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This is the easiest way to measure the specific gravity of a glaze if it is not in a container deep enough to float a hydrometer (or if it is too viscous to enable free movement). Just counterbalance the empty graduated cylinder to zero (you can buy these at amazon.com), fill it to the 100cc mark and the scale reading divided by 100 is the specific gravity. Be careful on cheap plastic graduated cylinders like this, check them with water and mark the true 100cc mark if needed. You could actually use any container, just fill it with water and mark the level, then fill to the same level with slurry and divide the slurry weight by water weight.
The specific gravity of a glaze slurry is simply its weight compared to water. Different glazes optimize to different specific gravities, but 1.4 to 1.5 is typical (highly fritted glaze are higher). To measure, counter-weigh a plastic measuring cup on your scale and fill it with 500 grams of water and note how high the water fills it (hopefully to the 500cc mark!). Fill the container with your glaze to the same place. Divide its weight by the number of ccs (in this case, 500) and you have the specific gravity. The more you weigh the more accurate is the test.
Potters sometimes call this a "floating thingy". Maybe, because of the problems it presents, it does not make a big enough impression to remember the name! Because of the length of the hydrometer the only container we have is this graduated cylinder. I have to fill it just the right level so it reads near the top. OK, fine. But the hydrometer needs to bob up and down to find home. However, this glaze has our desired thixotropy, which prevents free movement. OK, I will carefully help it find home by pushing it down a little. But then it doesn’t want to bob back up! Ok, I’ll pull it up ... then is doesn't want to go back down to where it should float. Not great. Next problem: The glaze is opaque, I can’t see the reading. Yikes! A better way would be to throw out the hydrometer and just tare the empty cylinder on a scale, fill it to 100 and read the SG as the weight/100. Or fill it to any mark and divide the weight by that.
Glossary |
Rheology
In ceramics, this term refers to the flow and gel properties of a glaze or body suspension (made from water and mineral powders, with possible additives, deflocculants, modifiers). |
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Glossary |
Deflocculation
Deflocculation is the magic behind the ceramic casting process, it enables slurries having impossibly low water contents and ware having amazingly low drying shrinkage |
Glossary |
Slurry
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Glossary |
Thixotropy
Thixotropy is a property of ceramic slurries of high water content. Thixotropic suspensions flow when moving but gel after sitting (for a few moments more depending on application). This phenomenon is helpful in getting even, drip-free glaze coverage. |
Glossary |
Specific gravity
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. |
Articles |
Understanding the Deflocculation Process in Slip Casting
Understanding the magic of deflocculation and how to measure specific gravity and viscosity, and how to interpret the results of these tests to adjust the slip, these are the key to controlling a casting process. |
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