|Monthly Tech-Tip |
In ceramics, the production of glazes and casting slips requires dispersing powders into water to create slurries. A good mixer is essential.
Key phrases linking here: blunging - Learn more
In ceramics, the mechanical mixing of a slurry is done with a blunger (usually a propeller mixer). Blungers of all different types are available. Some are simply a high-speed propeller on a shaft, others employ complex arrangements of paddles and container shapes. Clay slurries, which normally have very high specific gravities, require the application of considerable amounts of energy to achieve a suspension in which water has penetrated well between, and wetted, the surfaces of all particles. The rheology of incompletely-mixed slurries will change as they age, often quite drastically during the first few days. While industry generally has the mixers it needs to put the needed energy into their mixes to thoroughly disperse particles, potters most often rely on less-than-ideal equipment (like electric drills and paint mixers).
More thorough blunging is sometimes quoted as a way to achieve greater plasticity and workability in bodies. This applies most in industry, where there are advantages to using bodies of the lowest plasticity possible (since they have the lowest drying shrinkage and dry the fastest). Longer blunging will often improve plastic performance (however at considerable energy cost). But for bodies needing enough plasticity for hand-building, throwing on a potters wheel or jiggering a more drastic solution is needed. Achieving that almost always means more plastic clay in the recipe. Extended blunging of a throwing body of "totally crappy plasticity" will likely only improve it to the level of "crappy plasticity"!
Before slurrying a body or glaze it is important to mix the powders (especially ones where some ingredients are very fine-particled and tend to be agglomerated). Powder mixing is deceptively difficult. It might seem that shaking or tumbling some powders in a bag or container will mix them but this may not the case. Like slurries, the more energy applied the better the mix will be. The mixing and in-plant transport of powders is an entire industry (and science) unto itself. Companies constantly struggle with powder blending issues and often need to hire consultants to assess the issues involving the specific type of powders they must deal with.
All of the equipment has been washed in preparation for a porcelain run. Original container bags are broken in the dust-hood unit on the right and augered and elevated into the rotating blender/mixer. It feeds a vibrating screen (not visible) that removes paper and other contaminants. For wet clay bodies the screen feeds hoppers on the other side of the wall, they in turn feed the pugmill. For dry bodies and glazes the powder goes to one of the hoppers and that feeds a bagging unit. This type of equipment can handle 1200 lb batches (doing one every five minutes for some products, longer for others).
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.
|By Tony Hansen|
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