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).
A viscosity deflocculantion curve
As the amount of defloccuant is increased the viscosity drops and the slurry becomes more and more fluid. However, at some point, the slurry will begin to become more viscous with increasing deflocculant percentages. This underscores the importance and tuning your casting slip recipes to avoid this problem. It is actually better to deflocculate to a point before the curve reaches its minimum (where the slop is still downward). This "controlled state of flocculation" enables the slip to gel after a period of time (to prevent sedimentation) and avoids the issues that come with over-deflocculation.
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).
Over-deflocculated ceramic slurry forms a skin
In this instance, the slurry forms a skin a few minutes after the mixer has stopped. Casting recipes do not travel well. Over-deflocculation is a danger when simply using the percentage of water and deflocculant shown. Variables in water electrolytes, solubles in materials, mixing equipment and procedures, temperature and production requirements (and other factors) necessitate adapting recipes of others to your circumstances. Add less than the recommended deflocculant to try and reach the specific gravity you want. If the slurry is too viscous (after vigorous mixing), then add more deflocculant. At times, more than what is recommended in your recipe will be needed. After all of this you will be in a position to lock-down a recipe for your production. However flexibility is still needed (for changing materials, water, seasons, etc).
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.
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.
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.
All that powder with that little water for a casting slip?
This is 8.4L of water (in the bottom of that pail) and a 20kg bag 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 2/3 of this 5 gallon pail. How is this possible? That water has 100 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.
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.
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).
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.
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.
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