You open the kiln expecting a smooth finish and discover tiny holes, bare patches, or raised bubbles. Glaze defects are frustrating because the same glaze may behave differently on another clay body or in another firing. The key is to diagnose the defect before changing several variables at once.
Pinholes: tiny openings in the glaze
Pinholes appear as small holes in the fired surface. Gases escaping from the clay or glaze, application thickness, and the firing schedule can all contribute. Start by checking that bisque ware is clean and dry. Compare thin and thick applications on test tiles, and review the manufacturer's recommended firing range. Depending on the glaze and kiln, a carefully tested soak or cooling adjustment may help the glaze heal. Also there are rare occasions that if organics have not been completely burned off, refiring can help. Be advised some pinholes can be sharp so running your hand over is not suggested. And refiring can add stress to the piece but can also solve the pinhole problem.
Crawling: glaze pulling away from the surface
Crawling leaves exposed clay surrounded by islands of glaze. Dust, oils, overly thick application, poor adhesion, or excessive shrinkage of the dried glaze layer may be responsible. Wipe bisque surfaces with a clean, slightly damp sponge and allow them to dry before glazing. Apply even coats and avoid handling the surface with oily fingers.
Blisters: raised bubbles or broken craters
Blisters may develop when gases become trapped in molten glaze or when a glaze is overfired. Check whether the clay and glaze are compatible with the firing temperature. Rather than increasing heat automatically, test small changes in application thickness or the firing schedule never pop bubbles. Fired glaze is basically a type of glass. Popping bubbles makes the bubble very sharp.
Uneven color or patchy coverage
Inconsistent dipping time, brush loading, stirring, or coat thickness can create uneven results. Mix glaze thoroughly according to its instructions and record how many coats you applied. Remember that layered glazes can behave differently from either glaze alone.
Troubleshoot one variable at a time
Create a controlled comparison using the same clay body, glaze batch, and firing where possible. Label test tiles with application method and coat count. Change only one factor per group of tiles. A kiln witness cone can help you evaluate actual heatwork instead of relying only on the controller display.
If a fired surface has sharp defects, questionable glaze fit, or exposed areas where food contacts the pot, do not assume it is food safe. Follow glaze manufacturer guidance and appropriate durability testing.
Use The Potter's Mud Room to find patterns
The Potter's Mud Room gives ceramic artists a practical place to track glazes, pieces, test tiles, firing records, and Studio Notes. Photographing a defect and connecting it to a specific clay body and firing helps you compare results rather than relying on memory. Over time, your records can reveal which combinations are consistently successful.
Takeaway: Identify the defect, document the conditions, and test one change at a time. Reliable glaze results come from repeatable processes—not guesswork.