Wednesday, January 4, 2012

Gold Ore Fire Assays

 (Fire assays have been around for a long time as this woodcut from the Middle Ages shows.)

Fire Assays are the Way to Go

I've received quite a few queries here as of late from folks wanting to know the why's, how's, and where's of getting their ore samples assayed. So, I thought I'd write this as an interim post before continuing on with my series on gold in California's coastal ranges.

First of all, for those you who are strictly placer gold miners you probably are not going to need the services of an assayer at any point. However, if you are hard-rock gold miner (gold or silver), a rock hound, or a placer miner who has some ore samples you'd like to have tested then a fire assay is the way to go.

 Fire Assay Steps

What's a fire assay? It's the oldest, cheapest, and probably the most reliable method of determining precious metals concentrations in rock (ore) samples. The process received its name because back in the old days the sample was smelted along with lead oxide in a very hot fire. These days, electric furnaces are used for smelting.

There are essentially six basic steps to a fire assay:

1. Crushing (an ore sample is crushed and ground into a fine powder)

2. Splitting (the sample powder is split into additional samples)

3. Weighing (powder samples are weighed down to 30 gram units)

4. Mixing /Firing (lead oxide is added as a firing flux and any gold or silver present will mix with the flux. After firing and cooling, a solitary metallic "button" remains containing the lead oxide and any precious metals)

5. Cupelling (the "button" is placed into a cupel or dish made from bone ash and fired in what's known as a cupelling furnace)

6. Parting (here the lead returns to its oxide form and is drawn into the bone ash plate, leaving a tiny speck of metal called a "parting" bead)

Since Our Main Interest is Gold...

The "parting" bead is then weighed on a micro-balance. Since our main interest as miners is in gold and not silver, a further step is needed here and that is to heat the precious metal bead in hot nitric acid which dissolves the silver and leaves only the gold.
 (Gold bead remaining from fire assayed ore sample.)

The gold bead is then carefully weighed and this amount of gold is then compared to the original weight of the ore sample(s). This gold value will typically be expressed as "troy ounces to the ton" for larger mining enterprises (e.g., "This ore sample indicates .089 troy ounces gold per ton."). For you and I, the result of a fire assay on a single ore sample we submitted would probably be expressed in grams or fractions of grams of the gold contained in that sample alone.

Cost of a Fire Assay

How much does a typical fire assay cost these days? That depends. I've seen prices quoted as low as $30.00-$50.00 per sample on up to thousands of dollars per sample. In terms of my personal knowledge and experience, as an individual miner you're better off dealing with a lower priced "mom and pop" fire assay operation than some of the bigger name, corporate type assay businesses. The latter tend to cater to larger mining companies and charge much higher fees.

Gold Concentrators

Are there reputable folks and companies out there conducting fire assays? You bet. These are the folks you want to deal with because they are competent, cheaper, and won't rip you off. If you need a lead or two in that direction, let me know.

Best of luck to you all.

If you liked this post, you may want to read: "Gold in the Southwest: New Mexico (Part 4)"

(c)  Jim Rocha (J.R.)  2012

Questions? E-mail me at jr872vt90@yahoo.com


4 comments:

  1. Hey I can do this! Thanks for the post

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  2. So basically the fire assay will tell you the overall gold content, and no further detail as to what it could be associated with....?

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  3. A good fire assay can provide you with a pretty accurate accounting of the various constituents in the ore, not just the gold content. Best, J.R.

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