|Monthly Tech-Tip |
This mug was thrown. But the handle was cast from L4005D, the recommended recipe for an M390-compatible casting body. This is not a product you can buy, you have to mix it yourself, but we have plenty of instructions and pictures. The fired maturity of the two (fired shrinkage and porosity) matches very well. The casting process is superior for certain shapes and ware types. And now, with 3D printing, it is much easier to make many kinds of casting molds. This handle mold is made by pouring plaster into a 3D printed form. These are strong, the handle on this glazeless mug endured a couple of good taps with a hammer and stayed solid. With glaze, the strength would be much better. The body fires a little browner in color than M390. It would be redder if we included more iron oxide in the recipe, but that would gel the slurry and make it harder to work with. As a red-burning body, this one has better casting properties than any other we have used.
The body is Plainsman M390. These are commonly-used base glazes. The top one is an MgO matte, next down is a calcium matte. They react very differently to these additions. Notice also the difference when titanium dioxide is applied thickly. Tin oxide fires whiter than zircon (e.g. Zircopax). Each opacifier has issues. Tin is expensive. Titanium is difficult to mix into the slurry (screening required), not as white or opaque, variations in thickness produce more difference in results and it can turn blue. Zircon is more likely to cutlery mark, twice as much is required and it amplifies the color of any iron present.
These are made from L4005D red cone 6 stoneware. Both are cast and thin-walled (half of what a thrown piece would be). They were glazed only on the inside to encourage cracking/splitting if the glaze is under excessive compression (that is, the thermal expansion of the glaze is significantly less than that of the body). And that is what happened here. The piece on the left cracked after a couple of taps with a hammer. Notice how the crack has opened. The piece is "spring-loaded" (press it together and it reopens on release). The glaze is GA6-B. The piece on the right is glazed with G1214Z. It spontaneously blew in half, with a loud crack, a few 5 hours after exit from the kiln. On further taps with a hammer these pieces shattered into dozens of smaller ones! The white glaze is certainly under too much compression. Obviously, neither is under any danger of crazing. Is the compression too great on the dark glaze? It did not shatter the way the white one did on further taps. And, another thicker-walled piece exiting the same kiln was glazed inside and out with that glaze. It was very strong. The lesson: Glaze compression, if not too much, is good for ware strength - but pieces must be glazed both outside and inside. And, thin ware like this, is good for testing that compression.