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Specific gravity


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.

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. Hobbyists and potters typically target around 1.75 wheres in industry 1.8 or higher is needed (especially in sanitaryware). 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 a specific gravity measurement. That tells the technician if the issue is with the percentage of water or deflocculant.

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 (obviously highly deflocculated), something that would appear impossible to the average potter. 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. Raising the water content (thus lowering the specific gravity) and gelling the slurry (using vinegar for example) is a way to deal with this. Notwithstanding this, doing the opposite, deflocculating the slurry, is also a way to prevent sedimentation (provided it is still viscous). However a typical potter is not well equipped for the challenges of applying such a glaze.

Flocculating a glaze slurry is an effective way to make it easier to use, apply more evenly and drip less. To be flocculated a glaze obviously needs to be more runny than normal, even to the point of settling out. For many common partially-fritted glaze recipes this occurs around 1.43 to 1.45 specific gravity. The addition of epsom salts or vinegar to the slurry gels it and imparts the property of thixotropy (it gels somewhat upon standing). The important point here is that once a specific gravity value has been decided, that must be right before any attempt or flocculate the slurry.

A casting slip of 1.9 specific gravity. Should we use it?

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

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

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

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

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

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!

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!

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

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

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.

Out Bound Links

  • (URLs) Specific Gravity at Wikipedia

    http://en.wikipedia.org/wiki/Specific_gravity

  • (Videos) Thixotropy and How to Gel a Ceramic Glaze

    I will establish specific gravity first, then gel the slurry, then establish thixotropy. This will change your life! Glazes that you have never been able to suspend or apply evenly will work beautiful...

  • (Glossary) Viscosity

    The term viscosity is used in ceramics most often to refer to the degree of fluidity of a slurry or suspension (the term 'shear' is often used when discussing viscosity, theoretically engineers understand viscosity in terms of layers particles or molecules that exhibit a friction that resists latera...

  • (Glossary) Thixotropy

    Knowing about thixotropy will enable you to mix a glaze that stays in suspension much better. It does not drip alot when a piece is draining. It goes on evenly, of adequate thickness and does not run. It dries quickly (on porous bisque) and is just much nicer to use. The secret to all of this is not...

In Bound Links

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

    In ceramics, when we speak of deflocculation, we are almost always talking about making a casting slip. Glazes can also be deflocculated (to reduce water content and densify laydown). Deflocculation is the process of making a clay slurry that would otherwise be very thick and gooey into a thin po...

  • (Glossary) Casting, Slip Casting

    Forming pottery by pouring deflocculated (water reduced) clay slurry into plaster molds. In the process the absorbent plaster pulls water from the slurry and over a period of minutes a layer builds up against the mold surface. The slurry is then poured out and within a short time the item shrinks sl...

  • (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 Perkins Studio Clear (it contains alot more SiO2 and uses a frit instead of Gerstley Borate as the b...

  • (Troubles) Uneven Glaze Coverage

    The secret to getting event glaze coverage lies in understanding how to make thixotropy, specific gravity and viscosity work for you

  • (Glossary) Glaze Layering

    In hobby ceramics (at low temperatures), layering of glazes for decorative effects has been commonplace for many decades. Potters have traditionally used dipping and pouring techniques, but in recent years they have increasingly adopted commercial prepared brush-on glazes for their stoneware pottery...

  • (Glossary) Spray Glazing

    In the production of smaller bisque fired ceramics it is almost always possible to dip-glaze ware. However, this is seldom an option for single-fire ware (especially if large). This is the case in the sanitary ware industry, for example. Spraying is the only option, and it is a very effective one if...

  • (Troubles) Powdering, Cracking and Settling Glazes

    Powdering and dusting glazes are difficult and a dust hazard. Shrinking and cracking glazes fall off and crawl. The cause is the wrong amount or type ...

  • (Glossary) Rheology

    Rheology refers to the array of characteristics that a ceramic slurry exhibits: its density, flow, thixotropy, permeability, viscosity, stability, etc. Technicians seek to understand and control the dynamics of the slurries they use (to maintain consistency and optimize them for the product and proc...


By Tony Hansen




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