Never Trust a Glaze Until You Have Tested It

A test tile is a small piece of fired clay used to evaluate how a glaze looks and behaves before committing it to finished work. No matter how experienced you are, every new glaze (whether a recipe you mixed yourself or a commercial glaze you have never used) should be tested before it goes on a piece you care about.

Glazes behave differently on different clay bodies, at different cone ranges, and in different kilns. A glaze that fires beautifully in one studio may run, crawl, or look completely different in yours.

What Makes a Good Test Tile

A test tile should:

• Be made from the same clay body you will be using for your finished work.
• Have a vertical face so you can see whether the glaze runs. A flat tile lying horizontally will not show you how a glaze behaves when it is upright on a pot.
• Have a foot or base that can sit on the kiln shelf without the tile being glued to it if the glaze runs.
• Be bisque-fired before glazing, just like your regular work.

A simple and popular tile design is an extruded or hand-formed rectangular bar with a hole for hanging, a grooved texture on one face, and a slight curve or raised lip at the base to catch glaze drips without fusing to the shelf.

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Test tiles are especially important when working with glazes that behave differently across clay types: a glaze that looks beautiful on stoneware may craze or crawl on porcelain due to differences in thermal expansion. Systematic testing is what separates reliable studio practice from costly trial and error.

Test Tile System That Scales

As your glaze testing grows, standardise your tile format:

• Same dimensions
• Same texture band
• Same dip timing
• Same bisque temperature

Consistency in the tile itself is what makes comparisons meaningful.

Pro Tip

Fire duplicate tiles when possible. One can be archived and one can be handled during class, sales prep, or studio consultations.