|Monthly Tech-Tip |
If a glaze has already been mixed and gelled to give it thixotropy these things won't bob up and down to home in on the right level. If the glaze is watery enough there are other issues. The one on the right has a 1.0-1.7 scale. Since most pottery glazes need to be 1.4-1.5 specific gravity (40-50 on this scale) it is difficult to get a very accurate reading. And it is long, you will need a container tall enough to float it and enough glaze to fill it. The small hydrometer appears better, it has a scale of 1.2 to 1.45. But it really bobs up and down (so it is even more important that the slurry be runny and thin to give it the freedom to do so). But glaze slurries are creamy. So it is better to weigh a measured volume of glaze slurry and calculate the SG instead. The easiest is a 100cc graduated cylinder (from Amazon.com), if 100 ccs weighs 140 grams, that is 1.4 specific gravity. You don't even have to pour in 100cc, just pour in any amount and divide the weight by the amount.
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.
Deflocculation is the magic behind the ceramic casting process, it enables slurries having impossibly low water contents and ware having amazingly low drying shrinkage