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Rheology refers to the array of characteristics that a ceramic slurry exhibits: its flow, thixotropy, 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 process at hand). This is done by the control of specific gravity, water consistency and quality, selection of materials, temperature, mixing methods, and the addition of electrolytes.

For example, when it is desirable to have low water content in slurries deflocculation is employed. Flocculants can used for higher water concentration, sedimentation prevention or to create thixotropic characteristics.

The Old Hickory clay website has some excellent papers on understanding rheology in production of ceramic ware.

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.

Adding water actually made this white engobe run less? How?

Adding water actually made this white engobe run less? How?

The white slip (applied to a leather hard cup) on the left is dripping downward from the rim (even though it was held upside down for a couple of minutes!). Yet that slurry was viscous with a 1.48 specific gravity, on mixer-off the motion stopped immediately. Why? Because it was not thixotropic (it did not gel). The fix? I watered it down to 1.46 (making it very thin and runny) and did a cycle of adding a pinch of epsom salts (about 0.5 gm) and mixing vigorously watching for it to thicken enough to stop motion in about 1 second on mixer shut-off (bounce backward!). It is extremely difficult not to overdo the epsom salts (gelling it too much) so I keep ungelled slurry aside and pour some back in to dilute to overgelled batch. That works perfect to fine-tune the degree of thixotropy so it gels after about 10-15 seconds of sitting. So to apply it I stir it, wait a couple of seconds and dip the mug. By the time I pull it out it is ready to gel and hold in place.

Clay, feldspar, wollastonite, silica and frits are insoluble. Right?

Clay, feldspar, wollastonite, silica and frits are insoluble. Right?

Wrong! That is what the glaze was made of that was in this bucket. The scum on the inside is so hard that it is extremely difficult to remove, even using a scraper or a scrubber. Even lime-a-way does not remove it all. This is an example of how water-soluble materials can be. When this glaze settles out the water on top is brown (like this scum) yet all the material powders are white! So it is not surprising that glaze viscosity changes over time and things dissolve and impact rheology.

Fundamentals of Fluid Mechanics - book

Fundamentals of Fluid Mechanics - book

Many aspects of ceramic production relate to the control of fluids (mostly suspensions). This is also true of material production. If you want to solve problems and optimize your process this is invaluable knowledge. This book is available at amazon.com.

Out Bound Links

  • (URLs) Society of Rheology Website


  • (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 dipped into it. It goes on evenly 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 intuitive. It in...

  • (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) Flocculation

    The opposite of deflocculation. Flocculation in a slurry can be a desired or undesired property. For the latter, a ceramic glaze or clay slurry that would otherwise be thin and runny can be made into a gel by the simple addition of a flocculant. This is typically done to improve suspension prope...

  • (URLs) Malvern Instruments Rheology page


  • (URLs) Old Hickory website


In Bound Links

  • (Materials) Claytone - Bentonite-based rheological additive
  • (Materials) Garamite - Rheological additive
  • (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...

  • (Glossary) Water

    There is a need to discuss water in ceramic production as it related to a number of natural phenomena and production processes: Plasticity: Clays are plastic because water glues and lubricates the particles. The micro-dynamics of this are complex. Rheology: Suspensions (solids:water systems) e...

  • (Project) Ceramic Minerals Overview

    The materials we use are powders and we assess their physical presence on that level. However these powders are generally composed of microscopic mineral particles (except for frits of course). In man...

  • (Project) Ceramic Properties

    A property in this context is a created physical phenomenon in a glaze or body that can be achieved in a variety of ways (called mechanisms). For example, there are a number of ways to suspend a glaze...

  • (Glossary) Once fire glazing

    The practice of applying glazes to dried ware and firing in one operation. Obviously this is going to save money on energy. But it introduces extra problems also. In general, the thicker and heavier the ware and the greater its dry strength the greater the chance that it can be glazed easily in the ...

  • (Materials) Vinegar

    Vinegar Solution 20%

  • (Glossary) Bisque, bisquit firing

    Generally, bisque firing refers to the practice of prefiring ware without glaze to make it impervious to water, resistant to damage during handling and absorbent for glazing. The porosity of the bisque (generally more than 15%) makes it an ideal medium to absorb water from the glaze suspension and h...

By Tony Hansen

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