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Author Topic: Tips on doing a fire assay  (Read 14190 times)

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Offline Johnny

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Tips on doing a fire assay
« on: December 28, 2008, 06:15:37 AM »
Howdy fellow prospectors, I was wondering if any of you have a better education in chemistry than I do? ???
We've been prospecting and rock hounding for many years, naturally we have collected several pieces of equipment over the course of things. Finally decided to "bite the bullet" and buy a kiln so we can find out what is really in all those interesting pieces of ore.

Not wanting to spend a huge amount on equipment we really don't know how to use, we bought an inexpensive top loading propane fired kiln. It seems to get up to 2200 degrees or so and darn sure is able to melt anything I have put in there so far!

We've discovered that the kiln was just the start... tongs, gloves, face shield, apron, hammers, flux, scales, sifters, crushers, molds, and books aplenty.

One tip I can share allready is not to get greedy and put too much material in a crucible... it'll boil over and leave ya a mess in the bottom of the kiln. The batch I messed up with assayed nearly 40% gold... only thing is it's stuck to the inside of the lil kiln... sheesh ^#!

Offline Johnny

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Re: Tips on doing a fire assay
« Reply #1 on: December 28, 2008, 10:32:30 AM »
I was hoping that someone would have a little experience with the different recipies for flux... seems to make a lot of difference .... but as I said... it's been a long time since high school chemistry don'cha know!

Offline Seeker

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Re: Tips on doing a fire assay
« Reply #2 on: December 28, 2008, 01:16:04 PM »
Johnny, check out this link... The Art of Fire Assaying

Also think there is something in the downloads area about fire assaying  <-dont~know->

Offline Johnny

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Re: Tips on doing a fire assay
« Reply #3 on: December 29, 2008, 08:05:56 AM »
Howdy Seeker
Excellent site... Thank You! Kinda funny how there is a learning curve to everything! It's truly become an adventure in applied education. Back in school... nearly 40 years ago I never thought I'd ever use any of that stuff they made us learn... I mean really... Periodic Table of Elements... who'd a thunk it!

Offline Johnny

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Re: Tips on doing a fire assay
« Reply #4 on: December 31, 2008, 07:24:21 AM »
Here are the steps that I took to "assay" my gold from Walker AZ.

1. Examine the ore to see if you can tell first if there is gold, how much pyrite vs. silver for example.
2. Crush the ore into dust... the finer the better it seems. Measure out one oz. (this is the assay)
3. Mix the assay with 3 oz. of flux. Place the mixture in a 30g crucible.
4. Put the crucible into your pre-heated kiln at 2000 degrees F. Roast for at least an hour (60 minutes)
5. Using your safety equipment, tongs, face shield, welding gloves, apron, with a 5 gallon bucket of soda water nearby, (just in case) pour the molten assay into a cone mold.
I use a small pinch of borax glass in the mold to keep the assay from sticking.
6. When the button sets up remove it, can still be hot/warm so be sure to use your tongs.
7. Pound out the button into a "cube" to remove most of the slag.
8. Put the button on a cupel and place back into the hot kiln for as long as it takes for the cupel to absorb the "lead" leaving you a nice little bead of .999 fine
9. weigh the bead this and a little math will tell you how much gold there is per Assay ton.

We've done this with raw high grade ore and with panned concentrates. A little practice and you too can make your own gold bars.;-)

Offline iso9001

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Re: Tips on doing a fire assay
« Reply #5 on: December 31, 2008, 10:09:36 AM »
Safety tip for casting with any kind of molten metal:  Make sure your mold if free of any water unless your sand casting.  I once welded my eye shut while pouring molten aluminium into a steel ingot mold that was damp.  Steam formed a bubble inside, made a weird shrieking sound them Ka-Frickin-Boom!  I still have eyesight thanks to the increadible human blink reflex... and wear safety goggles, not just glasses like what I was wearing.

Offline deserdog

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Re: Tips on doing a fire assay
« Reply #6 on: December 31, 2008, 03:06:25 PM »
I found out the same thing when we were isothermic welding on a job. You use a graphite mold. And you always, always heat the mold before you use it! A little bit of water expands over a 1000 times when it turns to steam, so it only takes a tiny bit of water to cause disaster!
Cannot find if you do not look!

Offline Johnny

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Re: Tips on doing a fire assay
« Reply #7 on: January 24, 2009, 04:28:00 PM »
Absolutely right, safety first, I actually roast my crushed samples at 500 degrees for an hour to drive off all the sulfides... especially important if there is much pyrite. The sulfides will come off in a yellow cloud when it gets to temp. The particles turn black mostly iron left, use a magnet.
The point to all this is to be sure the sample is dry! If you bake it first you get no chance of water or sulfides!
I use a full face shield, welders gloves and long sleve jacket, apron, and tongs. Leather boots are best... sure don't want to drop molten ore on those $3 tennis shoes.
Be sure the mold you are pouring into is baked dry as well... once again the BBQ comes in handy.

Offline Chuxgold

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Re: Tips on doing a fire assay
« Reply #8 on: March 12, 2009, 04:50:20 PM »
Should always wear a respirator with active carbon filter. If you can smell it your breathing it in. Something I have said before but can't be said enough. Do not stand directly up wind as this causes a vacuum that will pull it right to you. Stand to one side allowing the air to pass by.
Chuxgold. <-thinking->
Give self, to gain wisdom,

Offline Johnny

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Re: Tips on doing a fire assay
« Reply #9 on: March 13, 2009, 04:06:05 PM »
Great safety tip Chuxgold!
I keep a 5 gallon bucket full of water and stir in as much baking soda as it will carry. Keep it a couple of steps away NEVER NEVER GET WATER NEAR THE KILN OR YOUR POURING AREA!
Be sure your pouring mold is pre heated to assure there is no moisture at all in it!
Wear an apron so there is at least 2 layers between you and any "accident", Wear a full face mask, wear your ball cap backwards to protect your neck. Naturally you should use welding gloves or furnace gloves and long tongs, not the greasy old channel lock pliars outa the garage.
It can be very hard to remove all the silica from your sample, when it melts to glass it can sorta spit and bubble in the crucible... kinda like thick spaghetti sauce. It's easier to get burnt than you think! REMEMBER SAFETY FIRST!
Happy Trails,
Johnny

 


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