Tech Talk Tuesday: Common Glaze Defects
- , by Haley Stanaford
- 3 min reading time
We all know the feeling of opening up the kiln with lots of excitement and anticipation only to find that the glazes did something weird we weren’t expecting. There are so many things that can happen to a glaze and for a variety of reasons and these are called glaze defects. Here are the most common glaze defects and how and why to make changes in the relationship of clay and glaze.
Crazing
The “We’re not a Good Match” defect
Image from digitalfire Crazing enhanced by an additive
Crazing, possibly the most common glaze defect, shows up as a bunch of cracks in the glaze. This appears because the glaze is shrinking more than the clay. This can mean that the clay and the glaze are not a good match for each other. This could be a glaze makeup issue where you can add more silica to your glaze recipe to combat this or the glaze could have just been applied too thickly! While you may not be going for that particular look, there’s always a match for someone out there and some people search out these glazes to put India Ink in the cracks to create an interesting finish, however the cracks make the piece noot food safe!
Shivering
The “It’s not you, it’s me” defect
Image from digitalfire
Shivering is the direct opposite of crazing, with the clay shrinking more than the glaze but this can be hard to detect true shivering. If the glaze is cracking and falling off BEFORE firing, it may look like shivering but it is not actually shivering. Shivering can be detected by looking at the edges of the broken glaze. If the edges are sharp, it is shivering; if they are smooth, it happened before firing and is not shivering. True shivering can be caused by too much silica in the glaze recipe.
Crawling
The “Poor Hygiene” defect
Image from digitalfire
Crawling is when the glaze pulls together instead of spreading and adhering where it was applied. This can happen because there is too much clay in the glaze recipe. However, the most common reason is that the bisque was dirty! Bisque can sit and collect dust and oils from our hands when we touch them, making the glaze not adhere properly when applied. Some glazes are designed to crawl and create an interesting surface, but these glazes are not food safe.
Pinholing
The “Passing Gas” defect
Image from digitalfire
Pinholing is when little, pinhole sized dots appear in your glaze, looking like the glaze just separated and stayed that way. Well, that’s kind of what happened. Ceramics release gasses when they are fired, causing the glaze to have to separate to allow those gasses to pass through, these are called blisters. Pinholing occurs when those blisters do not fully close. An excess of gasses can occur when the bisque is under fired or thick, not allowing all the gasses to pass.
For more information and more glaze defects, check out Bracker’s super helpful Glaze defect pdf!
Haley Stanaford is currently a Post Baccalaureate student in ceramics at the University of Kansas where she is currently taking a Clay and Glaze Formulation taught by Colby Charpentier in addition to her studio practice. She also is a Sales Representative at Bracker’s Good Earth Clays in Lawrence, Kansas.
