Well, I have been studying up on assays tonight, just because it is something I want to start doing for myself. I have a good number of prospects going, and it would be nice for me to be able to find out what I have without having to send a million samples out :P Well, not literally but I have enough rock on hand I probably could!
So I thought I would talk a little bit about what Fire Assays are, and how they are done, in general terms of course. Maybe I will give specific how to guide later on, but this is strictly for informational purposes to give you insight into how your ore results are derived. It is quite a fascinating process!
First step, is to take a representative ore sample... Generally you want to get an average over the entire exposed vein, that way you can have a more accurate estimate of what the value is, since you will tend to have about the same proportions of waste rock to actual metal, but it will vary from place to place along the deposit of course.
The next step is to finely pulverize the sample, and mix it thoroughly. Then a specified proportion is taken from it, usually 29.167 Grams, for which is called an "Assay Ton", results are reported in amounts per short ton which is 2,000 pounds, and there are 29,167 ounces Troy in a short ton. In this manner, calculating the results is simple. Every milligram of metal recovered is one ounce per ton. So if you have 9 milligrams of gold and 25 milligrams of silver, your ore is said to have 9 ounces gold and 25 ounces silver per ton.
The fluxes are mixed according to the type of ore and the form the metals are in, whether they be oxides or sulfides. Then the ore is placed on top of the flux, and heated at 2,000 degrees for a period of around 45 minutes... The molten charge is then poured into a conical mold, and after it cools, it is dumped out, and the metal cone, called a prill, is busted free from the slag with a hammer, and generally from there it is pounded into a cube, placed on a cupel, and that is heated until all the lead from the flux (which collected all the precious metals in the ore sample), is absorbed into the bone ash or magnesia cupel. Anything left on the cupel after the lead is gone, is precious metals. The silver can be parted from any gold by dissolving in nitric, and then precipitating it out, any metal left that didn't dissolve, is gold or platinum group metals.
What you end up arriving at is a sample of pure gold and a sample of pure silver that can be weighed and calculated. Of course there may be easier ways of determining the proportions of what comprises the metal left from cupelling, but for someone low tech like me, chemicals is probably the easiest way to go.
And that's pretty much how an assay works in a nutshell. I know there are people out there who want to know the nuts and bolts of how things work... A little later on I might go into more detail and talk about some of the chemical processes that happen inside the crucible when the smelting is going on... But it's a little more complicated than I feel like going into at the moment. Thanks for reading!
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