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Propeller Mixer

In ceramic studios, labs and classrooms, a good propeller mixer is essential for mixing glaze and body slurries.

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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 (either by sheer energy or by extended mixing times). 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 with a propeller is needed to smooth them back out.

For mixing glazes and casting slips a mounted, hands-free mixer that can run for an extended period at the desired speed is much better than a hand-held device, especially if it has a good propeller and enough power to turn it. 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).

A table-top model is best for mixing small test batches of a litre to a couple of gallons (a 1/20hp Lightnin mixer is about the right size). 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. However such mixers must be treated with respect, they are potentially dangerous and can inflict serious injury if not used carefully. You can buy table-top ones on Amazon (for $hundreds) or from laboratory supply companies (for $thousands).

As already noted, 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).

All this being said, even the most powerful mixer may be not able to handle some powder mixes (e.g. those with significant percentages of bentonite, hectorite, smectite). VeeGum containing slurries mixed to pourable viscosity using a normal lab mixer will still have tiny agglomerates of Veegum that an ordinary kitchen blender will easily remove. Even on low speed the blender will turn the slurry to a gel that requires up to double the amount of water to maintain pourability! Thus, for production situations, the slurry recipe may need to be compromised to what production mixers are capable of handling. This is an interesting situation where potters, because they work on a small scale, can do things impossible in a factory.

Related Information

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, it uses a 7.9mm dia (5/16") shaft. 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.

Home-made propeller mixer with mount and switch

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

Printing a prototype propeller for my Lightnin lab mixer

The worn-out stainless propeller was going to cost $500 to replace. But the size and pitch of the blades was not ideal anyway. So I drew them using Fusion 360 and printed in PLA plastic, this enabled experimenting with different sizes and pitches. How about getting a tight fit on the shaft? No problem. I measured it with calipers and printed that size. It was a little tight so I printed it slightly larger. One issue: Mixing slurries with hot water bends the blades and the collar loosens. If you would like this in Fusion 360 format, it is available in the Files Manager in your Insight-live.com account.

3D printed plastic and stainless steel propellers

I had this done at Shapeways.com. They offer an after-print polishing service, which I did not get. The plastic one on the left (actually printed from PLA filament) weighs 5 grams. The steel one weighs 45 grams! It cost $35 to print this. The quality is like regular stainless, this is incredibly hard! Fitting it on the shaft was the first issue. The shaft measures 8mm. My drawing sets the hole at 7.9mm (5/16"). On the 3D print with PLA I got 7.8mm, but this this arrived at 7.7mm. It required a lot of work to enlarge the hole to fit. Thus, if I were to print this again, I would set the drawing at 8.2. That should either fit or only require enlarging the hole slightly (using emery cloth). The second issue was the hole and tap for the set screw. Drilling it was very hard, the first bit broke. The second made it through, but we could not tap the threads. So we will glue it to the shaft. Do you have a suggestion on a better way to fix it to the shaft? Please let me know.

Variable speed lab mixers at Amazon.com

You need variable speed (not constant speed). Although some have timers these are not useful. The prices range from $100 to $thousands. They do not always come with the shaft and propeller (but it is easy to get a stainless steel shaft). A table-top device may be rated at 20L capacity, for example, but that is for thin liquids. For thick ceramic slurries, it likely will only handle 8-10L (if the propeller is suitable). Double check and question the RPM rating, the cheap mixers use stepper motors (they require minimal electronics) and only get a fraction of the claimed RPM (so they will obviously not mix slurries well enough). These mixers will only be useful with a large propeller having steeply pitched blades. Buying a propeller is not practical because one will likely cost more than the mixer, be the wrong pitch, wrong direction, wrong size. 3D-printing one yourself is the best way (keep reprinting until it works well). If the shipping weight of the package is 15-20 lbs much of that will be the heavy metal base.

Double auger mortar mixer

These are used for mixing drywall mud and can be very effective for mixing large batches of body and glaze slurries. They are durable, powerful and inexpensive. However, they are intended for plaster, not clay so mixes containing hyper-fine particled powders may not respond completely to this type of device. Aging a day or two and mixing again would certainly help.

Cheap Amazon glaze mixer, how does it stack up?

Make some adjustments and it is usable. First, it is very quiet and has lots of power. The plastic sliders ride smoothly and provide precise adjustability (but the plastic threads might not last). The vertical shaft is stainless steel and the cast iron base is heavy, sturdy, practical. The motor-to-shaft mounting collar is good quality (but must be tightened with a tool). The shaft is 1/4" (6.35mm) dia. It has a stepper motor that runs less than 300 rpm (not the 3000 advertised)! The timer switch will not likely last, better to leave it on and use on/off. It does not turn off completely on zero-speed setting. The propeller shaft is too short and the flapper on the end is useless in ceramic slurries. The shaft rotates opposite-to-normal direction. You have to 3D-print a large propellor (we can help you if needed), with that it will easily mix 2 gallons of thick, high-specific-gravity slurry (we replaced the 8" shaft with a 12" one).

Here is how vigorously a deflocculated ceramic slurry should be mixed

A video of the kind of agitation needed from a propeller mixer to get the best properties out of a deflocculated slurry. This is Plainsman Polar Ice mixing in a 5-gallon pail. Although it is quite plastic compared to industrial casting slips, it has a specific gravity of 1.76, is very fluid and casts well in the hands of a potter. These properties are a product of, not just the recipe, but the mixer and its ability to put high energy into the slurry.

The difference propeller-mixing makes in a titanium glaze

The glaze has 5% added titanium dioxide. These were fired at cone 6. The titanium in the one on the left remained agglomerated, it did not disperse in the slurry during hand mixing (the agglomerates can be seen as white particles floating in the glass). On high-speed propller-mixing the effect on the right was produced! Blender mixing would be even better. This incredible difference occurs because the mixer is able to break up the titanium agglomerates, dispersing and wetting all the surfaces of the incredibly tiny particles. In this state they do their magic during the firing, opacifying and variegating the otherwise transparent base matte glaze. Would sieving be as good? Only partially, the particles are 200+ times smaller than the openings in an 80-mesh sieve, high energy input is needed to separate them.

Don't waste expensive tin oxide, propeller mix glazes well

This is a cone 04 glaze on a terra cotta body. Two 300-gram test batches were made. Both have 5% tin oxide added to a base transparent, G1916Q. The one on the left was high-speed propeller-mixed for 10 seconds. That was not enough, small agglomerates appear as tiny white specks floating in the glass. The one on the right was blender mixed for 60 seconds. Now the incredibly small tin particles have been dispersed and can do their job of opacifying the glaze. Would sieving the slurry have worked? No, the particle agglomerates can be far smaller than the openings in an 80-mesh sieve (individual particles could be 1000 times smaller!). The thorough dispersion of the expensive tin oxide particles has real benefit: It enables making an accurate assessment of whether the 5% addition is enough. In this case it isn't, more is needed.

The clay aging myth: Prove it wrong by slurry mixing

Do you really need to age clay when you make your own? No. In ancient Japan they did not have power blenders and propeller mixers. We do. To illustrate: I just sieved out the +80 mesh and +200 mesh particles from this raw clay (from one of our stockpiles) and then propeller-mixed it as a slurry. That wetted the particles very well and made it easy to sieve. Then I poured the slurry on to a plaster table and thirty minutes later it was ready-to-use. Slurry mixing is just as good as deairing in a pugmill. No wait! Particles wet even better. The plasticity of this clay is wonderful, and, it will not get any better with aging. Ancient Japanese potters used non-plastic, coarse particled clays so they needed to squeeze every last bit of plasticity out of them. Today, fine particled plastic clay materials are readily available. And we have something else the ancients did not: Micro-fine bentonite. A few percent of that and any clay can be made super-plastic (provided you have a good mixer to wet and separate all the particles to release their full power).

Get a kitchen blender for mixing ceramic slurries

Blender mixing is invaluable in slurry preparation in small scale ceramics and testing. It is quick and so effective that not only are particle surfaces wetted much better but clay particles can actually be reduced in size (literally ground finer). Slurry rheology is also stabilized. For example, thixotropy can often be achieved naturally, without any additives. Materials that are otherwise impossible to mix into a slurry (e.g. bentonite, Veegum, CMC gum) or just difficult (e.g. titanium dixoide, tin oxide) are no problem. Even slurry-processed porcelains benefit, not only being more plastic, but firing to a more homogeneous surface and to greater density. Just visually, it is easy to see how much improved this MNP slurry is (a local clay with porcelaneous properties). It was mixed using our propeller mixer and seemed OK (on the left). But the improvement after only 20 seconds in the blender (right) is amazing.

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Here is a good start to doing serious ceramic production at home

Learn to draw and print a mixer propeller

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