|Monthly Tech-Tip |
Every serious potter needs a plaster table. The bigger the better. Plaster is porous, it can absorb alot of water. You can pour a clay slurry onto plaster and it sucks out the water. It is magic. And clay does not stick to plaster, so it is a perfect surface for wedging. And for setting freshly-thrown ware (to pull water out of the thicker, and thus slower-drying base).
Below are a series of pictures showing how I re-poured the 350 lb plaster slab into a frame that my father made for Luke Lindoe many years ago. That frame showed no signs of rot, he had treated it with a preservative. The construction is ingenious, the 1x12 outer cap is fastened very securely to the 4x4 legs and the inner 2x4s are secured to it. It is very strong, it withstood alot of my banging on it with a big hammer and chisel to get the old slab out of there. This type of frame gave me a lot of flexibility during the pour. I was able to do it alone in a couple of hours. I poured 2-bag batches in succession, the previous just barely setting before I poured the next. I had a 20 gallon plastic container on wheels, so I could power-mix it (using a propeller mixer) by our dust hood and wheel in to the table and use a bucket to empty it into the frame.
If you live in a wet climate, water may not evaporate from your table fast enough. You can solve that by using the industry practice of making molds air-releasable (in pressing operations, molds have internal ductwork attached to a compressed air line, on pressurization air travels outward through the plaster's capillary system, releasing the just-pressed object). A side benefit is that the escaping air also purges the plaster of water, it bubbles out with the escaping air. By incorporating molduct tubing (available at Lagunaclay.com) with a compressed air plug and coupler it is possible to quickly purge water in a plaster table, making it ready for another batch of clay slurry.
The old, worn-out plaster slab has been removed (using a sledge hammer) and everything cleaned up. One cardboard insert has been put in place.
4x4 legs. 1x12 side. 2x4 cross members.
2x4s are nailed to the 1x12s along the length to support the 2x4 cross members.
A plaster table wooden frame with cardboard retainers stapled in place and ready for the plastic liner. This will hold a 350 pound plaster slab. That slab will absorb 100 lbs of water from a slurry.
These liners have been covered in plastic tape. They extend 3/4 inch above the outside wooden frame. The plaster slab must rise above the frame.
Each has been cut and folded to size and stabled in place. These will bear the weight of the plaster when poured.
The plaster will push the plastic into place tightly against the frame.
The drying will take a couple of weeks. It can be accelerated using fans. The table is now very heavy, it cannot be easily moved. The plaster has found its own level, slurry poured onto it will not run in any particular direction.
This plaster slab is about 5 years old. I have used it to dewater many raw clays that contain significant soluble salts. Over time they sealed the surface with a hard scum. That not only made the table slower to dewater slurries but it also significantly slowed down the time it took for water to evaporate out of it. Using this carpenter's plane I removed about half a millimetre. The blade self-sharpened. It took some experimentation to set it at a depth that would effectively remove the hard layer. I thought it would produce grooves and unevenness in the surface, but it did not. Sanding was not necessary. And the first slurry stiffened quickly, like a freshly-poured slab, and there were no bits of plaster in it.
This is an example of a plaster table on wheels (made using angle iron). 150 lbs plaster and 92 lbs water were poured onto the plastic (which was supported by cardboard attached below). This one uses Duramold pottery plaster.
Pottery plaster is highly absorbent, it can remove the water from this thick slurry, made from a 5 kg powder mix, in an hour. Slurries of higher water content dewater much quicker, a 1 kg mix can be ready in minutes. This table weighs 400 lbs dry, but smaller ones are equally practical for smaller test batches. Plaster tables are much more practical in arid climates, it is dry here so one this size can supply enough clay for production of a potter. In wetter climates ductwork can be installed within the plaster and air pressure can be used to dewater the table. If you need one of these, photos are linked to our plaster table article.
The Laguna catalog is striking for the number of different kinds of plaster they stock (at the time of this writing: fifteen). This catalog is an eye opener to the number of applications in which plaster is used and the number of properties needed. Among these are ability to pressure purge, the wet strength or dry strength, setting expansion, setting speed, absorption, density, toolability, suitability for hand layup or carving or layering, ability to surface harden, high density for simulating stone, low water needs, fire resistance and others. Plastic notches and molduct tubing are notable also (these are not generally easy-to-find).
Although there are reasons to use various types of bats in ceramic production (e.g. wood, plastic, masonite), for throwing pieces that are too big or fragile to lift immediately, a plaster bat is the best solution. This is especially so for porcelaineous clays that are difficult to dry. The main reason for this to minimize drying cracks. The plaster pulls water out of the base of the piece in the hours it sits after throwing. That solves a fundamental problem, for example, with making large platters and bowls. On a wooden bat the rims of these need to be dried until the walls are stable enough to support the weight upside down. By the time that happens the rim is well ahead of the base and a gradient has been set up that can case a drying crack across the base later. These are fairly expensive, but it is easy to make your own.
A One-speed Lab or Studio Slurry Mixer
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.
Recylcing Scrap Clay
Guidelines for collecting, reprocessing, testing and adjusting scrap recycle clay in a pottery or ceramics studio or production facility.
How to make a forced moisture release plaster mold
Laguna web page with "Plaster/Mold Making Catalog" link
Slurry Mixing and Dewatering Your Own Clay Body