A comparison of the weights of equal volumes of a given liquid and water. Water has a specific gravity of 1.0. A ceramic slurry with a specific gravity of 1.8 is thus 1.8 times heavier than water. The best way to measure specific gravity is to weigh a container and record its weight, then weigh the container full of water and full of the liquid of unknown specific gravity. Subtract the weight of the container from each weight and divide the weight of the liquid being measured by the weight of the water.
A casting slip of 1.9 specific gravity. Should we use it?
A hydrometer is being used to check the specific gravity of a ceramic casting slip in a graduated cylinder. Common traditional clay-containing ceramic slips are usually maintained around 1.75-1.8. In this case the slurry was too heavy, almost 1.9. Yet it is very fluid, why is this? It has both too much clay and too much deflocculant. While it is possible to use such a slip, it will not drain as well and it will gel too quickly as it stands. It is better to settle for a lower specific gravity (where you can control the thixotropy and it is easier to use). It might have been better to simply fill a 100cc cylinder and weigh it to get the specific gravity (slurries that are very viscous do not permit hydrometers to float freely).
Measuring slip viscosity the easy way
A Ford Cup being using to measure the viscosity of a casting clip. These are available at paint supply stores. It drains water in 10 seconds. This casting slip has a specific gravity of 1.79 and we target a 40-second drain. Maintenance of viscosity and specific gravity are vital to an efficient process in slip casting.
Slip of the proper specific gravity and viscosity is so much better
This deflocculated slurry of 1.79 specific gravity (only 28% water) has just been poured into a mold. The mold is dry, the wall thickness of the bowl will build quickly and the liquid level will sink only slightly. The mold can be drained in minutes (for a wall thickness of 3-4 mm). The clay is not too plastic (too fine particle sized) so it is permeable enough to enable efficient water migration to the plastic face. If the specific gravity of this slip was too low (too high a percentage of water) the liquid level would sink drastically during the time in the mold, take longer to build up a wall thickness and water-log the mold quickly. If the slip contained too much deflocculant it would cast slower, settle out, form a skiln and drain poorly. If it had too little deflocculant it would gel in the mold and be difficult to pour out.
Measuring glaze slurry specific gravity
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 thick to float it properly). Just counterbalance the empty graduated cylinder to zero, fill it to the 100cc mark and the scale reads the specific gravity. Be careful on cheap plastic graduated cylinders like this, check them with water and correct the true 100cc mark if needed (using a felt pen). You could actually use any tall narrow container you have (if you mark the 100cc level). The hard way? A container that holds other than 100cc: you have to divide the slurry weight by water weight.
Specific gravity of a glaze using a scale and measuring cup
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.
What to do when glazes dry-drip like this on the rims of ware
These are the same glazes. The one on the left had a specific gravity of 1.45 and the slurry was creamy and appeared to be good. However when this bisque porcelain mug was pulled out of the slurry (after the dip) the glaze dried so fast that it would not even out around the lip (even though I rolled it). To fix this I added water to take it to 1.43 specific gravity, they I added epsom salts to gel it back to the same creamy consistency it was. This time it went on evenly, dried more slowly and stayed even. Notice the darker color, is it still damp. Although the piece dries enough to handle in less than 30 seconds, it does take longer to dry completely.
Glaze at 1.7 specific gravity on green-ware. Way too thick!
This is G2926B clear cone 6 glaze deflocculated with Darvan. Because the Darvan is thinning it, 2.5kg of glaze powder is suspended in only 1100g (1100ml) of water (half the normal amount). While the slurry in the bucket flows well and appears like it should work, a one-second dip produces twice the desired thickness. It dries slowly and it is very difficult to prevent runs. The lesson: Make sure the specific gravity (SG) of your glazes is right. What should the SG be? Measure it when your glaze is working well. Or take note of it in instructions that come with the recipes you use. For bisque ware: 1.43-1.45 with a flocculant (like Vinegar or powdered Epsom Salts) added to gel the slurry slightly.
Measure glaze specific gravity with these? Forget it!
Glazes need to be gelled, have thixotropy. That means these things won't bob up and down to find the right place. The one on the right is completely useless, the scale is too wide. Most glazes need to be between 1.4 and 1.5 specific gravity (40-50 on this scale). That is like reading seconds on the hour-hand of a clock. And it is too long, when are you going to have enough glaze to float that? And the small one? It has a scale of 1.2 to 1.45. But it is a real bobber, it needs a fluid that gives it the ability to move. That will never be the case with a glaze. It is much better to weigh a measured volume of glaze slurry and calculate the SG instead. The easiest is a 100cc graduated cylinder, if 100 ccs weighs 140 grams, that is 1.4 specific gravity.
Specific gravities on three commercial glazes might surprise you
The freshly opened transparent low fire glaze on the left has a specific gravity of only 1.34. Yet it is viscous because they add alot of gum. It needs three coats to be on thick enough and takes quite a bit of time to dry each one. When dipping, a very thick layer dries very thin. So when you buy this you are getting mostly water! Perhaps this is another good reason to mix your own transparent cover and liner glazes. The center Potter's Choice glaze, made by the same company, is 1.52. That is a much better deal. And it goes on nice and thick. The Celadon glaze on the right is lower, 1.46. So glaze manufacturers can make a very broad range of specific gravities work, they just adapt the percentage of gum to impart the viscosity they want.
The specific gravities on two commercial underglazes might shock you
AMACO and Crysanthos. 1.26 (67.5% water) and 1.22 (68% water). It is possible they might be thinking that you want to be able to apply these in a thinner layer to get some transparency. However, generally you likely want to be able to make a brush stroke and have it cover well without having to go over it a second or third time. Strangely, the Crysanthos, although having a lower specific gravity is more viscous and goes on thicker. But really, these products are mostly water. If you make your own underglaze colors (by adding stain powders to a base) you would be able to mix them to have double the amount of powder that these have (per liquid volume unit) and with the right amount of gum they would paint on much better. Plus it would need less gum and would dry more permeable and the clear overglaze would cover without pinholes and bare spots.
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