Preparing the Treasure

Metal Mercaptides (Gold)

The unsung heroes of the lustre world.

Before we talk clean decomposition curves and glossy mirror finishes, let’s rewind a few centuries. Early potters were basically cooking sulphur swamps in clay pots: pine resin, lumps of sulphur, a dash of gold chloride, and the optimism of someone who thought bad smells were part of the magic. In truth, they’d stumbled into primitive thiols and thioethers. Buried in those stinky stews, gold-bearing oils appeared that could cling to ceramics and reduce to metal in the kiln.

Fast forward a few hundred years and we can skip the swamp. We’ve got fume hoods, solvents, and a working knowledge of what a thiol actually is. Out of that comes something infinitely more civilised: mercaptides.

Why Mercaptides?

Because they work. Properly.

Mercaptides are:

• Pre-organised complexes, with gold already primed in a usable chemical state

• Sulphur-attached, so no need to sprinkle in elemental sulphur like seasoning

• Soluble, which means they slot into resin systems without high heat or long alchemical prayers

• Firing-ready, decomposing neatly into metallic gold instead of leaving behind charcoal confetti

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Unlike the swamp soup of old, mercaptides burn cleanly and predictably. That means:

• No sulphur stains

• No carbon crusts

• No tedious burnishing

• Just a thin, even film of gold

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Many reduce to metallic gold below 200°C. Yes, that’s low enough to decorate things that should never see a kiln: plastics, wood, paper. And unlike older lustres that look red from underneath, mercaptides stay golden both sides.

Two Tribes: Alkyl vs Aryl

Alkyl Mercaptides – the Waxy Characters

Long carbon chains, soft, flexible, and oily. Think “greasy butter” rather than “convenient powder.” Brilliant for one-pot syntheses, but awkward to isolate or weigh.

Examples:

• Dodecanethiol mercaptide – waxy, solvent-friendly, beloved of DIY tinkerers

• Octanethiol mercaptide – shorter chain, same oily charm

• 2-Ethylhexanethiol mercaptide – branched, slightly more volatile, easier on the nerves

Aryl Mercaptides – the Powdered Professionals

Built on aromatic rings. Brittle, crystalline, and so much easier to isolate. No sticky mess, just neat piles of yellow powder that industry loves because they behave themselves.

Examples:

• Thiophenol mercaptide – the classic, with a sharp decomposition curve

• 4-Mercaptobenzoic acid mercaptide – tweaks solubility

• 4-tert-butylbenzenethiol mercaptide – my personal favourite

Bonus curiosity: grapefruit mercaptan is a thing. Imagine lustres that shimmer gold and smell like a fruit bowl. I’m not saying I’ll try it… but I probably will.

The Recipe (Gold Mercaptide, Step by Step)

Step 1: Make Gold Chloride

• 2.5 g pure gold (old jewellery will do)

• 10 mL hydrochloric acid

• 3.5 mL nitric acid

• 100 mL beaker, stirrer, hotplate, fume hood

• PPE (unless you fancy eyebrows as a memory)

Mix hydrochloric acid and nitric acid to form aqua regia, add the gold, and heat gently to ~50°C. Within half an hour you’ll have a bright orange gold chloride solution.

Step 2: Purge the Nitric Acid

Gently evaporate at ~40°C until syrupy. Add fresh HCl, repeat three times. You’re left with clean gold chloride. Skip this, and your mercaptide will sulk.

Step 3: Reduce to Gold(I)

Dilute with distilled water. For 2.5 g gold, add ~2.5–3 mL dimethyl sulphide drop by drop. Stir 15–20 minutes. The solution shifts from orange to pale and opaque white. A deeply satisfying “something’s happening” moment.

Step 4: Make the Mercaptide

Add 10–15 mL chloroform. The solution splits: gold complex dives into the heavier bottom layer. In a second beaker, dissolve 2.16 mL of 4-tert-butylbenzenethiol in a splash of chloroform. Add this slowly to the first beaker. The bottom layer glows a striking yellow-green. Stir for an hour as it warms to room temp.

Wash, rinse, repeat: decant the top layer, add clean distilled water, stir 20 minutes, decant again. Keep going until no greasy thiol floats to the surface.

Step 5: Isolate the Treasure

Cool some methanol (fridge works fine). Stir it, then add your chloroform mercaptide solution drop by drop. It clouds, turns turquoise, then settles. Decant, wash with fresh methanol, filter, and repeat until the smell stops scaring the neighbours.

Dry overnight in the filter paper. Congratulations: yellow mercaptide powder, ready to go.

Lessons Learned

• Chemical names lie. CAS numbers don’t:
  Dodecanethiol – 112-55-0
  Tert-dodecyl mercaptan – 25103-58-6
  4-tert-butylbenzenethiol – 2396-68-1

• Aryl mercaptides are tidy powders that behave.

• Alkyl mercaptides are oily, flexible, but hard to store.

Safety & Kit

• Fume hood required (aqua regia is not kitchen-safe)

• Gloves, coat, goggles – obvious, but worth saying

• Borosilicate beakers, stirrer, hotplate

• Neutralise waste before disposal, unless you like surprise visits from regulators

Why Bother?

Because this turns a lump of gold into a paint-ready compound. No swamp stews, no bad smells, no blind luck. Just shimmering, mirror-bright lustres on ceramics… and maybe one day, grapefruit-scented gold for the potter who has everything.