Measuring Specific Gravity
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.
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.
3) Specific Gravity = (weight of 50ml glaze) minus (weight of the syringe), then divide by 50.
So, the glaze in the picture weighs 112g. Subtracting the 35g weight of my syringe gives me 77g. 77g divide by 50 gives us 1.54, which is the specific gravity of this glaze.
In general, a dipping glaze should have a specific gravity of 1.30 to 1.60. 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.
A few notes on measuring specific gravity.
1) With a newly mixed batch of glaze, the ingredients will continue to absorb water for quite some time. It is best to mix up a batch of glaze, get it close to your target specific gravity, then test the glaze again the next day and make adjustments if necessary.
2) A 50ml syringe full of water will have a specific gravity of 1.00
3) After many uses the black rubber plunger part can get a little sticky. A light lubrication with some vegetable oil will get it back in working order.
4) Here are links to videos I found helpful when figuring this specific gravity thing out:
A video by John Britt explaining specific gravity.
An AMACO video on specific gravity.