How Is Gold Refined? The Chemistry That Turns Old Gold Into 24K

Gold is refined by separating it from every other metal it was mixed with, using heat, chemistry or electricity. The Miller process blows chlorine gas through molten gold to reach about 99.5 percent purity, and the Wohlwill process uses electrolysis to reach 99.99 percent or higher. For readers in Palm Beach who wear gold every day, here is how the metal in your jewelry became pure enough to shape.

Why does gold need refining at all?

Gold rarely appears alone. In the ground it is mixed with silver, copper and other metals. In recycling, it comes back as old jewelry, workshop filings and electronics, already alloyed with whatever made it useful before. Before a jeweler can make 18K or 14K gold with a precise color and hardness, the gold has to be taken back to pure metal, so the new alloy can be measured exactly.

Gold makes this possible because it is chemically noble. It resists oxygen, most acids and corrosion. Refining uses that stubbornness: you attack everything else and leave the gold behind.

How do refiners measure purity? Fire assay

Before refining, you need to know what you have. Britannica describes fire assay as the most reliable method for measuring gold in ores and concentrates. The sample is melted with litharge (lead oxide), flux and flour. The lead collects the precious metals into a button, the lead is then oxidized away, and the remaining bead is boiled in nitric acid, a step called parting, to dissolve the silver. What is left is gold, and it is weighed.

It is old chemistry, and it is still trusted because it is so direct: separate, then weigh.

What is the Miller process?

Patented by Francis Bowyer Miller in 1867, according to Wikipedia, the Miller process works by blowing chlorine gas through molten, slightly impure gold. Nearly all the other metals react with chlorine to form chlorides, while gold does not at that temperature. The chlorides either evaporate or float to the top as a light slag that can be removed. Refiners know the process is finished when the color of the flame changes.

The result is gold of about 99.5 percent purity, which Britannica and Wikipedia both report. It is fast and practical, and good enough for many industrial uses, but not the highest purity possible.

What is the Wohlwill process?

For the purest gold, refiners use electricity. The Wohlwill process, introduced by Emil Wohlwill in 1874, places a cast bar of gold that is already more than 95 percent pure in a bath of chloroauric acid, a solution of gold chloride. When current flows, gold dissolves from that bar, the anode, and deposits as pure metal on thin cathode sheets. The other metals stay behind in the solution or fall away.

Britannica gives the result as about 99.99 percent purity, and Wikipedia cites up to 99.999 percent. Either way, this is the "four nines" gold that becomes the starting point for fine jewelry alloys.

Why can aqua regia dissolve gold?

Gold resists almost every acid alone, which is exactly why aqua regia, Latin for royal water, is so famous. Wikipedia describes it as a mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1 to 3. Neither acid can dissolve gold on its own. Nitric acid oxidizes a tiny amount of gold into gold ions, and chloride from hydrochloric acid immediately binds those ions into tetrachloroaurate, pulling the reaction forward so more gold can dissolve. Aqua regia is used mainly to make the chloroauric acid that serves as the electrolyte in the Wohlwill process.

How is recycled gold refined?

Recycled gold goes through the same chemistry. Britannica notes that gold in scrap can be recovered by leaching with cyanide or with aqueous chlorination before final refining. Once purified, recycled gold is chemically identical to newly mined gold, atom for atom. I wrote more about this in What Is Recycled Gold.

That is why I love working in 18K recycled gold. The Wavy Diamond Bracelet is a gold wave that rises and falls around the wrist, made in 18K recycled gold with ethically sourced natural diamonds. The Sahara Ring, from the RAW collection, is 18K recycled gold with an approximately 2 ct natural diamond, GIA certified.

From pure gold to 18K and 14K

Pure 24K gold is too soft for most fine jewelry, so after refining it is alloyed again. Britannica explains that each karat equals about 4.167 percent gold, so 18 karat gold is 75 percent gold. The other 25 percent, a mix of metals such as silver and copper, decides the color and strength. The Wavy Diamond Stud Earrings are a small example of that balance: 18K recycled gold, strong enough to hold diamonds, rich enough to stay warm in color.

To learn how to read the stamps that record all this, see Karat Stamps and Hallmarks Explained, and for the color chemistry, Gold Alloys Explained.

Key facts about gold refining

• Fire assay measures gold content by melting, separating and weighing.
• The Miller process (chlorine gas) reaches about 99.5 percent purity.
• The Wohlwill process (electrolysis) reaches 99.99 percent or more.
• Aqua regia dissolves gold only because its two acids work together.
• Refined recycled gold is identical to newly mined gold.

Margaret Krinetz Fine Jewelry, handmade in Tel Aviv and shipped to Palm Beach, works with this pure, re-alloyed gold in every piece.

Frequently asked questions

Is 24K gold used in fine jewelry?

Rarely for rings and bracelets, because it bends and scratches easily. 18K and 14K alloys are stronger.

Can you tell refined recycled gold from mined gold?

No. After refining, the metal is the same element with the same properties.

How long does delivery to Palm Beach take?

Every piece is handmade to order in 4 to 6 weeks and shipped with DHL Express in 3 to 5 business days. Shipping is free on orders over $1,500.

The gold on your wrist has been purified by fire, chemistry and electricity. Give it a shape that means something to you, and write to me if you would like help choosing.