Why use Specific Gravity (SG) measures? To any potter who invests time in making his or her own glazes or clays, the use of SG is a cheap, reliable tool that saves not only time, but a lot of money too. When we get a good glaze or a clay body, we want to be able to depend on it all of the time…all of the time. Ruining your work with glazes that are not properly hydrated is common and need not happen.
So, what is it and how does it work.
You will need a 100 ml (or cc) graduated cylinder, and an accurate gram scale…that is it. I use an older set of scales that holds up to shop abuse, but electronic scales are commonly available and undoubtedly work well too.
First, check the method as follows. Weigh the empty graduated cylinder and write it down. To arrive at a SG of 1.00, you must fill the cylinder with 100 ml of water, and it should weigh 100 g, plus the weight of the cylinder. An easier way is to adjust the counter weight on your gram scale or tare your electronic scale with the empty cylinder on it, then simply read the final result on the scale when it has the 100 ml of liquid in it.
If you weigh a glaze that is working for you now, put exactly 100 ml of it in your cylinder. You might get a weight of about 140 g, giving you the SG of 1.40. Now each time you re-make that glaze and are wondering if it is too thick or too thin, all you have to do is re-measure and you will have your answer in no uncertain terms. Some glazes want to be thicker, and some thinner for many reasons, and this is a way of getting the same reliable mix each time. If a glaze is too thick, it is easy to add some water, and it if is too thin, wait a couple of days for the solids to settle, and take some of the water off by siphon or sponge. How much to add or take off is told by the use of specific gravity testing. The same applies to slip casting bodies where it is even more critical to measure each batch and make scientific adjustments rather than a guess. Easy, cheap and accurate.
Harlan House RCA