Gravimetric Assay for the newb

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snoman701

Well-known member
Joined
Oct 8, 2016
Messages
2,108
Location
SE MI
I have spent a reasonable amount of time developing technique and understanding processing, where I haven't spent time developing skill is the assay. And it's been said more than once that you make your money in the assay.

I have no difficulty in the understanding of procedure for material like karat scrap, silver, or even the higher grade PGM's. Where it seems like it's all technique, is the assay of low grade scrap.

The idea of digesting a 2 gram pin full of copper and every other base metal to get MAYBE .02 grams out, and not making money unless you can do it to within .005 grams is obviously a bit intimidating, but I'd like to start.

So here's the actual questions:

Scorification vs Cupellation - Cupellation seems to work better for lower melting point impurities, but if it's a high copper/nickel material, scorification works better I believe. Given a low grade material, how many scorifications should one expect to do before they can blow off a reasonable portion of the copper? Is this where the addition of silver helps an assay? Loss of gold during this stage?

How difficult is the actual technique of digesting the copper bead after rolling it out? And not losing gold when decanting...as in my experience, it's not easy. Not at all. And it doesn't seem like it's enough gold to anneal in to a
 
I have never experienced significant gold losses when using the scorification technique on copper based bullion. And by using the right sample size to granulated lead ratio it usually is good after 1 fusion. This is determined by a lack of blue (or lack of significant blue) in the parting phase.

And the samples recovered are too small to anneal, although I always found it easier to handle the samples and transfer them to the balance pan after annealing.

These are instructions I had written up a while back to teach a client the process, is it similar to what you are doing?

Scorification is an oxidizing fusion in a shallow fire clay dish called a scorifier. It uses a small sample, a minimal quantity of acid flux and granulated lead. This process is used if a cupellation of a copper based bullion produces a bead with visible black copper oxide on the button. The excess lead in this process will oxidize the base metals along with the lead collecting the precious metals in the lead while the base metals form a clear slag which contains a glass metal oxide mixture which is easily separated from the lead.

• Prepare the metallic sample by rolling as with karat samples and cut into small pieces.
• The sample you will be weighing out can be placed directly in the scorifier as the scorifier and its charge are put into the furnace cold.
• For gold filled or copper base bullion, add 5 grams of sample to an empty scorifier.
• Weigh out 2.5 grams of assay silver and add it to the scorifier.
• Cover the sample and silver with granulated lead, the proper charge for a 3 inch scorifier is 60 grams.
• Sprinkle a teaspoon of borax glass on top of the lead.
• Prepare all of the samples and charge them into the furnace at the same time.
• The samples should be charged into a furnace preheated to 900 degrees C.
• After about 10 minutes the hole in the furnace door can be unplugged to provide air to the reaction.
• Unlike a cupellation which produces a visibly spherical bead when done, the scorification is complete when the molten pool of lead is about half of the starting volume. Generally this will take an hour.
• To finish the scorification and aid in the pouring, raise the temperature of the furnace to 1050 degrees C for the last 10 minutes.
• Grasp the scorifier with a pair of crucible tongs and pour the slag and lead into a depression in the assay cone mold. After pouring hold the inverted scorifier over the mold to drain all of the metal and slag. The lead will fill the mold and the slag will overflow the mold. A metal tray with a lip will keep the hot slag from running about the floor.
• Cover the glassy slag with a used crucible to allow it to cool. Often the glassy slag will shatter while cooling and the crucible will keep it from flying about.
• When the glass is cool, invert he mold and drop out the cone shaped lead buttons.
• Hold the lead button in a gloved hand and place it on edge on the bead anvil. Strike it sharply with a ball peen slag hammer at the slag, lead interface to break off the slag.
• Using the hammer, pound the bead into a cube shape. This will loosen any remaining slag which should be removed with a bead brush.
• The lead cubes are now cupelled in a manner identical to the lead boats used for karat assays with the one exception being you will use the larger 1.5 inch diameter cupels.
• Because you will be driving off more lead than in a typical bullion assay, the time for driving the lead will be a bit longer.
 
I've probably run 10,000 fire assays in my life, mainly on electronic components, and can't remember scorifying more than a half dozen times. There are tricks you can use. For hi copper, you can put a spoonful of yellow sulfur on the sample in a crucible, heat it for about 30 minutes, which converts the copper to a sulfide. Then, add the flux and run a regular fusion. I think that's about right. It's been a long time. Also, I seem to remember you can get away with as much as 2g of copper without doing anything special. I didn't need to treat the Cu that often and I could be off on the details. I do remember that it worked. You can always tell when there's a lot of Cu in the lead because it flattens out

It's all in the books. I don't remember what is in what book. Get a copy of the Bugbee assay book and the Shepard and Dietrich assay book from Legend. They're the best 2, in my opinion. They're only $20 each, but the last book I bought from them, they sort of gouged me on the shipping, I thought, Also, there are a few assay books on Google books that are "full view", that you can download. Probably the best modern author is Beamish. I once had a photocopy of his "big" book that ran over 1000 pages. He had some interesting methods using a nickel sulfide collector for PGMs, instead of litharge, which is not that great for PGMs. NiS is supposed to work quite well.

Fire assaying is where you make the money. Not doing assays commercially, but for buying and selling. The person that knows the most about the value of the material, always wins. Study the art and math of sampling. Most $20 scientific calculators do simple statistics functions. If you have a math bent, learn to use those functions and apply them to sampling and assaying. With a bad sample and a perfect assay you end up with bad results. Assaying is easy - sampling is hard. Fire assaying is very forgiving when it comes to fluxing. Never waste money on pre-mixed fluxes. Just line all the flux ingredient containers in a row and go down the row with a teaspoon. The sugar or flour to reduce 20-30g of lead require a much smaller measure.

For electronic components, you must do both fusions and cupellation. For karat gold, cupellation only, in triplicate.
 
I wasn't able to find the sulfide conversion in bugbee, but that doesn't mean it's not there.

They do describe scorification, but I wasn't sure how effective it is, and what kind of losses can be expected. Not sure if I could get my little furnace to hold 1050 with the door cracked. It's a bit wimpy unfortunately.

,



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So whaddya say we turn this in to a tutorial. I will order the stuff I'm told I need. I'll start with a simple fire assay of karat scrap, then if you guys are willing to help, I will move on to more technically challenging fire assays of polishings, gold filled, electronics, etc.

I will not do a video, but will take pretty pictures.

So as of right now, I have some #3 Mabor cupels, but no bone ash. Should those work?

https://www.lmine.com/index.php?main_page=product_info&cPath=1_164_268&products_id=4844

Flux components, I have bone ash, silica flour, borax...need potassium carbonate and litharge

I think I have lead sheet...it's not silver/gold free, but I'm not going to be good enough to notice the difference yet!

And I can move my furnace outside so it's got good ventilation. That will work for the time being. I'm well versed in the dangers of lead poisoning...if you are reading this and you aren't, you should educate yourself.

I have a Mettler AT201 balance, even though I don't know how to use it.

I'll stop at the jewelry store tomorrow to see if I can buy some scrap, I just sold everything yesterday and all I have on hand are a few gold crowns right now.

Other things I know I'll need, and please make additions...

litharge
fire clay crucibles (what size? 30 gram?)
scorifier dishes (2.5" work?)
 
Actually,

I think this is a great idea. We need a fire assaying tutorial.

First thing people need to remember is that it is in fact lead, so where a respirator, dispose of the solid waste appropriately, and oh yeah, goggles, gloves, don't eat popcorn after, and...

have fun! Fire assaying is a quintessential skill.


Let's do lead then if you really want, we can do some NiS fire assays at my place Jake because we have ICP to finish.

NiS is nice because you can eliminate a lot of matrix with an HCl soak.

Another technique that people should really learn to use is the use of tellurium. A mighty fine collector for all things precious! I only have ever used for gravimetry (TeO2/SnCl2 in HCl) but apparently some of the big PGM refiners used to use it for waste scavenging!
 
Lou said:
Let's do lead then if you really want, we can do some NiS fire assays at my place Jake because we have ICP to finish.

Sounds fun! I've read about NiS but not past the "maybe someday" stage.

But...always thinking of the alternative option...

You can buy Dr Goch a new ICP with all the bells and whistles, and then you can just bring the old tired one up here and I'll recycle it for you. I've got just enough room on my balance table.
 
goldsilverpro said:
I've probably run 10,000 fire assays in my life, mainly on electronic components, and can't remember scorifying more than a half dozen times. There are tricks you can use. For hi copper, you can put a spoonful of yellow sulfur on the sample in a crucible, heat it for about 30 minutes, which converts the copper to a sulfide. Then, add the flux and run a regular fusion. I think that's about right.

Found a version of it in Bugbee, page 229.

"Sample down the finely ground bullion to about 0.25 assay ton and adjust the weight of the sampled portion to exactly 0.25 assay ton. Place in a 20 gram crucible and mix with 1.2 grams of powdered sulphur. Cover this with a mixture of 15 grams of sodium carbonate, 240 grams of litharge, and 8 grams of silica; but do not mix with the sulphur and copper, which should be allowed to remain in the bottom of the crucible. Cover with salt or flux mixture and place in a hot muffle so that the charge will begin to melt in 6 or 8 minutes. The fusions should be quiet and ready to pour in 25 or 30 minutes."

It goes in to further detail which states that as soon as the copper melts, it combines with the sulfur to form a matte, then as the litharge melts it oxidizes off this matte into the slags.

It's a lot of litharge, and per Bugbee, using that much litharge for a sample does increase chances of silver loss.
 
I am not sure if this Assay learning program ever got started but I am interested as well.

I have some of the materials on hand and would need to order others.

I am waiting on a company to rebuild my electric box furnaces controller. (It is a Linberg. Specs say 1200 c. Should be enough I think?)

I would also be willing to contribute some material like litharge, lead foil etc. That I have extra in stock if it helps get this rolling.

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rickzeien said:
I am not sure if this Assay learning program ever got started but I am interested as well.

I have some of the materials on hand and would need to order others.

I am waiting on a company to rebuild my electric box furnaces controller. (It is a Linberg. Specs say 1200 c. Should be enough I think?)

I would also be willing to contribute some material like litharge, lead foil etc. That I have extra in stock if it helps get this rolling.

Sent from my LG-H872 using Tapatalk

heh...I have to rebuild my furnace as well. I left it on on accident and burnt it up.

Rebuilding them is dead simple, based on your other posts, you should be able to handle it.
 
It is a vintage version I picked up from Old Dominion University. I think it has been fired a few times.

I havn't found documentation for it. The temp controller is not functioning. I am thinking of just replacing it with a Honeywell UNC 3000 I have.

Just havn't taken time to put it on the bench yet.

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