|Monthly Tech-Tip |
These are the same dipping glaze. The slurry of the one on the left had a specific gravity of 1.45, it was creamy and appeared to be good. However, when this bisque porcelain mug was pulled out after the dip it dried so fast that it would not even out around the lip (in spite of my efforts to roll it). To fix this I added water to decrease the specific gravity to 1.43 (making it quite watery). Then I added Epsom salts to induce thixotropy (gel it), bringing it back to a creamy consistency. This time it went on evenly. Notice the darker color, it is still damp. Although the piece dries enough to handle in less than 30 seconds, it does take longer to dry completely because there is more water in the slurry. An alternative way to slow down drying would be the addition of CMC gum, but it would be very tricky to get just the right amount (otherwise dripping could be worse than this).
CMC gum is indispensable for many types of ceramic glazes. It is a glue and is mainly used to slow drying and improve adhesion and dry hardness.
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 glaze coverage.
In ceramics, the specific gravity of slurries tells us their water-to-solids ratio. That ratio is a key indicator of performance and enabler of consistency.