How trace elements like rhodium, chromium and tin influence gold lustre behaviour

MICRO ADDITIONS TURNING "WORKING INTO "OUTSTANDING"

Once the main system was finally behaving, the nature of the problems changed. The binder was no longer sabotaging itself. The gold was depositing where it should. Firing stopped being hit and miss. What remained were smaller, more irritating issues: a tiny black spot that ruined an otherwise good piece, gold that looked fine at first glance but didn’t quite match the reflectiveness of commercial lustres.

This was no longer about getting something to work, that part was nailed. It was about understanding the little things that can improve it.

At arm’s length the gold looked good. Bright, continuous, and already magnitudes better than anything I’d managed with older resin-heavy systems. Under magnification, the surface told a different story. The film was subtly crazed, and although difficult to see. What looked like a continuous film was, in reality, a mosaic held together by optimism and ageing eyesight.

The cause was obvious PMIB films were cracking as they dried. Sometimes the cracking was subtle enough to miss before firing, but the kiln never missed it. As the temperature rose, the film shrank, the cracks opened, and the gold deposited itself along exactly the same fractured pattern. This wasn’t a gold problem, and it wasn’t a firing problem. It was a film-stress problem that happened long before the piece ever reached the kiln.

PLASTICISERS: THE IDEA THAT DIDN'T QUITE WORK

The first instinct was plasticisers. On paper, they exist to do exactly this job: soften brittle films and relieve drying stress. In very small amounts they helped, but only marginally. Each improvement demanded more, and before long they stopped being micro-additions and started reshaping the entire system.

At that point they were past fine-tuning behaviour, they were introducing new problems. The system became less nstable without actually becoming better. Plasticisers didn’t fail spectacularly, they just quietly ran out of road and were dropped.

Pine Rosin: Stress relief

The breakthrough wasn’t clever chemistry, it was practical. I reached for a resin that had been sitting on the shelf from my first experiments and added it in very small amounts to see what happened.

It worked.

Pushing traditional rosin far enough to stop cracking revealed a clear trade-off. As rosin content increased, brightness fell. The film survived the firing, but the gold dulled. It didn't quite need burnishing, yet still wanted a light polish.

That was the clue. Raw rosin was helping to control PMIB cracking, but it brought a degree of uncleanliness with it, leaving more residual carbon in the finished film. That observation led me toward modern esterified and hydrogenated rosins. They retain the useful tack and flexibility while avoiding the drawbacks of raw pine resin. Oxidation, darkening, acidity, carbon.

Used sparingly, these act as stress-relievers rather than structure. Push them too far and the system slides straight back toward traditional resin-heavy lustres. Too little, and the cracking returns.

Finding that balance mattered far more than it sounds. In my case, the sweet spot landed around a 4:1 ratio. For every 4 g of PMIB in the stock solution, 1 g of rosin gives enough flexibility without dragging the system backwards into duller gold territory.

RHODIUM FOR CLEANER BURN OUT

Rhodium wasn’t a random addition. Older lustre recipes frequently describe rhodium compounds producing unusually bright or “self-burnishing” gold films. Obvious language which gave hints at what rhodium brings to the table.

In theory, making a usable rhodium compound yourself is possible. In practice, it involves boiling rhodium in concentrated sulphuric acid for days at a time. That was one rabbit hole I was happy to walk right past. Instead, rhodium entered the system in a sensible, controlled form: rhodium octoate dimer (CAS 73482-96-9) off the shelf and added to some toluene to make a stock solution that could be carefully measured and added to my mix in very small portions.

In this system, rhodium isn’t acting as a flux, and it doesn’t reduce to metallic rhodium during firing. It remains as an oxide. Its role is catalytic. Rhodium compounds promote more complete and better-timed burnout of organic residues, reducing carbon-related dulling and allowing the gold film to consolidate more cleanly.

The result is cleaner, brighter gold films. Seen through that lens, those old claims about “self-burnishing” start to make sense. Rhodium isn’t polishing the surface, it’s stopping the process from dirtying it in the first place.

COPPER: UNDERSTOOD BUT SET ASIDE

Copper also earned its time on the bench. Like rhodium, it can act as a micro-addition that influences burnout and surface chemistry rather than forming the main metal film. Historically, copper compounds have played similar supporting roles in lustres, so it was worth understanding what it brought to the system.

The copper used here was a copper resinate made in-house. Functionally, it behaved much like rhodium. It integrated cleanly and helped tidy up the surface chemistry during firing. But the difference showed up in the colour of the gold film. Copper consistently nudged the gold toward a warmer, redder tone. That isn’t wrong, and in many contexts it’s exactly what’s wanted. For my work, though, I preferred the paler, truer gold produced by the rhodium-based system.

There’s also a historical and economic angle that’s hard to ignore. In the 19th century, rhodium was effectively a by-product of platinum refining, with few practical uses and very little market value. Today, thanks largely to its role in automotive catalytic converters, it’s one of the most expensive metals on the planet. Copper, by contrast, is cheap, familiar, and easy to justify at scale. That shift alone goes a long way toward explaining why copper appears so frequently in modern, lower-cost lustre formulations.

Interestingly, this circles back to chemistry. Rhodium’s modern industrial value comes from its ability to promote cleaner burn-off at lower temperatures in car exhausts, which is exactly the role it appears to be playing here in a lustre.

UV BRIGHTNER

This is included for one reason only: visibility during application.

Of everything in this guide, this is probably the one thing I genuinely feel I’ve contributed to the lustre world. It solves a boring but persistent practical problem that somehow nobody ever bothered to fix: being able to actually see where the lustre is while you’re applying it.

I only know of one manufacturer who approached this problem by adding a red dye. In my view, that’s a fairly clumsy solution. It works on some glaze colours and is actively unhelpful on others. I fully expect the UV tracer approach outlined here to quietly surface in commercial products at some point. If it does, you’re welcome. A thank-you card wouldn’t go amiss guys.

Modern lustre systems can be almost colourless on the pot, which makes judging coverage on certain glazes frustrating at best. To get around this, I add a trace amount of a Fluorescent Brightener 135 (CAS 1041-00-5), to my solvent stocks.

The brightener is dissolved into solvent at roughly 3 mg per mL, and those solvent stocks are then used when preparing polymer solutions or when topping up a lustre batch. Under UV light, the applied film becomes immediately visible, making missed areas and thin spots obvious without guessing.

The brightener plays no role in the fired lustre. It burns away completely during firing and exists purely as a handling aid.

At present, this particular brightener dulls noticeably once the gold mercaptide is introduced. It remains usable, but it could be improved, and I expect to replace it with a brighter compound in the near future.

Where this leaves the system

Micro-additions only matter once the fundamentals are already behaving. Their job is to nudge, smooth, and clean up, not to perform miracles.

By the time these pieces came together, the lustre stopped feeling experimental. Application became more forgiving. Edges improved. Carbon-related defects dropped away. No single test was a dramatic change, but the cumulative effect was unmistakable.

After I introduced these small tweaks I felt i had finally reached a point where i could seriously challenge the commercial gold lustres and in some ways have improved on them.