|Monthly Tech-Tip |
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.
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.
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.
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.
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 clean 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 much 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 bentonite, a few percent of that and any clay can be made super-plastic in minutes.
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.
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.
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.
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 I printed a propeller for my mixer using my first budget 3D printer. It has already been drawn 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 ran the printer. Within Simplify 3D the object can be positioned on the bed. A moving slider can animate how the printing will be done, layer-by-layer. Finally, after making sure it is connected to the printer, it is just a matter to click to initiate. That begins the process of preheating the printing bed and head, which took about 5 minutes. After that, the actual printing takes about 10 minutes.
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.
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.
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|By Tony Hansen|
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