|Monthly Tech-Tip |
The process of propeller-mixing powder into a slurry and then dewatering that slurry on a plaster table is not practical on an industrial scale. But producing a workable plastic body in this way is certainly practical on a hobby or small business scale. A plaster table in your studio or work area not only makes it possible to make your own clay bodies but is also useful for many other things. The flexibility of having control of your clay body recipe is among the most compelling reasons to do this process. And you can make special-purpose clay bodies that are impractical for ceramic suppliers to make.
The use of a plaster table is especially practical if you live in a drier climate (where the plaster can dry out naturally). That being said, by incorporating flexible braided tubing (called molduct) into the plaster slab (with a compressed air fitting), you can literally blow the water out of the slab! A heavy, super sturdy and absolutely level plaster table is useful for so many other things in ceramics also. Set up a propeller mixer and a batching table with dust hood to make body batches with relative ease. The slurry-mixing process has a number of advantages. No aging is needed, water is intimately mixed with the particles. Better plasticity is achieved. Recipes that are almost impossible to plastic-mix (e.g. those high in bentonite) are no problem with slurry mixing. Less dust is generated and less space is needed than for other methods. Getting the exact stiffness is an integral part of the process. Reprocessing scrap is easy. This process enables you to utilize clays native to your own area, ones that you find and excavate as lumps and break down by slaking in water. Slurry processing enables using a magnet to remove particles of iron. And slurries can be screened to remove particulates. Having slurry-mixing equipment also enables you to make casting slips.
As noted, this process is simple. Propeller-mix the powder (and optionally a percentage of scrap) into a slurry in a sufficiently-sized bucket or barrel. Optionally put it through a sieve (e.g. 80 mesh). Then age it overnight. Then, as needed, ration it out onto the plaster table for dewatering and fine-tuning of the stiffness. Everything from the very finest of porcelains to hyper-plastic heavily grogged sculpture bodies can be done this same way.
I have already entered the recipe into my account at insight-live.com, assigned it a code number and printed a mix ticket for the total I want. I have a scale that tares to zero after adding each material. The slotted dust hood sucks the dust away as I add each. When I pour the dust into the water in the pail, a lot of dust is generated and this hood is even more important. Normally I used a big enough bag that I can seal the top and shake the mix together before adding it to the water.
Leave it mixing long enough under a mixer to thoroughly wet the surfaces of all the particles.This is a powerful mixer that can put a lot of energy into the slurry and it only takes a few minutes for it to be silky smooth.
This table weighs 400 lbs dry and it can remove the water in an hour. If you need to make a plaster table you can find photos here on how to do it.
Using a rubber tool I make cut lines. Even before the center sections are ready I am able to peel them up and turn them over (as has been done on the far right). About half an hour after that it is possible to wedge them.
I combine sections into manageable sized pieces and put them back down to stiffen more. Finally I layer them and repeat a cycle of cutting across the layers and slamming the mass down onto the table each time. Ten cycles of this produces a thousand layers. After a final wedging the clay is ready-to-use, as good as produced in a vacuum de-airing pugmill!
Throw the clay and feel how smooth and plastic it is!
An example of a custom-made dust collection hood in the material repackaging area of a supplier. The slots along the front suck particles into the duct, the suction comes from an exhaust fan downstream where the pipe exits the building. It has a wall switch and a sliding damper (where the hood enters the pipe) to enable stopping all airflow (to prevent heat loss in the room during cold days). Notice it is located above the scale and heat sealer where most dust is generated during weighing and packaging. Working in front of a system like this enables me to mix glaze recipes without breathing any dust at all.
How to Find and Test Your Own Native Clays
Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.
Formulating a body using clays native to your area
Being able to mix your own clay body and glaze from native materials might seem ridiculous, yet Covid-19 taught us about the need for independence.
In ceramic studios, labs and classrooms, a good propeller mixer is essential for mixing glaze and body slurries.