Specific gravity is the weight of a volume of glaze or slip compared with the same volume of water. Potters measure it to keep glaze and casting slip at a consistent thickness, so results stay the same from batch to batch.
What Is Specific Gravity?
Water has a specific gravity of 1.0. A glaze that weighs 1.45 kg per litre has a specific gravity of 1.45. Because glaze and slip are made of heavy mineral powders in water, the more powder there is, the higher the figure.
The easiest way to measure it is to weigh a known volume, such as 100 ml in a measuring cylinder, and divide the weight in grams by the volume in millilitres. A hydrometer floated in the glaze gives a reading directly.
Many dipping glazes work well somewhere around 1.4 to 1.5, while casting slips are typically higher, often around 1.7 to 1.8, but recipes and makers give their own targets. Adding water lowers the figure, and letting the glaze settle and pouring off water raises it.
Checking specific gravity is one of the simplest ways to stop glazes behaving differently from week to week as water evaporates from the bucket. Record the figure on the bucket each time you adjust it.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.