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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 lateral displacement against each other). Viscosity is the opposite of fluidity, a term also commonly used, viscous slurries are thick and thus lack fluidity. Laboratory instruments that measure absolute viscosity (that can be quoted on a data sheet) are called viscometers and they express the result in a unit called the poise. Higher poise numbers mean a more viscous slurry. Units of fluidy are taken as 1/poise, thus 2 poise = 0.5 rhe (water has a fluidity of 100 rhe). However much simpler devices can also be practical for quality control and comparative studies (e.g. a Ford Cup used for paint simply times the drain of a liquid through a small hole in the bottom).

The viscosity of a slurry can be reduced by the addition of a deflocculant, fluid slurries of remarkably low water content can be produced. Deflocculants work their magic by imparting electrical charges to the surfaces of particles to make them repel each other. Conversely, the viscosity of a slurry can be increased by the addition of a flocculant that makes it gel (if its specific gravity is high enough). Soluble materials within a powdered mix can impede or block the action of deflocculants and particle properties like size, size distribution, shape, surface area, surface reactivity, density, etc. all affect their action. See the Potters Dictionary under Fluidity for a detailed and easy-to-understand discussion of this (especially relating to the dynamics imparted by flat particles with differing end and flats charges).

Molten glazes also exhibit viscosity, but the term 'fluidity' is normally used.

Controlling the viscosity of casting slips is vital to efficient production. However, it is critical that the correct specific gravity first be achieved, then it becomes apparent if more or less deflocculant is needed. Ideally, a slurry needs to be thin enough to pour and drain easily, but it needs to thixotropic enough to form a gel after some time (e.g. 1/2 hour) so that it does not settle out.

Viscosity of glaze slurries is an important factor in their performance. It seems obvious that ones of high viscosity will apply thicker to ware and vice versa. However in practice, the viscosity must be considered in consort with the specific gravity (and thixotropy). Since viscosity can be controlled by adding flocculants and deflocculants, it makes sense to first establish the correct specific gravity of a glaze, then adjust its viscosity. For typical bisque fired pottery a raw or partially fritted glaze works well at a specific gravity of about 1.45 (taking it too much higher could lead to settling, too thick application, tendency to drip, etc). Vinegar or Epsom salts can be added to increase the viscosity to that which works best (gives an even layer of glaze on a fairly quick dip).

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.

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.

Out Bound Links

  • (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) Melt Fluidity

    Glazes become fluid when they melt, they are molten. The fluidity (or viscosity) of this melt needs to be considered, especially when troubleshooting problems. While two different fired glazes may appear to have melted a similar amount (even on a vertical surface), one may be radically more fluid th...

  • (URLs) Viscosity at Wikipedia


  • (URLs) ViscosityJournal.com


  • (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...

In Bound Links

  • (Articles) A Low Cost Tester of Glaze Melt Fluidity

    This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.

  • (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...

  • (Tests) RHEO - Rheology of a Ceramic Slurry
  • (Tests) AVSC - Apparent Viscosity (cps)
  • (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...

  • (Glossary) 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 t...

  • (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) 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...

By Tony Hansen

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