How raw gold is transformed into a soluble, paintable form used in studio gold lustre

GOLD MERCAPTIDES

If there’s one ingredient that marks the gradual shift from historical lustres to modern bright gold systems, it’s this one: gold mercaptides.

Early lustre makers didn’t know what a mercaptide was, but they were circling the same chemistry. Pine resin, sulphur, gold salts, heat, patience, and a heroic tolerance for smell all went into the mix. Somewhere inside those sulphur-heavy brews, gold was being converted into sulphur-containing compounds that could cling to a glaze and reduce to metal during firing.

It worked, but it wasn’t controlled. Those early systems bundled multiple roles into the same ingredients, and while the results could be spectacular, they were hard-won and inconsistent.

Modern mercaptides pull that chemistry apart and put it back together cleanly. Instead of generating gold–sulphur compounds by chance, we start with well-defined gold compounds that reduce cleanly, reliably and predictably. The gold exists as a separate, quantifiable component in the recipe with known metal content and a predictable decomposition temperature, which makes the whole system far easier to control at the bench.

A mercaptide is simply a metal atom bound to sulphur via a thiol. Thiols are sulphur-containing organic compounds, and they’re notorious for one reason: they stink.

They’re so potent that they’re deliberately added to natural gas so leaks can be detected by the human nose. In one well-known incident, a thiol spill at a factory in France was reported by the public across parts of the UK, with emergency service calls spiking as a result.

Rotten eggs would be an upgrade. 

That matters here, because odour isn’t a footnote. It becomes a very practical consideration when choosing a gold compound you’re going to make, store, weigh, dissolve, and share your life with.

Modern gold mercaptides take that sulphur chemistry and put it on a leash. The gold is already in the correct state. The sulphur is already attached. The compound is soluble, predictable, and decomposes cleanly to metallic gold at relatively low temperatures. No guessing. No swamp brewing. No burnishing rituals.

One quiet but important consequence of this is how little heat is actually required to convert these compounds into metal. Gold mercaptides reduce to metal well below red heat, which is why the same chemistry is used well beyond ceramics, in plastics, polymers, and other materials that would not tolerate high temperatures. In a lustre firing the kiln temperature is dictated by fluxes and glaze interaction, not by the gold itself.

This low-temperature behaviour shows up visually as well. Traditional sulphur–resin lustres, particularly on glass, often take on a purple or reddish cast when viewed from the underside. Mercaptide-based gold films do not. The gold remains metallic in character and reads as gold from all angles, a property that also makes the same chemistry useful well beyond ceramics.

When choosing a gold mercaptide for a lustre system, a handful of very practical factors matter far more than elegant chemistry diagrams:

• Gold content: how much of the compound is actually metallic gold

• Decomposition temperature: how easily it reduces to metal during firing

• Physical form: does it isolate as a dry powder or a waxy oil

• Odour: whether it will quietly sit on your bench or clear the neighbourhood 

I’ve worked with two so far:

• 4-tert-butylbenzenethiol (CAS 2396-68-1)
  This isolates as a fine yellow powder, is easy to dry, easy to weigh, easy to store, and doesn’t stink. It contains roughly 54 % metallic gold by weight and decomposes cleanly at relatively low temperatures at 190-230 celcius. For all practical purposes, it behaves exactly how you want a gold compound to behave. This is my preferred choice and the example used throughout this guide.

• tert-dodecyl mercaptan (CAS 25103-58-6)
  This forms a waxy oil rather than a solid. It contains 49.4% metalic gold and works perfectly well in lustre systems, but it’s harder to measure accurately and more awkward to store. Still useful, just less friendly on the bench. This one decomposes to metalic gold at 160-210 celcius

Neither of these are especially exotic or outrageously expensive and both avoid the one thing that will make you abandon a material instantly: unbearable smell.

From here on, I’ll always include the CAS number when talking about specific chemicals. Chemistry naming is a minefield. The same substance can appear under half a dozen different names depending on who’s writing, selling, or patenting it. A CAS number cuts straight through that noise and makes sure you’re actually buying (and using) the thing you think you are.

The rest of this page walks through how I turn metallic gold into one of these compounds, step by step, using equipment that fits on a bench rather than in a research lab.

Before you even think about trying it: go and do some background reading on gold refining. The internet is absolutely flooded with guides, videos, and hard-won safety advice. And for once, that’s a good thing. Nothing from this point on is “have-a-go” chemistry. Treat it with respect and work safely.

GOLD MERCAPTIDE SYNTHESIS

Before starting, it helps to have everything laid out. The process is straightforward, but only if you’re not hunting for things mid-step.

Chemicals

• Metallic gold (Au, CAS 7440-57-5)

• Hydrochloric acid (CAS 7647-01-0)

• Nitric acid (CAS 7697-37-2)

• Distilled water

• Dimethyl sulphide (CAS 75-18-3)

• Chloroform (CAS 67-66-3)

• 4-tert-Butylbenzenethiol (CAS 2396-68-1)

• Methanol (CAS 67-56-1)

Equipment

• Borosilicate glassware (beakers, stirring rods)

• Hotplate with magnetic stirrer

• Fume hood

• PPE appropriate for corrosive acids and volatile solvents

• Filter paper

• Containers for waste and aqueous washes

Step 1: Dissolve the Gold

You begin by converting metallic gold into gold chloride.

Materials

• 2.5 g pure metallic gold

• 10 mL hydrochloric acid

• 3.5 mL nitric acid

• 100 mL borosilicate beaker

• Hotplate and stirrer

• Fume hood and PPE

Place the gold directly into the beaker.

Slowly add the hydrochloric acid to the gold, followed by the nitric acid.

Heat gently to around 50 °C with stirring. Over 20–30 minutes the gold will dissolve, forming a clear, bright orange solution.

If any gold remains undissolved, add nitric acid drop by drop and continue heating until everything has gone into solution.

Step 2: Remove Excess Nitric Acid

Nitric acid causes problems later. It must go. Gently evaporate the solution at around 40 °C until it thickens noticeably. I often use a small fan nearby to encourage evaporation without cranking the heat.

Add a small amount of fresh hydrochloric acid, swirl to dissolve any residue, then evaporate again. Repeat this two or three times.

When finished, you should add hydrochloric acid until you have 50 mL of gold chloride solution in hydrochloric acid.

Step 3: Dilution and Reduction to Gold(I)

This step puts the gold into a more cooperative chemical state before the final mercaptide is added.

Gold is produced in aqua regia as gold(III). While it’s possible to react gold(III) directly with a thiol, doing so is inefficient and messy. Gold(III) requires significantly more mercaptan to reduce and generates additional sulphur-containing side products, which makes the final purification harder than it needs to be.

To begin, dilute the gold chloride solution by adding 50 mL of distilled water, bringing the total volume to 100 mL.

With stirring, add dimethyl sulphide (CAS 75-18-3) slowly, drop by drop. For 2.5 g of gold, around 2.5–3 mL is plenty. Continue stirring for 1 hour

A brief apology is warranted at this point. Dimethyl sulphide smells like Satan’s onion soup. It will stink out the fume hood, the lab, and quite possibly your soul. Even with a respirator, it has a way of lodging itself firmly in your memory. This is, regrettably, normal.

The consolation prize for enduring the smell is that dimethyl sulphide is relatively easy to remove later. Far easier, in fact, than excess mercaptan. That alone makes this step worth the suffering.

As the reduction proceeds, the solution will shift from bright orange to a pale, opaque white waxy solid and start to aggrigate.

Step 4: Transfer into Chloroform

Add 10–15 mL of chloroform to the reaction mixture.

Chloroform plays a critical role here. It dissolves the newly formed gold(I) species extremely well, but does not mix with water. This creates a clean, separate lower phase that gathers all the gold chemistry into one manageable place.

Allow the system to settle. You’ll see two layers form, with the gold moving into the lower chloroform layer.

In a separate container, dissolve 2.5g of 4-tert-butylbenzenethiol (CAS 2396-68-1) in a small amount of chloroform.

Slowly add this mercaptan solution to the gold-containing beaker with gentle stirring.

Allow the mixture to stir for around 2 hours at room temperature

Step 5: Washing the Gold Layer.

This stage removes everything you don’t want.

Repeated water washes strip out:

• Residual acids

• Dimethyl sulphide

• Excess mercaptan

• Water-soluble sulphur by-products

The process is simple, but not quick:

• Decant the upper aqueous layer

• Add fresh distilled water

• Stir for 15–20 minutes

• Allow layers to separate

• Decant again

Repeat until the aqueous layer no longer smells like bad onions.

You’ll also notice a visual cue as things clean up: the chloroform layer will gradually change from cloudy to clear, transparent green. That clarity is a very good sign your washes are working and nearing the end point

Step 6: Isolate the Gold Mercaptide

This step exploits a simple solubility trick.

Chloroform dissolves readily in methanol. Gold mercaptides do not, so will precipitate out as a solid.

Pre-chill 100-200ml methanol (CAS 67-56-1) in a fridge.

With stirring, slowly add the chloroform solution to the cold methanol. The chloroform disperses into the methanol, while the gold mercaptide crashes out as a fine yellow powder.

Allow the solid to settle, then decant the methanol.

Filter the remaining solid through filter paper and wash repeatedly with fresh cold methanol while it sits in the paper. This strips out the last traces of impurities.

Allow the powder to dry naturally.

Once dry, transfer to a labelled jar and store appropriately.

END RESULT

You now have gold 4-tert-butylbenzenethiolate as a dry yellow powder.

It’s:

• Easy to weigh

• Easy to dissolve

• Comparatively low-odour

• Well behaved in modern lustre systems

From here, it’s ready to be incorporated into the vehicle and polymer system.

WHY THIS WASN'T THE HARD BIT

This stage of the process was relatively well documented and, compared to what came later, fairly straightforward. I already had some experience in gold refining, so adding a few extra steps to arrive at a usable gold compound wasn’t a huge leap.

Once I had a reliable way to make a gold mercaptide, it simply worked. It dissolved cleanly. It applied predictably. It fired exactly as expected. And, crucially, there’s a long history of gold mercaptides being used successfully in lustre systems.That mattered more than I realised at the time. It meant that when things failed later, the problem wasn’t vague or mystical. The gold was doing its job. If something went wrong, the fault had to lie elsewhere in the system.

That realisation is what pushed me toward binders, solvents, fluxes, and eventually the micro-additions that make or break a modern lustre.

Gold wasn’t part of the puzzle anymore.

It was the baseline.

And once that baseline was solid, everything else finally had something stable to stand on.