Rheology
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Rheology refers to the array of characteristics that a ceramic slurry exhibits: its density, flow, thixotropy, permeability, 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). Their task is complicated by variations in water consistency and quality, materials changes, temperature variations, growth of mold and mixing procedure inconsistencies. In a factory setting a casting slurry, for example, is typically maintained to perform well if its fluidity, one-minute thixotropy and density all fall within certain ranges.
For example, when it is desirable to have low water content in slurries, deflocculation is employed. Flocculants can be used for higher water concentration, sedimentation prevention or to create thixotropic characteristics. At time both are used, to both reduce water content and gel a slurry.
The Old Hickory clay website has some excellent papers on understanding rheology in production of ceramic ware.
Fundamentals of Fluid Mechanics - book
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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.
A casting slip of 1.9 specific gravity. Should we use it?
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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).
Confirming slip viscosity using a paint-measuring device
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A Ford Cup is being using to measure the viscosity of a casting slip. These are available at paint supply stores. This is a #4 (4.25mm opening), it holds 100ml and drains water in 10 seconds. This casting slip has a specific gravity of 1.79. Having made it many times, our experience indicates 40-seconds as a drain target (after high energy mixing). In production situations, the seconds-value this test produces enables an audit-trail for quality control and problem solving. When first mixing a slurry, under-deflocculating and eye-balling the viscosity is typical, during that period the slurry gels while draining and Ford cup measurements are not valid. When the mixing process has been perfected and viscosity stabilized the Ford Cup becomes practical.
Don’t overthink this type of measurement or the type of cup or opening size (you can even make your own cup). You must still determine the optimal flow rate based on experience with your process. This technique is more about maintaining to ongoing adherence to a standard you define.
Slip having proper rheology is so much better
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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. It 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 skin and drain poorly. If it had too little deflocculant it would gel in the mold and be difficult to pour out. The rheology is just right.
Are cheap plastic graduated cylinders useful?
Yes, you can calibrate them yourself.
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Graduated cylinders like this are inexpensive (e.g. Amazon.com). Of course, these cheap plastic ones are not accurate. But, there is one factor that makes them useful: They are light and can thus be used to measure specific gravity on a 200g scale. There is a way to make these accurate enough for a potter: Calibrate them by checking with 100g and 50g of water, just mark the levels using a Sharpie.
How stop dripping and running:
Add water! Then make it thixotropic.
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The white slip on the left, L3685Z2, (applied to a leather hard cup) is dripping downward from the rim (even though it was held upside down for a couple of minutes!). Yet that slurry was very viscous with a 1.48 specific gravity. Why? Because it was not thixotropic. The fix? I watered it down to 1.46 (making it runny) and added pinches of powdered Epsom salts (while mixing vigorously) until it thickened enough to stop motion in about 1-2 seconds on mixer shut-off. But that stop-motion is followed by a bounce-back. That is the thixotropy. It is easy to overdo the Epsom salts (gelling it too much), I add a drop or two of Darvan to rethin it if needed. When the engobe is right, it gels after about 10 seconds of sitting, so I can stir it, dip and extract the mug, shake to drain it and then it gels and holds in place. Keep in mind, this is a pottery project. In industry, they deflocculate engobes to reduce water content and then impose thixotropy, but that is more technical than the average potter would want.
Clay, feldspar, wollastonite, silica and frits are insoluble. Right?
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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.