Digitalfire Ceramic Glossary

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Specific gravity

A comparison of the weights of equal volumes of a given liquid and water. 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.

Specific gravity is very important in the production of casting slips where low-as-possible (or, more accurately, low-as-practical) water content is needed. It is typical to target 1.75 to 1.8, this is achieved using the deflocculation process. When slurries are not behaving normally (e.g. settling, gelling, casting too slowly or unevenly, not draining from the mold properly, not releasing) the first step in isolating the reason is to do a specific gravity measurement.

Glazes do not have a universally desirable specific gravity range like casting slips. The same glaze can be used effectively by different people and in different processes having quite different specific gravities. We have seen glazes with a specific gravity approaching 1.75, others at less than 1.4. Some industries even prepare their glazes up to 2.0, something that would appear impossible to the average potter. This is not only due to the differences in glaze materials and the way they suspend and interact in the slurry, but it is also common to add flocculants and deflocculants to glaze slurries. Caution is needed here. Some glaze recipes, when mixed to the apparent correct viscosity (having no additives), will have a fairly high specific gravity (e.g. 1.55-1.6). These commonly settle out into a hard layer on the bottom. It is better to raise the water content (thus lowering the specific gravity) and gel the slurry (using vinegar for example). A suspension of low specific gravity will settle less and respond better to raising of its immediate thixotropy (or tendency to gel right away when not in motion). This is a key property to making a slurry easier to use and apply more evenly. As an example, here are the instructions for one commercial glaze sold as a powder:

"Adjust the amount of water so you have a specific gravity of 1.45. Then add vinegar (if needed) to gel it. You can tell if it is gelled by stirring and then stopping and watching it. It should continue moving for a few seconds, then stop and bounce back slightly. For a gallon of glaze it might take 3-4 capfuls of vinegar. Stir after each one to see if there is an effect (be careful not to put too much). When it is right you will be able to dip a piece for 1-2 seconds, withdraw it and get an even layer without dripping or rapid drying."

The above is just a suggestion. The bottom line with glazes is to get an even laydown of the proper thickness that is well-stuck to the ware. Different technicians can achieve that using different specific gravities.


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 fill 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).

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.

Out Bound Links

In Bound Links

  • (Tests) RHEO - Rheology of a Ceramic Slurry
  • (Glossary) Deflocculation

    In ceramics, when we speak of deflocculation, we a...

  • (Glossary) Casting, Slip Casting

    Forming pottery by pouring deflocculated (water re...

  • (Recipes) G2926B - Cone 6 Whiteware/Porcelain Transparent Base Glaze
    A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
    2014-02-06 - This is an adjustment to an original recipe named ...
  • (Troubles) Uneven Glaze Coverage
    The secret to getting event glaze coverage lies in...
  • (Glossary) Glaze Layering

    Glazes are often layered for decorative effects. T...

  • (Glossary) Spray Glazing

    In the production of smaller bisque fired ceramics...

By Tony Hansen

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