Assaying & Refining Gold
Hold a bright yellow nugget up to the light and a quiet question follows it like a shadow: how much of this is actually gold? Colour alone has fooled people for thousands of years — pyrite glints, alloys flatter, the eye is easy to bribe. So we learned to ask the metal a harder question and demand an honest answer. That answer is an assay: a careful, repeatable test that turns a hopeful "it looks like gold" into a number you can stake money on. Refining is the next chapter — the patient work of taking metal that is mostly gold and coaxing it toward almost entirely gold.
This page stays conceptual and historical. It is not a how-to. Assaying and refining use furnaces, strong acids, and toxic gases, and they are the work of trained specialists in proper facilities.
What an assay actually is
An assay is a measurement of how much precious metal a sample really contains — not what it looks like, but what it is.
- It replaces appearances with evidence: a tested, weighable result.
- It is how ore, scrap, jewellery, and bullion are all held to the same standard.
- A good assay is repeatable: run it twice, get the same answer.
The result is usually expressed as purity. In the jewellery world that means karat (pure gold is 24 karat), and in the bullion world it means fineness — parts per thousand, so "three-nines fine" is 99.9 percent gold. (See Fineness, Karat & Purity for how those two languages line up.)
The fire assay: ancient, and still the gold standard
The fire assay is thousands of years old and still considered the most reliable way to measure gold, silver, and most platinum-group metals in an ore or concentrate. The idea is beautifully stubborn: don't trust the surface, melt the whole thing down and let physics sort it out.
- A weighed sample is fused in a crucible with fluxes and a lead source.
- The lead gathers the precious metals and sinks, forming a dense "button" at the bottom while the worthless rock floats off as glassy slag.
- That button is then cupelled: heated in a porous dish (a cupel) under a blast of hot air, so the lead and other base metals oxidise away — partly vaporised, partly soaked into the cupel's pores.
- What is left is a tiny shining bead of precious metal. Boil it in acid to dissolve the silver (a step called parting), weigh the gold that remains, and you have your answer.
It is a lovely piece of logic: lead is the workhorse that carries off everything unworthy, then sacrifices itself, leaving only the metals that refuse to burn.
The touchstone: assaying by "touch"
Long before furnaces ran all day, there was a faster trick — and it still works. The touchstone is a black siliceous stone (the old "Lydian stone"), and testing by "touch" was one of the earliest ways anyone judged precious metal.
- Rub the suspect piece across the stone to leave a streak; rub a known sample beside it.
- Treat the streaks with acid, which attacks impurities and sharpens the contrast.
- Compare the marks — the truer the gold, the better it survives.
It is quick, portable, and surprisingly honest for gold. It is not laboratory-exact, but as a field guide to quality it has earned its keep for over two and a half thousand years.
Refining: from "mostly gold" to "almost all gold"
Mined gold arrives carrying company — zinc, copper, silver, iron. Refining sends those companions away. Two classic processes do most of the heavy lifting.
- The Miller process (chlorine, patented 1867): melt the impure gold and blow chlorine gas through it. Almost every impurity grabs the chlorine more eagerly than gold does, so the base metals turn to chlorides and rise off as a skim. It is fast and cheap and lands gold around 99.5 percent pure — fine enough to cast into market bars.
- The Wohlwill process (electrolysis, introduced in the 1870s): dissolve impure gold as an anode in an acid-and-gold-chloride bath, then plate pure gold onto a cathode while the impurities drop out as slime. Slower and more costly, but it reaches roughly 99.99 percent — the realm of the very purest gold.
In practice the two are partners more than rivals: Miller for speed and scale, Wohlwill when the world wants four-nines metal.
In Lodester
Out in the field, Lodester is your sampling companion: it reads the ground, weighs the signals, and shows you favourability — where the "colours" are worth a closer look. But remember what the map is and isn't. Favourability is an invitation, never a promise; it points you toward ground that deserves an assay, it doesn't hand you a karat number. The assay is the rigorous next step, the moment a hopeful streak becomes a real figure — and that step belongs to trained assayers with the right furnace, acids, and care. Lodester helps you find and sample; the lab tells you, finally and honestly, what you found.
General educational guidance only — not instructions for handling furnaces, acids, or toxic gases. Assaying and refining are hazardous, specialist work for trained professionals in proper facilities.