# Can you Use a Steel/ Ferrous Crucible for a Fire Assay ?



## Photobacterium (Dec 18, 2011)

So far I have done 2 fire assays using a graphite-clay crucible from Rio Grande - This one -

http://www.riogrande.com/Product/Salamander-Clay-Graphite-No-3-Crucible-85-lbs/704156

It's about 5 inches tall, 4 inches across, and is rated as "8.5 pounds of brass".


The assay was successful in the sense that the lead produced contained metal that "so far we think" was a low-gold alloy/ TBD.

I also did a 'calibration assay' where I just melted a sample of the lead in the cupel, to see if any of the metal was from contamination in the lead. That assay produced no bead, all the lead was soaked up, so it appears that my lead source is clean.


One part of the first assay, using the lead from the "ore/ borax/ lead in the furnace" process, was that* it was real hard to remove from the crucible.*

Somehow the crucible survived, I swear I was using a screw-driver as a make-shift chisel (along with a hammer) to chip away at the glassy material on top, to get to the lead underneath.


I would like to do another assay. But hopefully with a little smoother access to the lead when it's cooled off.

I want to get the lead out 8) without my blood pressure going up !

The Salamader graphite/ clay crucible has some draft angle, but apparently not enough.

Long story short - I want to do an assay using a ferrous crucible. I pick ferrous because -
* it melts above 2500 F and
* I know I can get a shape with a LOT of draft angle because I have access to a machine shop. Of course I would rather not make a steel crucible with a roughly "truncated cone" shape, I'd rather buy it off the shelf - but, either way.


*So my basic question is - is steel a suitable material for a lead-ore-borax type fire assay ?*


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## butcher (Dec 18, 2011)

I do not assay (beside some small expieriments), so some other member would be more help for you,

white clay crucibles are cheaper, and if you break one now and then, you would not get blood pressure too high.

Why such a big crucible when doing an assay? normally assays do not need much material in melt (about 29 grams of ore (assay ton) or so and flux, with room for foaming.

personally I would save that grafite clay crucible for more valueable jobs.

pouring your melt while hot to sooted cone mold can also help save your crucible.

Edit to add: melting in steel is a terrible idea unless you like soldering your values to the steel.


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## Photobacterium (Dec 18, 2011)

butcher said:


> I do not assay (beside some small expieriments), so some other member would be more help for you,
> 
> white clay crucibles are cheaper, and if you break one now and then, you would not get blood pressure too high.
> 
> ...



*OK - sounds like steel is not a good material choice.*

I filled the crucible up about halfway. In that case I got about 120 grams of lead, but my cupels only took about 15 grams. So I poured the lead into a sheet, and cut it up into pieces and weighed out 15 grams.

True, I may be using an adequate amount of lead.

*Anyway, I would like to find a crucible that is compatible with the materials, and has a generous draft angle so that it is easier to get the end-result out of the crucible.*

I don't expect the glassy material to part easily from the lead, but it would be helpful to be able to let the material cool, take the crucible out of the kiln, let it cool some more ... then turn it upside down and have it just fall out of the crucible - without the wrestling match.


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## butcher (Dec 18, 2011)

http://www.actionmining.com/Catalog.PDF


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## Photobacterium (Dec 18, 2011)

butcher said:


> http://www.actionmining.com/Catalog.PDF



thanks a lot !

what an *awesome *selection of gear.


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## jimdoc (Dec 18, 2011)

You can check here also;
http://www.legend-reno.com/

Jim


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## goldsilverpro (Dec 18, 2011)

Forget the iron crucibles. Really bad idea.

These are assay crucibles. The 30 gram ones cost about $1.50 each.
http://www.lmine.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LMS&Category_Code=fire_clay_crucible

In your 1st post, I see no indications of any sort of proper assay procedure. It's like you're just melting the stuff in a hi-dollar crucible and throwing in whatever. You're adding lead in some form, but where is the cupellation? You seem to have no idea of what you are doing.

If you really want to fire assay, I would suggest getting a book and studying it. The ones by Bugbee or Shepard and Dietrich in this link will straighten you out.
http://www.lmine.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LMS&Category_Code=assaying


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## Harold_V (Dec 19, 2011)

Photobacterium said:


> *Anyway, I would like to find a crucible that is compatible with the materials, and has a generous draft angle so that it is easier to get the end-result out of the crucible.*


Sigh!
Did you read the comment about a cone mold?
Need I say more?

Why aren't you using clay crucibles, which are meant for the purpose? Graphite clay crucibles are too expensive to be wasted the way they're being used in your process. 
Lead is generally added in the way of litharge. Does your operation lend itself to that lead source? It does more than just collect values, as you likely know. 

Harold


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## Dirtdiggaler (Jan 4, 2012)

No on the steel. I am guessing that you are trying to do a smelting process for concentrates, with a lead collector and industrial cuppeling that you only use if you do not have 30% metal in your concentrates not a fire assay . The proper way to do an assay is to use 29.16 grams of ore. That way every milligram of metal that you recover equals 1 ounce per ton. You need to use litharge. Litharge is powdered oxidized lead that mixes with the ore easily and hopefully contacts every particle of precious metal in your ore. When you mix your flux you add flour that at 1000C wants to burn but can't because at that temperature it is a reducing atmosphere with no oxygen so the flour uses the oxygen from the litharge which reduces the litharge to elemental lead. I believe that at that temperature the lead is highly corrosive and it corrodes all the metallic minerals into it, but it could also alloy with the metals either way when the temperature of the charge starts to reach the melting point of the ore the tiny lead beads with the precious metal in them start to drop to the bottom of the crucible and collect. After about 45 minutes you pour the charge into a conical mold. After the charge cools, you have a lead prill that you measure to see how much lead you reduced. Hopefully, if you had your flour right, the cupel will handle it. When you cupel the lead you leave the furnace door open a bit so the lead and other metallic minerals can start to oxidize which breaks the surface tension of the liquid so it can be absorbed into the cupel. The noble metals that you are looking for in the assay do not oxidize at the cupeling temperature so you are left with a little bead that you weigh. If you have a 10 milligrams of bead and you did your assay right, you have ten ounces per ton. If the bead is silver colored you dissolve the silver away in nitric acid, then weigh the remaining metal to see how much silver you have. You add other things to your flux so it pours properly and it is the proper PH - but it can be overwhelming to try to learn assaying all at once. The best thing to do is buy your flux and a manual to get a better handle on assaying. Buy flux, 30 gram clay crucibles, a scale and 30 gram cupels. Hope this helps - it is complicated but with the proper training and supplies, it is the best way to operate as a hard rock miner because it is an accurate way to see if your ore has what you think it has in it and to see if your recovery method is working. Good luck!


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## Photobacterium (Jan 4, 2012)

dirtdiggaler said:


> No on the steel. I am guessing that you are trying to do a smelting process for concentrates, with a lead collector and industrial cuppeling that you only use if you do not have 30% metal in your concentrates not a fire assay . The proper way to do an assay is to use 29.16 grams of ore. That way every milligram of metal that you recover equals 1 ounce per ton. You need to use litharge. Litharge is powdered oxidized lead that mixes with the ore easily and hopefully contacts every particle of precious metal in your ore. When you mix your flux you add flour that at 1000C wants to burn but can't because at that temperature it is a reducing atmosphere with no oxygen so the flour uses the oxygen from the litharge which reduces the litharge to elemental lead. I believe that at that temperature the lead is highly corrosive and it corrodes all the metallic minerals into it, but it could also alloy with the metals either way when the temperature of the charge starts to reach the melting point of the ore the tiny lead beads with the precious metal in them start to drop to the bottom of the crucible and collect. After about 45 minutes you pour the charge into a conical mold. After the charge cools, you have a lead prill that you measure to see how much lead you reduced. Hopefully, if you had your flour right, the cupel will handle it. When you cupel the lead you leave the furnace door open a bit so the lead and other metallic minerals can start to oxidize which breaks the surface tension of the liquid so it can be absorbed into the cupel. The noble metals that you are looking for in the assay do not oxidize at the cupeling temperature so you are left with a little bead that you weigh. If you have a 10 milligrams of bead and you did your assay right, you have ten ounces per ton. If the bead is silver colored you dissolve the silver away in nitric acid, then weigh the remaining metal to see how much silver you have. You add other things to your flux so it pours properly and it is the proper PH - but it can be overwhelming to try to learn assaying all at once. The best thing to do is buy your flux and a manual to get a better handle on assaying. Buy flux, 30 gram clay crucibles, a scale and 30 gram cupels. Hope this helps - it is complicated but with the proper training and supplies, it is the best way to operate as a hard rock miner because it is an accurate way to see if your ore has what you think it has in it and to see if your recovery method is working. Good luck!



Thanks for the many replies.

The first procedure that I did, I used -
1/4 cup lead powder
1/4 cup ore sample
1 ounce borax

2 hours @ approx. 1850 F in a kiln, the temperature rising asymptotically to 2000 F. 1 1/2 hours with that kiln won't quite melt silver (1700+), 3 hours will melt gold (approx. 1950 F).

I found out real well what litharge is because I inadvertently made a lot of it.

The procedure I used was from a mining textbook on the Internet, before I knew about GRF.

Like I said in the first post, the major problem was the lack of draft in the container used to do the procedure. I am used to parts coming out of a 'mold' much easier than that.

The cupels I used (for the lead/TBD metal mixture) were bone ash. I got a gold-silver color bead, less than .1 grams.

I since got both kinds of crucibles. In the long run I will use the cheap ones. For the next assay I will tend to pick whichever one has the smoothest sides & closest to a straight "truncated cone" shape because I don't want to wrestle with the lead-glass 'casting' again.

Do people ever use real cheap crucibles for fire assays & just smash them when it's time to get ahold of the lead with the TBD metal in it ?


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## Harold_V (Jan 5, 2012)

Photobacterium said:


> I since got both kinds of crucibles. In the long run I will use the cheap ones. For the next assay I will tend to pick whichever one has the smoothest sides & closest to a straight "truncated cone" shape because I don't want to wrestle with the lead-glass 'casting' again.
> 
> Do people ever use real cheap crucibles for fire assays & just smash them when it's time to get ahold of the lead with the TBD metal in it ?



So then, from your remarks, I gather you're not here to learn anything, just to report on what you're doing? (Not a problem. I'm just curious, in particular when you're fighting with something that has a ready solution.)

Have you wondered why others who perform assays don't have the same problem? 

Did you happen to notice my comment about a cone mold?

Are you having a problem understanding what has been mentioned? If so, would a basic explanation help you understand?

Harold


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## Dirtdiggaler (Jan 5, 2012)

"The first procedure that I did, I used -
1/4 cup lead powder
1/4 cup ore sample
1 ounce borax

2 hours @ approx. 1850 F in a kiln, the temperature rising asymptotically to 2000 F. 1 1/2 hours with that kiln won't quite melt silver (1700+), 3 hours will melt gold (approx. 1950 F).

I found out real well what litharge is because I inadvertently made a lot of it.

The procedure I used was from a mining textbook on the Internet, before I knew about GRF.

Like I said in the first post, *the major problem was the lack of draft in the container used to do the procedure.* "



Many ores are oxides of metals; when you are trying to reduce an oxide ore you need to have a reducing atmosphere with no oxygen. So " lack of draft in the container " is not your problem.its any draft at all. You would be better off just adding carbon to the crucible with your ore and making sure you have no draft. When metallic oxides are heated to sufficient temperature in a zone where there is a deficiency of oxygen the oxygen in the ore itself separates and joins with the free carbon ions, and this reduces the ore to an oxygen free metallic state. The resulting metal would drip down through the carbon and form a metallic slug on the bottom of your crucible. If your ore is a carbonate or a hydroxide it will convert to a oxide from the heat. If your ore is a sulfide you need to convert it to an oxide before you try to extract the metal by roasting it. To roast it heat it in a iron pan with the lid off over a fire stirring it until it stops smoking. Then put the roasted ore in the crucible with the carbon and heat. If you don't have enough metal in your ore it won't collect in the bottom of the crucible and you will have little beads that you can pan out after the crucible cools.


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## qst42know (Jan 5, 2012)

You are not getting the point.

While molten the material is poured from the crucible into an iron mold. The button reports to the bottom.

A mold like this.

http://www.lmine.com/mm5/merchant.mvc?Screen=PROD&Store_Code=LMS&Product_Code=18390&Category_Code=slag_molds


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## Westerngs (Jan 5, 2012)

Ore assay procedure:

29.16 grams sample
60 grams litharge
30 grams soda ash
15 grams borax glass (can also use 5 mol borax, etc.)

Use one of crucibles here http://www.lmine.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LMS&Category_Code=fire_clay_crucible. I prefer the AP green crucibles, they are more durable.

Fuse muffle furnace for 1 hour then remove crucible and swirl to mix contents. Pour into cone mold like one of these http://www.lmine.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LMS&Category_Code=assay_molds. Let cool then break slag away. Weigh . You want a lead button in the 30 gram range.

If you get a lead button in the right range then flux mix is OK. Repeat assay except this time you want to add pure silver inquart about 0.2 grams. Weigh accurately to 4 decimal places. 

If lead button too large, add potassium nitrate. Each gram will reduce the lead button by approx 4 grams. If lead button too small, add flour. Each gram flour will increase lead button by approx. 10 grams. Once you find good flux mix, repeat assay with pure silver inquart as above.

Cupel lead button at 1750 for approx. 1 hour in muffle furnace. Remove cupel from furnace, cover with empty hot cupel and let cool. Weigh bead. Flatten and dissolve silver in 1:7 nitric acid (1 part nitric 7 parts distilled water) for 1 hour on hot plate at just slightly below boiling point. pour off acid and add 1:1 nitric acid, heat again for about 1 hour. rinse well. Collect gold, dry and weigh.

If the bead did not dissolve in the nitric acid, you will need to repeat the assay with increased silver. You need at least a 3:1 silver to gold ratio for the silver to dissolve. I prefer a 4:1 ratio.

As Dirtdiggaler said, every mg of gold recovered equals 1 oz per ton in the ore. Same with the silver. Remember to subtract the silver added as inquart.


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## Photobacterium (Jan 6, 2012)

Westerngs said:


> Ore assay procedure:
> 
> 29.16 grams sample
> 60 grams litharge
> ...



thank you for sharing that procedure.

>> http://www.lmine.com/mm5/merchant.mvc?S ... slag_molds

beautiful, thanks.

i also found the ceramic (or ceramic-ish) cone shaped crucibles.


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