The Binder Blunders

🔥 From Sulphur and Smoke to Precision Polymers

Choosing the Right Binder for Gold Lustre Work

Historically, gold lustre binders — the sticky stuff that holds everything together before the firing — were brewed from natural oils and resins boiled with sulphur. These were noxious, volatile mixtures. Smelly, sulphur-rich, and short-lived, they were used to suspend gold salts and glue them onto ceramic surfaces.

The potters behind Islamic, Hispano-Moresque, and Italian majolica lustres didn’t know the chemistry, but what they’d created were likely primitive thiol or thioether systems — crude predecessors to the gold mercaptides we use today.

And while these early brews worked, they came with baggage:

• Short shelf lives

• A tendency to gel or separate

• Wildly inconsistent performance

• And unless burnished within an inch of their lives, they often gave only dull, reluctant results

They were a glorious mess. Effective, yes — but unpredictable, fussy, and foul.

So What’s Changed?

These days, we understand that lustre isn’t just about gold — it’s about timing. The binder isn’t just a carrier; it’s a structural scaffold, a thermal shield, and a stage manager. It needs to:

• Form a smooth, cohesive film

• Hold its shape as solvents evaporate

• Burn off just as the gold compound begins to decompose

• Leave no residue

• And most importantly, it must match the thermal behaviour of the gold thiol

This was the breakthrough: realising that a “good” binder isn’t just about application — it’s about burnout profile and glass transition temperature. I struggled with this part for several months, testing and retesting, i was sure the choosen binder was good and that some other issues were at play, but as they say "You dont know, what you dont know"

The Binder Trials

Here’s what I tested — and why most didn’t make the cut:

Pine Resin (Colophony)

• A historical nod, used in many early gold resinate formulas

• Easily sourced, romantic to use, but...
  ❌ Chemically unstable — reacts with gold thiols
  ❌ Not shelf stable — oxidises, gels, and separates
  ❌ Burns dirty — leaves carbon, smells horrific
  ❌ Softens too early — Tg too low, causes film collapse
  ✅ Works in a pinch, or if you're writing a medieval spellbook

Ethyl Cellulose

• A classic binder used in traditional ceramic media
  ✅ Forms a usable film
  ❌ Pre-fire films were weak and required a complex solvent blend to give an even film
  ❌ Poor compatibility with gold dodecanethiol
  ❌ Left carbon residues after firing
  ❌ Decomposed too late to sync with gold burnout
  ✅ Soluble and safe, but ultimately unreliable for high-quality lustres

Paraloid B67 (Poly(isobutyl methacrylate))

• Initially very promising
  ✅ Forms a strong, flexible, glossy film
  ✅ Excellent brushability and solvent compatibility
  ✅ Burns cleanly in most kilns
  ❌ Tg ~60 °C — too soft; can sag before solvents fully flash off
  ❌ Decomposes too late (330–370 °C) — after gold thiol has already burned
  ❌ Resulted in pinholes and surface defects when used with alkyl gold thiols
  ✅ A good polymer… but in the wrong time zone

The Realisation

The polymer doesn’t just need to behave during drying - it must decompose in lockstep with the gold thiol.

This is where things clicked. Gold dodecanethiol (my chosen compound) starts to break down around 250–270 °C, fully decomposing by ~320 °C.

So the binder must:

• Hold its shape ideally until 250 °C (high enough Tg)

• Begin to decompose around 270–280 °C

• Burn off completely before 330 °C

• Leave zero residue to interfere with gold development

Paraloid B67 — and other common resins — just didn’t meet that profile.

The Next Generation: Timing is Everything

With that in mind, I began testing binders based not on brushability alone, but on thermal decomposition curves and glass transition temperatures.

Two polymers rose to the top:

Elvacite 2776

• PMMA-co-n-butyl methacrylate

• Tg ~105 °C — remains rigid during drying

• Decomposes ~260–310 °C — in sync with gold dodecanethiol

• ✅ Glossy, smooth films

• ✅ Excellent burnoff

• ✅ Compatible with my solvent system (cyclohexanone, toluene, etc.)

Paraloid B44 (PMMA-co-ethyl acrylate)

• Tg ~55–60 °C

• Burnout: ~270–320 °C

• ✅ Near-perfect thermal overlap

• ❌ A bit soft — works better blended with higher Tg resins

• ✅ Good gloss, smooth flow

Conclusion: Your Binder Is a Thermochemical Dance Partner

The goal isn't just a pretty film — it's a film that knows when to bow out.

A good lustre binder needs to:

• Stay rigid until the gold is ready to reduce

• Decompose just as the gold breaks free

• Leave no trace behind

That’s what early potters could never control — and what modern polymers let us fine-tune. So while pine resin had its moment, the 21st-century alchemist is after more than romance. We want timing. We want chemistry. We want lustre that performs every single time. We dont like surface defects, we dont want to burnish, we just want a reliable defect free mirror bright gold layer on the pot