skills/pottery-mentor/references/glaze_firing_troubleshooting.md

# Glaze and Firing Troubleshooting

## Start with the defect, not the fix

Collect:
- clay body;
- bisque cone;
- glaze/product/recipe;
- application method and thickness;
- glaze firing target;
- witness cone result;
- atmosphere;
- hold/cooling;
- kiln position;
- layering;
- refire history.

## Crazing

Visible network of glaze cracks.

Often related to glaze/body thermal-expansion mismatch.

Do not assume cooling speed alone is the root cause.

For functional ware, evaluate fit and intended use rather than treating decorative crazing as automatically acceptable.

## Shivering

Glaze chips/flakes from rims, edges, or corners.

This can create sharp fragments and is a serious concern for functional ware.

Treat as a glaze/body fit issue until evidence shows otherwise.

## Crawling

Glaze pulls into islands/beads leaving bare clay.

Possible contributors:
- poor adhesion;
- dusty/greasy bisque;
- excessive application;
- high drying shrinkage;
- glaze layering behavior;
- composition/surface tension.

Check application and surface preparation before changing chemistry.

## Pinholes / pitting

Small holes/craters in the glaze surface.

Possible contributors:
- outgassing;
- body/grog;
- application;
- under-melted or viscous glaze;
- firing cycle;
- glaze chemistry.

Do not blindly increase peak temperature; the glaze could instead be overfired or otherwise unsuitable.

## Blistering

Raised bubbles/craters from gas trapped during glaze melt.

Possible contributors:
- thick application;
- overfiring;
- body/glaze decomposition;
- glaze composition;
- gases not healing before freeze.

## Running

Check:
- glaze thickness;
- layering;
- overfiring/heatwork;
- flux balance;
- vertical vs horizontal test behavior.

Use cookies/catch plates for uncertain tests and respect kiln-shelf safety.

## Test strategy

Before changing a production glaze:
1. reproduce on small tiles;
2. keep the current recipe as control;
3. change one variable at a time;
4. use exact weights;
5. fire together when comparison requires it;
6. use witness cones;
7. photograph and label results;
8. retain notes.

For chemistry changes, prefer small percentage steps and do not casually add materials with significant toxicity or handling hazards.

## Key principle

A glaze defect is an interaction among:
- clay;
- glaze;
- application;
- firing;
- cooling;
- geometry.

There is rarely one universal correction.