Digitalfire Ceramic Glossary

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Deflocculate, deflocculation, deflocculant

The process of making a clay slurry that would otherwise be very thick and gooey into a thin pourable consistency. It is the opposite of flocculation. Deflocculants are electrolytes liquids or powders (like sodium silicate, Darvan, Calgon) that are added in small amounts, they work their magic by imparting electrical charges to clay particles making them repel each other (more accurately it is said to be a condition where repulsive forces predominate). Deflocculated slurries tend to settle slowly into a hard layer at the bottom.

To deflocculate a slurry properly it is very important to be able to measure its specific gravity and viscosity accurately. Yet it is very common for slip casters to be tied to a recipe and have little understanding of how to control their slip. Many will work for years with substandard slip without knowing it, others will throw away all scrap rather than reprocessing it simply because they do not understand slip rheology. It is common for potters to mix slips using clays intended for modeling or sculpture. Far better casting mixes can be made using mixes of materials that emphasize permeability instead of plasticity. The best slips are made by bringing the slurry into a state of controlled flocculation whereby the solids content is high but there are also enough repulsive forces present to cause the system to gel after a period of time (this prevents it settling out). Such systems stop short of complete deflocculation and the lowest possible water content in the interests of achieving a slurry with better working and suspension properties.

Once you have used a slip that has been properly formulated and deflocculated for casting you will never go back to using an inadequate one.

Sometimes glazes are deflocculated to reduce their water content, this is most likely where glaze is being applied to once-fire ware.


Viscosity deflocculantion curve

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 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.

An example of how an over deflocculated ceramic slurry forms a skin a few minutes after the mixer is stopped.

Properly deflocculated slip is so much better to use!

A deflocculated slurry of 1.79 specific gravity (only 28% water) has just been poured into this mold. In a dry mold it will form a 2mm thick wall in 2 minutes and the level will sink only slightly. For improperly mixed slip (with too much water and therefore too low a specific gravity) the level will sink drastically during the time in the mold, take far longer to build up a wall thickness, drain poorly and waterlog the mold quickly.

Can you mix all this powder into that little water?

This is 568cc of water and 1400 grams of Polar Ice porcelain casting clay. Amazingly enough it is possible to get all that powder into that little bit of water and still have a very fluid slurry for casting. The volume will increase to only 1065cc. How is this possible? That water has 13 grams of Darvan 7 deflocculant in it, it causes the clay particles to repel each other such that you can make a liquid with only little more water than is in a throwing clay! All it takes is 15 minutes under a good power propeller mixer (in a bigger container of course).

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).

Over deflocculated slip causes instability in toilet tank

Sanitary ware factories optimize their slips to have the lowest possible specific gravity for production volume reasons. Potters would be happy with 1.7 SG whereas numbers approaching 1.9 SG are common in factories. They often teeter on the edge of issues like this (sections softening causing localize warping) and inexperienced technicians can be unaware of the critical balances needed to prevent loss in production.

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By Tony Hansen

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