Fire assaying: An introduction

Finally! I had the time and motivation to write, edit and finally finish this STEM blog. I’ve been meaning to publish this one since January, unfortunately, I’ve been all over the place since and now I’m back in the city lab.

For today, and probably for the next few days, I’ll be writing about fire assaying, which is the analysis I had to go to the mountain site to be trained for.

this is what the start up process looks like. Without the sample preparation and all. That's when the sample is already smelted in the muffled furnace.
Source: https://www.911metallurgist.com/blog/fire-assaying-smelting-process since I wasn't authorized to take pictures on site

So Fire assaying. This is counted as one of the most ancient methods or artforms that can still accurately measure the gold contents in gold alloys or ores. In the lab, this is one of the most sensitive analyses we have. Everything needs to be exact here yet there’s still room for trial and error. We call it an artform because it’s not something you can simply read about then perform like the usual laboratory procedures we have in the city lab, no. This one needs experience, plenty of experimenting and plenty of samples. It takes years and years of experience before one can actually perfect it, or at least understand what went wrong when something goes wrong in the process and not all the gold was extracted. You also need a better understanding of what each component contributes to the process and what’s lacking or in excess whenever the metal separation becomes incomplete.

I do find it funny though since evidence does point that although fire assaying is widely used for gold, yet the first metal this method was used for was for silver purification. And I’m a bit proud to say that its origins can be traced in minor Asia in the B.C. era. Sadly, I can’t find a rough estimate of when, but in the books we had in the mountain site, the fire assaying method was refined after the manufacture of lead monoxide from galena (lead ore). And the first few artefacts that can be dated on the use of this method are the silver-lead buttons found at Mahmatlar. These artefacts are now on display in the Hittite museum in Ankara, Turkey and I want to see that one day because the lead they used back then was still unrefined and unstandardized so who knows what the lead buttons looked like back then.

But that bit of information was in its starting phase, when only silver was the known application of this method. In the mid 1400s, the method was revisited, reassessed and redefined. It was in the book of “De Re Metallica” (Agricola, 1556) It was in this book that the application of the method expanded to other noble metals such as gold, palladium, platinum, and in some cases, mercury since it’s a prominent interference. This era was definitely the golden era of mineralogy since methods used in the field still refer to the methods of this era, and I find that truly amazing because of how sophisticated the processes were already in the 1400s.

The original manuscript of the book, though we have the translated version in the lab then here's a link to the book in case you're interested in giving it a look: https://www.gutenberg.org/files/38015/38015-h/38015-h.htm

I kept mentioning lead monoxide before and to explain that, you must understand that Lead oxide is a key component in the fire assaying method since this is where your metals (whether it be base metals, heavy metals or precious metals) bind to while your organic matter tends to form a crystalline top, that which we call slag. This has been used since the discovery of its alloy forming properties with most metals and it’s still used today, but now it’s referred to as litharge. This material is basically why when we have gold extraction, we get confined to only certain rooms for the duration of the assaying session. On top of that, we get blood checked every 3-6 months to ensure that the lead doesn’t get absorbed by the skin, and if so, it doesn’t reach toxic levels. And that level is 5 micrograms/ deciliters. Anything exceeding that is already considered toxic and is in need of detoxing. Luckily, the hazard suits are enough protection that the litharge doesn’t get inhaled nor absorbed by the body.

This is red and yellow litharge. The ones mostly used in site was the yellow one, but these lead oxides are often used as dyes too, aside from actually being used in assaying
Source: https://www.gutenberg.org/files/38015/38015-h/38015-h.htm

This method is definitely toxic, yes, and it’s questionable why it’s still in practice, but then… I don’t think there's a different method that can replace this classic method. Even aqua regia and then electrolysis has its limit like when the gold or silver concentration gets too high, then we have no choice but to do a gravimetric assay just to make sure we’re getting the values right.

Sources:
http://dx.doi.org/10.13140/RG.2.2.22438.78408

Battaini, P., Bemporad, E. & De Felicis, D. The fire assay reloaded. Gold Bull 47, 9–20 (2014). https://doi.org/10.1007/s13404-013-0101-1

Smith EA (2003) The sampling and essay of the precious metals 1947, 2nd edn. Met-Chem Research, Albuquerque, Reprinted 2003

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So this will be just a brief intro, the next part, I'll go at it by step and by component used in that process. Thank you for stopping by~~
I shall go sketch now UwU

Posted with STEMGeeks