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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 (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 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 (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).

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).

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). 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). Question the RPM rating, the cheap mixers use stepper motors (they require minimal electronics) and only get a fraction of the claimed RPM. These will only be useful with a large propellor 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.

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.

Tin Oxide is expensive, do not waste it by not mixing well

Tin Oxide is expensive, do not waste it by not mixing 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. The one on the left was high-speed propeller-mixed for 10 seconds on a closed container. That was not enough, small agglomerates appear as white specks floating in the glass. The one on the right was mixed for 60 seconds. Now the tin particles, which are incredibly small, have been dispersed and can do their job of opacifying the glaze. Notice that 5% is not quite enough, more is needed.

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 right anyway. So I draw them using Fusion 360 and print them in PLA plastic, enabling experimenting with different sizes and pitches. While I could have one printed in stainless at shapeways.com I do not need to because these plastic ones are surprisingly durable. How about getting a tight fit on the shaft? No problem. I measured this shaft with a callipers and printed that size. It was a little tight so I printed slightly larger and it fits very tightly. One issue: If you mix slurries with hot water, it will travel up the shaft and the blades will bend.

Double auger mortar mixer

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.

Cheap Amazon glaze mixer, how does it stack up?

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). 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).

3D printing a propeller

3D printing a propeller

Here is how I print a propeller for my mixer. I have already drawn it using Fusion 360. The process involved choosing Make -> 3D Print, then selecting the propeller and clicking OK. This hands it off to Simplify 3D, the software that actually runs the printer. Within Simplify 3D I can position it on the bed and move a slider to animate how the printing will be done, layer-by-layer. Finally, after making sure it is connected to the printer, I click to initiate. That begins the process of preheating the printing bed and head, which takes about 5 minutes. After that, the actual printing takes about 10 minutes.

The difference propeller-mixing a glaze can make

The difference propeller-mixing a glaze can make

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! 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.

Out Bound Links

In Bound Links

  • (Articles) Recylcing Scrap Clay

    Guidelines for collecting, reprocessing, testing and adjusting scrap recycle clay in a pottery or ceramics studio or production facility.


By Tony Hansen




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