This mug was thrown. But the handle was cast from L4005D, a recipe for an M390-compatible casting body you can make. The fired maturity of this (fired shrinkage and porosity) matches very well with M390. The casting process is superior for certain shapes and ware types (this sacrifices no casting properties because there is no iron oxide in the recipe, it uses Redart instead). These are strong, the handle on this glazeless mug endured a couple of good taps with a hammer and stayed solid. The body fires a little browner in color than M390, which is a product of using Redart, a low-temperature clay, but as long as ware is glazed it looks identical.
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 G1214Z1. 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.
These two bowls are L4558, a 70:30 mix of M370 and IMCO C-Red clay. They were fired to cone 6 using the C6DHSC slow cool schedule. The left bowl is glazed with our standard G2926B, it contains about 20% frit. It is fluxing the clay surface, transforming it to an ugly brown (which is what that body looks like when fired to cone 8). The amber transparent GA6-B Alberta Slip glaze on the right is not doing this at all, even though it also contains 20% frit. In addition, it is much glossier. Both bowls have underglaze sprayed banding, the GA6-B is transmitting that better also.