Alternate Names: EPK, Edgar Plastic Kaolin, EPK Kaolin
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|PCE - Pyrometric Cone Equivalent||35|
A secondary water washed kaolin mined in Florida, USA. Very commonly found in glaze recipes in North America. If you need to substitute for another see the next paragraph.
EPK is considered by many to be the best North American kaolin for use in suspending glazes as it tends to produce a smooth thixotropic slurry. 15-20% will normally make the glaze gel slightly, thus when a piece is immersed in the slurry and removed it drips very little and the wet glaze layer stays in place for the time it takes to dewater. Since EPK is very close to theoretical kaolin chemistry, it will substitute for any other pure kaolins (e.g. Pioneer). Conversely, most other kaolins will not change the fired properties of most glazes that specify EPK. If other kaolins do not suspend the slurry well enough, just gel it to make it thixotropic (see link below).
EPK is and very white firing compared with most other American kaolins. It has excellent casting properties. But it is not plastic enough for throwing bodies, it must be augmented by a more plastic kaolin (like 6Tile) or by a plasticizer like bentonite or hectorite. Or, if whiteness can be sacrificed, by a ball clay.
This material is mined and processed in Edgar Florida. It is extracted hydraulically (a high pressure stream of water is used to wash the clay/sand mix from the bank into a small lake). The slurry is pumped from the bottom and transported to vibrating screens that separate out the sand and grade it into sizes. The clay that passes the screens is pumped into a pond and settled with the aid of a flocculant. The slurry is then dried and the flocculant neutralized. The main product at this mine is high quality white sand, the clay is a byproduct. The company is confident that there are reserves for the next hundred years or more and the quality will be constant throughout the life cycle of the deposit.
This was plastic and moldable two days ago, now it is incredibly sticky. It is being compared with 5 other kaolin:nepheline mixes, none of them have reacted in this way.
From these (SHAB test bars) EP kaolin appears to have a much higher fired shrinkage. But half of that happened during drying. Still, EPK shrinks 4% more during firing. Yet Grolleg produces more vitrified porcelains. The EPK bar also appears be whiter. Yet in a porcelain body Grolleg fires much whiter. That higher drying shrinkage proves that EPK is much more plastic, right? Not really. Throwing porcelains containing either require plasticity augmentation using similar percentages of bentonite. What do we learn? To compare materials like this we need to see them "playing on the team", in a recipe working with other materials. Don't rely on material data sheets, do the testing.
Out Bound Links
The most fundamental clay mineral. This mineral is found in nature in its purest form as kaolin. However it makes up at least part of all ceramic clays. The purest deposits are created as it is weathe...
The hazards of using this material in the ceramic industry and process
Knowing about thixotropy will enable you to mix a glaze that stays in suspension much better. It does not drip alot when a piece is draining. It goes on evenly, of adequate thickness and does not run. It dries quickly (on porous bisque) and is just much nicer to use. The secret to all of this is not...
In Bound Links
Understanding glaze slurry rheology is the key to solving problems and creating a suspension that does not settle out, applies well, dries crack free.