The Wee Extra Bits

The Wee Extra Bits

Or: How to Let It Shine

When people talk about “fluxes” in gold lustres, they’re usually stretching the truth. A proper flux lowers the melting point of a glaze and gets the whole thing flowing. The tiny additives here don’t melt anything. They’re more like film whisperers, the quiet finishing touches that coax a hair-thin layer of gold into its most brilliant, reflective, mirror-like self.

These micro-additions can:

• Encourage that self-burnished glow

• Steer the colour warmer, cooler, redder, paler

• Refine the metallic structure so the surface looks smoother and brighter

They’re the wee tweaks that separate “nice gold” from “good grief, that’s gold.”

A Cast of Many (Mostly Retired) Characters

Historically, potters tried everything under the sun: platinum, palladium, silver, bismuth, iridium, iron, antimony, even cobalt. Some were stolen from enamel recipes, others were pure alchemy-by-guesswork. With so many possible tweaks, my own journey has only just begun. Now that I’ve got the basics nailed down, I’ll revisit these characters in the future to see what each one can really do.

For now, I’ll stick to two of the main players.

Chromium: The Warmer Touch

The Secret Red-Gold Push

Chromium usually sneaks into lustre recipes as an organometallic such as chromium octoate or chromium resinate. We’re talking fractions of a percent here, so it’s not a pigment and it won’t leave oxide effects. Chromium acts more like a structural engineer for the gold layer.

What it does:

• Influences how the gold crystallises, giving a denser, flatter layer with better adhesion and brighter reflection

• Pushes the tone toward that deeper, redder gold you see in modern commercial lustres

• Costs far less than rhodium, which is why industry leaned on it when rhodium’s price exploded

My own test: I went with chromium resinate. Easy enough to prepare if you don’t mind a bit of kitchen-sink alchemy. Take pine resin, saponify with sodium hydroxide into a soap, dissolve in water. Dissolve chromium alum (chromium(III) potassium sulfate dodecahydrate) in another jug of water. Slowly mix the two, and what drops out is a fluffy green hydrophobic solid. That’s your chromium resinate. It dissolves nicely in most lustre solvents, and if you like green fluff, it’s oddly satisfying to make.

(Video coming soon if you want to see the green fluff magic in real time.)

Rhodium: From Spare Change to Pricey Guest Star

Rhodium is the brightener. Added in trace amounts as rhodium resinate or rhodium octoate dimer, it:

• Sharpens reflectivity for a mirror-like finish

• Helps burn away carbon residues

• Lightens the gold tone to a crisp, paler shade

Once upon a time, rhodium was a throwaway by-product. Then catalytic converters came along, demand skyrocketed, and suddenly it was one of the most expensive metals on the planet. Manufacturers swapped rhodium’s silvery brightness for chromium’s cheaper warmth.

I don’t usually shy away from tricky chemistry, but rhodium nearly had me running. It’s so unreactive even aqua regia shrugs at it. One prep I read about involved boiling rhodium in concentrated sulphuric acid for days. Not much rattles me, but keeping a pot of boiling sulphuric acid on the hob for 72 hours sounded like a fast-track to an obituary. Add in the price tag and I wasn’t about to gamble.

So I cheated. I skipped the heroic chemistry and bought something ready to use: rhodium octoate dimer. At first glance the green powder feels painfully expensive. But 100 milligrams is enough to make around 20 grams of lustre. Suddenly, the numbers don’t look nearly so terrifying.

Beyond Metals: Other Micro Tweaks

Metals aren’t the only game in town. A few other additives can make or break the film long before it hits the kiln.

Plasticisers: Keeping It Flexible

PMIB is a fantastic polymer for lustres: strong scaffold, burns away clean. Its fatal flaw? It dries like spun sugar, brittle and crack-prone, sometimes even peeling off. To the naked eye the fired gold looks fine. Under a lens you see a crazed micro-pattern, like a gold desert floor. Not exactly the flawless mirror finish I promised myself.

Old-school potters softened their resins with turps, eucalyptus, or other oils. That worked when pine resin was the scaffold. But PMIB is a different beast. I tried eucalyptus oil for nostalgia’s sake. It helped, until it didn’t. Too much and I had cloudy solutions and little precipitate corpses sinking to the bottom of the vial.

Modern answer? Acetyl Tributyl Citrate (CAS 77-90-7). Not the obvious choice, but it caught my eye for being food-safe and already at home in nail polish. By sheer luck, it solved my problem first go. A measly 0.5% was enough. The film dried glassy, consistent, and stuck beautifully to the glaze.

Pro tip: if you want to see the cracks, test on a semi-matte glaze. Glossy hides everything. Semi-matte exposes all sins.

Stabilisers: Shelf-Life Insurance

This one I haven’t played with yet since I make tiny batches, but industry swears by them. Butylated Hydroxytoluene (BHT, CAS 128-37-0) is the classic. Think of it as antioxidants for your lustre: it mops up free radicals and keeps the mix fresh.

Why All This Matters

On their own, these micro-additions look trivial. Together they push a lustre glaze into that sweet spot of colour, shine, stability, and usability.

At the start of this journey, I called lustre a chemical symphony. If that’s true, then gold, bismuth, and the vehicle are the soloists. These micro-additives are the hidden backbone: the conductor keeping time, the percussionist adding punch, the stage manager making sure the curtain actually goes up.

Without them, the stars are still on stage, but the music never quite reaches brilliance.