Specific Gravity

Written By Jake Corboy

Measuring the specific gravity of your glaze is very important. It allows you to mix different batches of the same glaze consistently. It helps you decide if you should add more water to a glaze to thin it down, or deflocculate it with a dispersal instead. It also helps when water has evaporated out of a glaze that has been sitting around for awhile. This way, you're able to bring it back to its original consistency. Basically, measuring specific gravity is telling you how much water is in your glaze.

There are several ways to measure the Specific Gravity of a glaze. I prefer using a 50ml plastic plunger syringe, because it is fairly quick, accurate and easy to measure with and clean.

Here is how to go about measuring the specific gravity of a glaze with a 50ml plastic syringe:

1) Weigh the plastic syringe and write it down. The syringe should be clean and completely dry when doing this.

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2) Mix the glaze to be tested thoroughly. Then suck up the glaze to the 50ml line (make sure there are no big air bubbles in the syringe). Clean the glaze off the outside of the syringe with a sponge. Then weigh the glaze filled syringe.

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3) Specific Gravity = (weight of 50ml glaze) minus (weight of the syringe) times 2, then divide by 100.

So, the glaze in the picture weighs 112g. Subtracting the 35g weight of my syringe gives me 77g. 77g times 2 = 154. Then divide by 100 (or just move the decimal to the left 2 spaces) and that gives us 1.54, which is the specific gravity of this glaze.

In general, a dipping glaze should have a specific gravity of 1.40 - 1.70. Each glaze is different and each person will have different preferences as to how thick they like the glaze. So, testing and adjusting the glaze according to your preferences is necessary.

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Deborah Schwartzkopf

Rat City Studios is the workspace of Deborah Schwartzkopf, Seattle based studio potter and instructor. Her mission is to engage and build community through clay – one person, one neighborhood, and one experience at a time. In service to this pursuit, she offers studio assistant positions for emerging artists, connects people through social events, instructs classes and workshops, and maintains a lively career in the ceramic arts!