|Monthly Tech-Tip |
The body is buff burning Plainsman M340 (cone 6). The amber colored glaze is 80% Alberta Slip (raw:calcine mix) with 20% of each frit. The white engobe on the inside of mug 1 is L3954A (also glazed inside using transparent G2926B). These frits are producing an amber gloss glaze of high quality. On the outside of mug 1 we see the 3195 version on the white slip until midway down, then on the bare buff clay (the other has the 3249 version). These mugs are fired using a drop-and-soak firing schedule. A couple of caveats: Frit 3249 has a very low thermal expansion, use it on bodies that craze other glazes (like Plainsman P300), it could shiver on stonewares like this. Both of these frits prevent the formation of bloating blues (with additions of rutile or titanium).
The body is dark brown burning Plainsman M390 (cone 6). The amber colored glaze is 80% Alberta Slip (raw:calcine mix) with 20% of each frit. The white engobe, L3954B, on the inside of two of the mugs is L3954A (those mugs are glazed inside using transparent G2926B). The Alberta Slip amber gloss glaze produces an ultra-gloss surface of high quality on mugs 2 and 3 (Frit 3249 and 3195). On the outside we see it this glaze on the white slip until midway down, then on the bare red clay. The amber glaze on the first mug (with Frit 3124) has a pebbly surface. These are fired using a drop-and-soak firing schedule. Some caution is required with the 3249 version, it has low thermal expansion (that is good on bodies that normally craze glazes, but risks shivering on ones that do not).
Rather than the normal 80:20 AlbertaSlip:Frit3134 recipe, this one substitutes Frit 3249 (super low expansion). The glaze is less runny and even glossier on these Plainsman porcelains. They are fired at cone 6 in a cool-and-soak firing. They survived an BWIW test (boiling water:ice water) without crazing (likely because of the low expansion of frit 3249). The finish is dazzling, a brilliant amber glass with no defects and perfectly even coverage. Of course, the iron in the glass prevents the colors of the blue underglaze from showing through. But the black is great.
Left is Plainsman M340. Right is M390. Each mug has been white-engobed inside, using L3954B, and half-way down the outside. The insides have been glazed using G2926B clear. The inside surface has more depth and has a richer appearance than could be achieved using a white glaze (especially over the dark burning body). The outside of the left one is Alberta Slip base GA6-B. The outside glaze on the right is the clear plus 4% iron oxide. This technique of using the engobe enables porcelain-like functional surfaces on the insides and striking visual contrast and character on the outside of the dark body mug.
L3954B - Cone 6 Engobe (for M340)
Dry and firing shrinkage fitted to Plainsman M390, M340
GA6-A - Alberta Slip Cone 6 transparent honey glaze
An amber-colored glaze that produces a clean, micro bubble free transparent glass on brown and red burning stonewares.
Ferro Frit 3249
A magnesia borosilicate frit having very low thermal expansion and melting point. Invaluable in pottery as to increase the MgO in glazes to prevent crazing.