Pinholing
- Details
- Last Updated: Thursday, 19 June 2014 19:43
This issue we are going to discuss a glaze surface defect called “pinholing”. Do you ever notice small pitting in your glazes? Very small pits are called pinholes and are most commonly caused by out gassing of under-fired bisque during the glaze fire. (There are other causes discussed below). If your clay body is not fired to maturity in the bisque fire, then during the glaze fire your ware will reheat causing more organics in the ware to be released through the glaze surface. As a result, your glaze will not “heal” in the cooling stages, leaving pinholes.
For low fire work, Mayco recommends that bisque ware be fired 2 cones hotter than the glaze. For instance, clay should be bisque fired at cones 03/04 for a 05/06 glaze fire. This ensures that your bisque has been fired to a hotter temperature, so that when you bring your glaze up to temperature your ware does not reheat to a level where additional out gassing occurs.
Of course, this recommendation is not always practical for a busy art educator who has limited kiln space, limited time, etc. We know you have to cut a few corners at times. So, how do you save some time but still achieve good results? We have two recommendations:
- Stroke & Coats® can be applied to clay and fired to cone 04. Leave an area – bottom or back – unglazed to allow an area for out gassing. No clear glaze required. Your students’ art will have a gloss finish. Not recommended for dinnerware projects.
- Apply Stroke & Coats® to bisque ware, leaving the bottom (as for dry footing), back or an area unglazed. Out gassing should occur in the unglazed area, leaving your surface smooth and unblemished. Not recommended for dinnerware projects.
Other causes of pinholing:
- Dust or other debris on the ware surface
- Application of wet glaze over wet surface
- Over application of glaze does not cause pinholing but can aggravate the problem of out gassing, making it more difficult for trapped gases to escape.
If you are assured that your ware is properly fired but you continue to experience pinholing, you may want to add a hold or soak at the end of the firing cycle.
As always, we would recommend that you follow best practices whenever possible. If you have experienced problems or have technical questions, please contact us at This email address is being protected from spambots. You need JavaScript enabled to view it.. For information about the use of shelf or witness cones used in kiln firing, visit the Edward Orton, Jr. Ceramic Foundation at www.ortonceramic.com.