Every potter eventually hits the glaze wall. Maybe you have a shelf full of test tiles that taught you nothing because you cannot remember what you did. Maybe you are scared to deviate from the one glaze that works. Maybe you are mixing recipes from the internet and getting wildly different results than the photos promised.
The difference between potters who understand their glazes and potters who are guessing is not talent or years of experience — it is having a system.
Why Most Glaze Testing Fails
The most common approach to glaze testing looks like this: mix something, dip a tile, fire it, look at it, and either like it or do not. The problem is that one test tile in isolation tells you almost nothing. You do not know:
- How the glaze behaves at different thicknesses
- How it responds to different clay bodies
- What happens when it overlaps with another glaze
- Whether the result is repeatable or a one-time fluke
- What variable actually caused the result you got
Without controlling your variables, you are just rolling dice with chemistry.
Building a Simple Testing System
Step 1: Standardize Your Test Tiles
Make a batch of identical test tiles from the same clay body. Give them texture — a carved line, a stamp, a smooth area, and a rough area all on one tile. This lets you see how the glaze breaks over edges, pools in recesses, and looks on different surfaces.
Make them all the same size and shape. Fire them all in the same spot in the kiln if possible. The more consistent your tiles, the more meaningful your comparisons.
Step 2: Change One Variable at a Time
This is the golden rule. If you change the glaze recipe AND the application thickness AND the firing temperature all at once, you have no idea what caused the result.
Pick one thing to test:
- Thickness (one coat vs two coats vs three coats)
- One ingredient change (add 5% more silica, or swap one flux for another)
- Application method (dipping vs brushing vs spraying)
- Layering order (glaze A over B vs B over A)
- Kiln position (top shelf vs bottom shelf)
Step 3: Record Everything
This is where most people fall off — and it is the most important part. For every test tile, record:
- The full recipe (by weight, not volume)
- The clay body used
- Application method and thickness
- Firing cone and schedule
- Kiln position
- The date
- What you were testing (your hypothesis)
Write it on the back of the tile with underglaze AND in a notebook or app. Tiles get separated from notes. Notes get lost. Having both saves you.
Step 4: Fire in Sets, Not Singles
Whenever possible, fire a series of related tests together. Five tiles testing increasing amounts of one colorant. Three tiles testing the same glaze on three different clay bodies. A grid of layering combinations.
Sets give you a visual spectrum that is infinitely more useful than scattered one-offs.
Step 5: Review and Build
After firing, lay your set out and actually study it. What changed? What direction do you want to push further? What surprised you?
Then — and this is key — do the next round based on what you just learned. Testing is iterative. Each round narrows your focus and builds real understanding of how your materials behave.
Common Glaze Testing Mistakes
- Testing too many things at once — you end up with a pile of tiles and no clear takeaways
- Not labeling tiles before firing — after firing, unmarked tiles are just mysteries
- Ignoring your clay body — the same glaze looks completely different on porcelain vs stoneware vs dark clay
- Only testing on flat tiles — glazes behave differently on vertical surfaces, inside curves, and over edges. Include at least one curved test form
- Giving up after one bad result — a failed test that you recorded and understood is more valuable than a lucky success you cannot repeat
From Testing to Palette
The goal of systematic testing is not to test forever — it is to build a working palette of glazes you understand and trust. Most production potters work with 3-8 glazes that they know deeply: how thick to apply them, how they interact with each other, how they look on their specific clay body, and what firing schedule brings out their best qualities.
That depth of knowledge only comes from organized testing. And once you have it, you can make intentional choices instead of hoping for the best every time you open the kiln.
Log every glaze test in the Mud Room app — recipe, thickness, clay body, firing details, and results. Over time, your test log becomes the most valuable reference in your studio.