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Propeller Mixer

An important, even essential tool in ceramic labs, studios and classrooms for mixing test and production slurries (body and glaze) is a good propeller mixer. Particles in ceramic powders can be exceptionally small (and often agglomerated) and wetting all their surfaces requires the injection of energy into a slurry that only a device such as this can do. This is especially important when the slurry is deflocculated and thus has a low water content. In addition, slurries get lumpy during use, a good mixer is needed to smooth them back out.

For mixing glazes and casting slips a mounted mixer that can run for hours at the desired speed is much better than a hand-held device, especially so if it has a good propeller. This is for one special reason: Observability. Ceramic slurries often need carefully balanced rheologies (where you are fine tuning specific gravity, viscosity and thixotropy). An important tool in achieving that is being able to observe changes in the way the moving slurry behaves as you add things. Being able to adjust the angle and speed of the mixer shaft such that maximum energy is being injected but without sucking any air bubbles is very important. At this state the behaviour of the slurry is very sensitive to changes, especially with deflocculants. For example, if nothing changes when some deflocculant is added you will know the slurry is over deflocculated. Even changes in the texture and reflectivity of the surface become evident. Another capability a mounted mixer presents is being able to shut off the mixer and watch the behaviour as the slurry comes to a stop (e.g. a thixotropic glaze should stop in a few seconds and bounce back slightly). Or, being able to slow it down and observe if the edges stop motion while the centre continues (indicative of too little deflocculant).
You should have a table-top model for mixing small test batches of a litre to a couple of gallons (around 1/20hp motor). A floor model for mixing larger amounts (e.g. 3-5 gallons) is also important (around 1/3 to 1/4hp motor). You can make your own large mixer inexpensively.

These machines must be treated with respect, they are potentially dangerous and can inflict serious injury if not used carefully.

It is important to have a good mounting system that enables leaving the mixer running for a period of time (minutes to hours). The mount should offer good control over vertical position and angle of the shaft (adjustment of these parameters enables mixing at maximum RPM without sucking air bubbles into the slurry).

Variable speed lab mixers can be very expensive to buy, a small high quality "Lightnin brand" can cost many thousands of dollars! However bargains for these can be found on ebay. Better yet, search "lab mixer" or "lab stirrer" at Amazon.com and you will find many more (see attached photo).

Home-made propeller mixer with mount and switch

Home-made propeller mixer with mount and switch

This is a heavy-duty unit, home made, with a 1/3 hp motor. It that can handle 5 gallons of high density suspension glaze or body slurry.

Get a variable speed lab mixer at Amazon.com

Get a variable speed lab mixer at Amazon.com

You need variable speed (not constant speed). Many of these have timers also. The prices range from $100 to thousands of dollars. Spend a little more and you will not regret it. A table-top device would normally be rated at 10L or 20L capacity. However, it is obviously easier to stir a thin liquid than a thick ceramic slurry so get a machine with plenty of capacity. If they label it a "lab mixer" then it is not too large to fit on a table top, a 20L capacity should assure that it will handle any glaze or clay slurry bucket that you would mix with it (e.g. 1-2 gallons). It should be capable of at least 1500 RPM. If the shipping weight of the package is 15-20 lbs (7-9 kg) it should be about the right size. They do not always come with the shaft and propeller, watch for that.

A must-have: Laboratory variable speed propeller mixer

A must-have: Laboratory variable speed propeller mixer

If you are at all serious about testing glazes and clay bodies, you need one of these. There are other methods, but nothing else comes close to this. It is the most valuable and frequently used tool in any ceramic bodies and glazes testing lab or classroom. These are expensive new, this Lightnin 1/20 hp variable speed cost more than $1000 many years ago, now it could be $4000! But you can get them used on ebay.com. I adapted a mount (to give it vertical adjustment) from a hardware store. Propellers are also expensive, but you can design and 3D print them yourself or have them printed at a place like shapeways.com.

Optimimal casting slurry properties impossible without good mixing

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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.

Printing a prototype propeller for my Lightnin lab mixer

Printing a prototype propeller for my Lightnin lab mixer

An example of how handy the ability to print in 3D can be. The worn-out stainless propeller costs $300 to replace. But the size and pitch of the blades is not ideal for the work I do. I print them in PLA plastic, this enables experimenting with different sizes and pitches. When one works well then I can have a stainless one printed at shapeways.com. These plastic propellers are surprisingly durable and it was easy to print one the with a hole the exact size needed, it fits tight on the shaft and never moves.

Out Bound Links

By Tony Hansen

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