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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 slightly and can be removed from the mold.

The sanitary ware industry produces the largest tonnage of products using this process. Water closets and sinks are made by casting porcelains in very complex many-part molds. Molds are heavy, cast sections are thick and take considerable time to release and extract from the molds. By contrast, fine delicate porcelain items can be cast very thin and quickly.

But almost any type of powdered ceramic can be made into a slurry and cast. If the slurry can be suspended and deflocculated, it shrinks enough on drying and has enough strength to hold itself together as it pulls away from the mold, it can be cast. Even non-plastics like calcined alumina and silicon carbide can be cast by incorporated small additions of plasticizers and binders. Since the whitest burning clays and low plasticity, casting lends itself to producing white burning ware.

In industry, casting rate is important. Items need to cast quickly (build up a layer against the mold) and extract from the mold quickly (shrink away and hold together as extracted). Molds need to be dried rapidly. Much automation has been devised to speed these processes. In addition, much can be done to fine-tune the balance of permeability and plastic strength of a body. It is important to understand the role of each material in the body recipe and other options for each of those materials. A typical industrial casting body has much more kaolin, less ball clay and usually no bentonite. A plastic throwing body, by contrast, could have nothing but ball clay as the plastic component in the recipe. Some industrial casting bodies go even further, selecting kaolins of large particle size, since these are more permeable to water. This is not to say that high ball clay (or other plastic material) bodies cannot be cast, they can, it just takes longer. The standard low fire white casting body recipe is 50:50 ball clay and talc.

Because casting bodies are relatively non-plastic they have low drying shrinkages and ware can be dried quickly with much less likelihood of cracking.

A slip cast bowl just removed from its plaster mold

A slip cast bowl just removed from its plaster mold

With a simple open shape like this a thin wall (2-3mm) bowl can be cast in minutes and removed from the mold in minutes more. No other method can produce such thin and even ware with this kind of ease.

Scale, calipers and fired test bars to be measured for shrinkage

Scale, calipers and fired test bars to be measured for shrinkage

These are part of the procedure for the SHAB test. The length of the bars is entered into a recipe record in your account at insight-live.com. When Insight-live has these numbers it can calculate the drying and fired shrinkages.

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.

Cast to only 1mm wall thickness? NZ Kaolin+VeeGum can.

Cast to only 1mm wall thickness? NZ Kaolin+VeeGum can.

This cast bowl (just out of the mold and dried) is 130mm in diameter and 85mm deep and yet the walls are only 1mm thick and it only weighs 89 gm! The slip was in the mold for only 1 minute. What slip? A New Zealand Halloysite based cone 6 translucent porcelain. This NZ material is fabulous for casting slips (it needs a little extra plasticizer also to give the body the strength to pull away from the mold surface as it shrinks).

Can you mix all this powder into that little water?

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).

Optimimal casting slurry properties impossible without good mixing

A video of the kind of agitation you need from a power mixer to get the best deflocculated slurry properties. This is Plainsman Polar Ice mixing in a 5 gallon pail using my mixer. Although it has a specific gravity of 1.76, it is very fluid and yet casts really well. These properties are a product of, not just the recipe, but the mixer and its ability to put energy into the slurry.

Crawling glaze on slip cast ware is common

Crawling glaze on slip cast ware is common

This cone 6 white glaze is crawling on the inside and outside of a thin-walled cast piece. This happened because the thick glaze application took a long time to dry, this extended period, coupled with the ability of the thicker glaze layer to assert its shrinkage, compromised the fragile bond between dried glaze and fairly smooth body. To solve this problem the ware could be heated before glazing, the glaze applied thinner, or glazing the inside and outside could be done as separate operations with a drying period between.

Out Bound Links

  • (Glossary) Rheology

    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 d...

  • (Articles) Understanding the Deflocculation Process in Slip Casting

    Understanding the magic of deflocculation and how to measure specific gravity and viscosity, and how to interpret the results of these tests to adjust the slip, these are the key to controlling a casting process.

  • (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) 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...

In Bound Links

  • (Materials) Boron Nitride


  • (Glossary) Slip

    In ceramics, this term can refer to a number of things: -A clay slurry poured into molds to be cast into shapes. The slip is deflocculated to minimize water content and fine tune viscosity. The deflocculation process involves using special chemicals that enable you to create a fluid clay-water slur...

  • (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...

By Tony Hansen

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