I have been struggling with this problem for about a year now. Can anyone please help with any suggestions or comments? I am getting a variety of blister sizes. The areas affected seem to be verticle surfaces, the underside of plate rims, edges of plates, but interestingly, not the rims of taller vessels.I am using a clear bright glaze recipe which melts totally. This is what I have found so far...
- I may have started experiencing this problem when I reduced the whiting content in my glaze by around 3-5%. I don't have enough documention on this; it's just one possibilty I need to test. I was glaze firing to ^6, bisque to ^04.
- I discovered in one glaze fire that the cooler shelves had significantly less blisters, so I started doing all the glaze fire to ^5. This reduced the incidents significantly. Still firing bisque to ^04.
- My large witness cones placed in the center of each shelf typically read one cone less than the junior cone in the kiln sitter. Bottom-most shelves maybe yet another half cone cooler.
- Bubbles in the glaze application don't seem to factor at all. The water soaking and deflocculating of the glaze related to this seems not a factor either. This is because these techniques just seem to result in a thinner glaze application, which does, in fact reduce the problem. However, this doesn't really help me because I am going for a very thick application.
- I have not been able to reproduce this problem on test tiles except in one special case. I suspected that I was contaminating the clay body with plaster from my wedging board. I wedged bits of this plaster into a large test tile, bisqued, then glazed, and voila, got beautful blisters. Until I find a better surface, I am doing all my wedging on canvas. Sadly, I am still experienceing this problem on accasion from plaster-free wares.
- I am also suspecting contamination from kiln shelves; kiln-wash powder dropping onto the wares. Another possibilty I need to thorougly test.
- I don't have a digital controller, so I can't really do a soak which is another possible solution.
- I am just beginning to venture into bisq at higher temps, such as ^03. Can't say anything about this.
- Try looking at your pinholes with a magnifying glass. See if there are any foreign particles in the center.
If you have any ideas or comments, I would very much appreciate it.