In the last post, we covered specific gravity — how to measure it and why it matters for glaze consistency. But SG only tells you half the story. You can have two glazes sitting at the exact same specific gravity and watch them behave completely differently the moment you dip a piece. One flows off cleanly. The other grabs, drags, and coats unevenly.
That difference is viscosity — and once you start tracking it alongside SG, your glaze results get a lot more predictable.
What Viscosity Actually Measures
Viscosity is the resistance of your glaze to flow. A high-viscosity glaze is thick and slow-moving — it clings to the surface and doesn’t level out easily. A low-viscosity glaze is thin and fluid — it runs off quickly and can leave you with bare spots or drips at the foot.
Unlike specific gravity, viscosity isn’t just about how much material is in your glaze. It’s about how those materials interact with each other in suspension. Flocculation, particle size, clay content, and even how long your glaze has been sitting all affect viscosity independently of SG. That’s why you need both numbers.
SG tells you what’s in the bucket. Viscosity tells you how it behaves when you use it.
Why Viscosity Matters
Glaze application is where most consistency problems actually originate — not in the kiln. If your viscosity is off, you’ll see it in uneven coating thickness, crawling, pinholing, or runs. And because these problems show up after the firing, it’s easy to blame the glaze recipe when the real issue was application.
Tracking viscosity gives you a second checkpoint before the piece ever goes in the kiln. If your SG is right but your glaze is still behaving strangely — dragging, setting too fast, or not adhering well — viscosity is almost always where to look.
How to Measure Viscosity
The most practical tool for studio potters is a Zahn cup — a small cup with a precisely sized hole in the bottom. You fill it, hold a finger over the hole, release, and time how many seconds it takes for the glaze to drain through. That number is your viscosity reading. No expensive equipment required.
Cup size matters. For most dipping glazes, a Zahn cup #2 is the right tool — the orifice size is well matched to the viscosity range most glazes fall in. If you’re measuring slips, step up to a Zahn cup #4 — slips are thicker and need the larger opening to drain at a measurable rate.
What you need: Zahn cup #2 (glazes) or #4 (slips) · Stopwatch or phone timer · Your glaze, well stirred
Fill the cup completely with your glaze. Hold your finger firmly over the hole so nothing drains before you’re ready. Release your finger and start the timer simultaneously. Stop the timer the moment the stream breaks — when continuous flow turns to drips. That elapsed time in seconds is your viscosity number.
Write it down alongside your SG. A few things to keep in mind: always stir your glaze thoroughly before measuring, temperature affects viscosity (a cold studio will give you different readings than a warm one), and consistency matters more than the absolute number. What you’re building is a personal baseline — once you know what viscosity produces your best results, you can return to it reliably every time.
How to Adjust Viscosity
Viscosity is controlled through flocculation and deflocculation — the degree to which glaze particles clump together or stay dispersed in suspension.
To Increase Viscosity (Thicken the Glaze)
Add a flocculant. The most common options are Epsom salt (magnesium sulfate) and calcium chloride. Start small — dissolve a pinch of Epsom salt in a little warm water, add it to your glaze, and stir thoroughly before measuring again. Flocculation causes particles to loosely bond together, increasing resistance to flow without significantly changing your SG.
To Decrease Viscosity (Thin the Glaze)
Add a deflocculant. Sodium silicate and Darvan 7 are the two most common choices in studio ceramics. Add a few drops at a time, stir well, and re-measure before adding more. Deflocculation disperses particles more evenly, allowing the glaze to flow more freely.
Both flocculants and deflocculants change how your glaze behaves without dramatically shifting your specific gravity — which is exactly why SG alone isn’t enough. You can flocculate a glaze and watch the viscosity climb while the SG barely moves.
Water is not a substitute for a deflocculant. Adding water to thin a glaze lowers your SG, thins your coating, and throws off the dry-to-water ratio you’ve worked to establish. If your glaze is too thick to apply properly, reach for a deflocculant first — not the water bucket.
Building a Two-Number System
The goal is simple: every time you use a glaze, record both the SG and the viscosity. Over time you’ll build a reference for exactly what numbers produce your best results with each glaze — and you’ll be able to return to those numbers reliably, batch after batch.
When something goes wrong in the firing, you’ll also have data to work backwards from. Did the glaze crawl? Check your viscosity notes — was it higher than usual? Did coverage look thin? Was the SG lower than your baseline?
Good studio practice isn’t about eliminating variables entirely. It’s about knowing which variables you’re working with. Viscosity and specific gravity together give you that. Start measuring both, and the guesswork mostly disappears.
