Mid-Fire Coffee Black is made from a base similar to H450 with 10% added raw umber powder (a powerful naturally-occurring pigmented clay). The umber fluxes the body so that it vitrifies at cone 6. It is smooth and easy to throw. It accepts glazes very well.
Mid-Fire black (right) beside M390. The upper sections of the mugs are white engobed. The glaze is Plainsman Alberta Slip base GA6A using Frit 3195.
Problems With Black Bodies From Other Suppliers
Making a good black burning cone 6 clay body requires plenty of development work. In our work we tested the existing products from a number of manufacturers and found serious issues (bloating, warping, glaze bubbling, washed out glazes).
Of course, this material is messy to work with, it obviously does not make a good companion to porcelain production. There is an alternative. By substituting the 10% Zircopax for 10% Mason 6600 black stain in our L3954B cone 6 engobe you can work with M340 instead, just staining the sections you want. It is much cleaner to work with, you will get a blacker color and even better glaze response.
This body works well with the standard white version of our L3954B engobe (visible on some of the pictures here).
Coffee contains 10% raw umber. Umber contains about 10% manganese. As with manganese-stained glazes, this means that manganese fumes could be released during firing (as would be release from any glaze containing manganese). These fumes are hazardous, use a kiln vent. The higher you fire and the more clay surfaces left unglazed the more fumes will be present. We are recommending cone 6 maximum. This makes the body a strong stoneware that works well with glazes but not so dense that issues with bloating, warping occur. Use cones to verify that your electronic controller is actually firing to cone 6.
Bisque firing can be done at the same temperature as other bodies. We bisque at 1850F.
Glazes that work on M340 should work on this (e.g. G2926B whiteware clear, Alberta Slip and Ravenscrag Slip glazes). Of course colorants in the body will bleed into glazes, often making them appear completely different than they would on lighter burning bodies.
We do not supply a thermal expansion value. The reason is that such numbers often mislead users. First, a body has different thermal expansion characteristics when fired at different temperatures, schedules and atmospheres. Dilatometers are only useful when manufacturers can measure bodies and glazes over time and in the same firing conditions. If a chart is supplied here, please view only as a way to compare one body with another.
Another significant issue is that many customers compare measured thermal expansion numbers with calculated values of glazes in efforts to fits those glazes to a body. This does not work. Calculated values are relative only and have limitations that must be understood. The best way to fit glazes to your clay bodies is by testing, evaluation, adjustment and retesting. For example, if a glaze crazes, adjust its recipe to bring the expansion down (using your account at insight-live), fire a glazed piece and thermal stress it (300F into ice-water). If it still crazes, repeat the process.
If we recommend a base clear or glossy glaze, try calculating the expansion of that as a rough guide to know whether your glazes will fit.
The mug has been white engobed on the inside and partway down the outside during leather hard stage. After bisque it was clear glazed on the inside giving a flawless surface (using G2926B) and dipped in GA6-A Alberta Slip base amber-clear. The GA6-A over the black clay produces a very deep, rich, almost black ultra-gloss surface.
Left: Mid-Fire Black with L3954B white engobe inside and partway down the outside. Inside glaze is Plainsman whiteware clear G2926B. Outside glaze is Alberta Slip Floating Blue. Right: M340 with burnt umber replacing the Zircopax in the engobe recipe. It is glazed inside and out with Alberta Slilp base GA6A using Frit 3195 as the flux.
Coffee and M340 marbled. Inside is L3954D engobe and G2926B clear glaze. Outside glaze is Alberta Slip base GA6A using Frit 3195. The GA6A is used because it does not develop micro-bubble clouds as do normal clear glazes on this body.
These Coffee coffee mugs are glazed using Alberta Slip rutile blue (and fired to cone 6). The color is a very deep, vibrant and glossy blue (yet there is no cobalt). The inside surface of the mug on the left is white-engobed and clear-glazed.
M340 mugs with L3954B black engobe (it uses Mason 6600 black stain instead of Burnt Umber for a blacker color). They are overglazed with Alberta Slip Rutile blue on the outsides and Alberta Slip GA6A on the insides.
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|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508