|Monthly Tech-Tip |
These are the same glazes. The one on the left had a specific gravity of 1.45 and the slurry was creamy and appeared to be good. However when this bisque porcelain mug was pulled out of the slurry (after the dip) the glaze dried so fast that it would not even out around the lip (even though I rolled it). To fix this I added water to take it to 1.43 specific gravity, they I added epsom salts to gel it back to the same creamy consistency it was. This time it went on evenly, dried more slowly and stayed even. Notice the darker color, is it still damp. Although the piece dries enough to handle in less than 30 seconds, it does take longer to dry completely.
Thixotropy is a property of ceramic slurries. Thixotropic suspensions flow when you want them to and then gel after sitting for a few moments. This phenomenon is helpful in getting even, drip free coverage.
In ceramics, the specific gravity of casting slurries and glazes tells us their water-to-solids. Body slurries especially require tight control of this property for performance reasons.