or Cupellation furnace, heated to about 2,000 deg F and the lead is absorbed into the cupel, leaving only a tiny gold bead. The cupels are removed from the furnace, allowed to cool and the gold bead is weighed."

I will try it again. I don't think I got it up to 2000 F.

Pics -

http://oi55.tinypic.com/5x0s3b.jpg

Crucible + Lead + Slag.


http://oi55.tinypic.com/2z556s9.jpg

Lead


http://oi55.tinypic.com/25zm9fo.jpg

Lead Oxide on Rim of Cupel.

[E. Haney]

The short answer is no, that won't work.

It appears you have a measuring cup of metallic lead, you need to use lead oxide (PbO).
Mixing ore with metallic lead and borax will result in the lead melting (at approx. 622°F) and sinking to the bottom of the crucible, as the heat is increased (Yes, you should start with the furnace cold or fairly warm, but not at fusion temperatures and gradually raise the temperature) at about 1000°F the borax will begin to melt (the use of 20 Mule Team or any hydrated borax is discouraged, as it heats the water boils off and the borax 'grows' and can overflow the crucible taking ore and flux components with it) and begin to form compounds with the bases of the ore. But the 'collector', the lead, is now sitting at the bottom of the crucible where it is of little use.
The use of lead oxide (a powder) thoroughly mixed with the powdered ore and powdered flux components forms an intimate mix where as the lead oxide reduces beginning at approx. 950°F, the resultant microscopic beads of molten lead alloy with the Ag and Au contained in the ore/flux mix, the base metals oxides for the most part dissolve into the flux as the temperature of the furnace is increased the flux becomes more fluid and the heavy lead alloy drops to the bottom and collects as a button.
A simple flux/ore charge:
1 part by weight Ore
1 " " " Sodium Carbonate (washing soda [baking soda {1.6 parts} can be substituted, but like 20 MT borax is hydrated and contains CO2, so it will 'grow' as well and release carbon dioxide gas])
1/2 part by weight Borax
2 1/2 parts lead oxide (PbO)
1/2 part flour (ordinary baking flour)
this is thoroughly mixed until it is homogeneous, added to the crucible, heated with increasing temps to fuse (dull red) when it is no longer bubbling, heated to fluidize at a bright yellow heat.
Allowed to settle, then removed from the furnace and poured into a cone mold.

As BeefSteak pointed out this is a procedure that is done outdoors or with a ventilated furnace due to lead vapor.

As far as you being able to do a cupellation, I doubt you can do it unless your furnace is a front door vertical opening model.

Your best bet for crucibles are clay smelting crucibles, cheap, disposable, non-reactive, the carbon in your graphite crucible will react with any oxides in the ore, a good thing for the fusion, but it eats the crucible thinning it, a bad thing if it breaks while removing it from the furnace or pouring.

[E. Haney]

Is that a #5 cupel?
How much lead did you load? A #5 absorbs 16.6g Pb.

From your photo, you are clearly too cold, the lead oxide needs to melt and absorb into the cupel, some has as you can see a yellow staining.
What you have is known as a frozen assay, they cannot be recovered.

I guess I should ask what your intentions are.
Are you attempting to extract values from ore in this manner? If so, it's not really the way to go, and will be quite expensive and time consuming.
If you are just experimenting, more power to you, be careful, read, try things, but realize you will lose some gold and silver, it's inevitable.
If you are trying to do accurate assays, I hate to say it, but accuracy in fire assays is not a 'home-made' venture, can it be done? Yes. But the proper equipment is required.

[gunDriller]

Is that a #5 cupel?
How much lead did you load? A #5 absorbs 16.6g Pb.

From your photo, you are clearly too cold, the lead oxide needs to melt and absorb into the cupel, some has as you can see a yellow staining.
What you have is known as a frozen assay, they cannot be recovered.

I guess I should ask what your intentions are.
Are you attempting to extract values from ore in this manner? If so, it's not really the way to go, and will be quite expensive and time consuming.

Yes, it is a #5 Cupel. The store guy told me it would absorb its own weight in lead; it weighs about 33 grams. maybe if i had a perfect lead sphere 33 grams would fit in the shallow cup on the cupel.

15 grams makes much more sense.

i think it was when i was talking to a few metal detector guys, one of them told me about fire assays.

as a way to see if there is gold in a particular ore sample. then i did a web search and realized i had most of the tools.

from the mine-engineer.com/mining/assay2.htm
write-up, they make it sound like a production-ready technique (if you have the right tools).

the most sensitive scale i have is a Salter Brecknell PB-500, which goes down to .1 gram. if i found .1 gram of gold in a 15 gram lead sample, well that's a lot of ounces per ton, i sure don't expect to find that.

for starters, i need a more accurate scale.

i measured the lead before & after the assay. 115 grams before, 96 grams after. it didn't spill but my guess is, some of those fumes infused lead into the fire-bricks in the furnace, or maybe went into the orange glass on top of the metallic lead in the bottom of the graphite crucible.


mainly, yes, i am doing it to learn.

If you are just experimenting, more power to you, be careful, read, try things, but realize you will lose some gold and silver, it's inevitable.
If you are trying to do accurate assays, I hate to say it, but accuracy in fire assays is not a 'home-made' venture, can it be done? Yes. But the proper equipment is required.

THANKS MUCHO everybody - Beefsteak, E. Haney, & Quixote !

i just started 2 more with about 14 grams in each one.


on the safety issue - obviously something that is easy to overlook in the enthusiasm/ gold fever of the moment.

for most of the work around the kiln, which is in the garage, i held my breath while i was in the garage.

i would work for a minute or 2, then go outside and get a breath, etc. i made a mistake and i left the hot crucible near the plastic meter on the oven, i didn't want to breathe that smell anyway.


work mode #2 - i was using a propane-oxygen torch to try & get the lead to "ball up" so it would fit in the cupel better. i also held my breath for that. at one point i put the lit torch down, while i stepped away to get a breath. and it burned the concrete step and sent a little chunk of it flying. so i know i held my breath for that part.


at the same time, i am building a fume hood workbench in the garage. about half an hour after i turned off the kiln, i went back in the garage and worked on that for about a half hour. now i wonder if i might have gotten any lead fumes then.

it makes me think about water - it doesn't boil till 212 F, but obviously air can contain water at lower temperatures.

i did see lead vapor rising off the lead in the crucible when i re-melted it and took it out of the kiln, hot, and poured it on the steel sheet. referring to the 96 grams of lead that was in the crucible beneath the orange glassy stuff.

i wonder what happens to that lead vapor. does it fall back to the ground (the garage floor in this case) because lead is heavy ?