Mid-Fire Coffee Black is made from a base similar to H450 with 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.
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M390 vs. Coffee clay. The upper sections of the mugs are white engobed (using L3954B) under the amber-clear glaze. The glaze is Plainsman Alberta Slip base GA6-A (with Frit 3195).
Problems with black bodies from some suppliers: Making a good black burning cone 6 clay body requires plenty of R&D. We tested existing products from a number of manufacturers and found serious issues (over-maturity producing bloating, warping and glaze bubbling; washed out glazes).
This clay is plastic, it throws and forms well (even if soft). It generates plenty of slip on throwing. The body is made from 60% of our native A3 stoneware clay; to that we add kaolin, silica, bentonite, raw umber and talc. Raw umber is a natural soil/clay, so it has the same handling precautions as other clays.
Coffee clay is plastic. This 6" high vessel was thrown from fairly soft material. It has not been trimmed. Look how thin the walls are! The photo has been enhanced on the left to make it easier to see.
To develop the right color Coffee clay needs to be fired to cone 6 (center bar). By cone 7 the color greys. Below cone 6 it is brown.
Coffee contains approximately 0.5% of MnO. As with manganese-stained glazes, this means that manganese fumes could be released during firing (much more likely if over-fired past cone 6). These fumes can be hazardous, use a kiln vent. This body fires as a strong stoneware not so dense that issues with bloating, warping occur. Use cones to verify that your electronic controller or kiln sitter is actually firing to cone 6.
Bisque firing can be done at the same temperature as other bodies (we bisque at 1850F).
To get the best defect-free surface please consider using a drop-and-hold firing schedule, for example the PLC6DS schedule. If crystallization during cooling is not an issue, glazes will give optimum results if slow-cooled also (e.g. the C6DHSC schedule).
Commercial brush-on glazes offer many colors and surfaces. For functional ware check for glaze fit (vital for quality functional ware). Do not assume food safety of brightly colored glazes in your kiln and with layering without a leach test (e.g. GLLE test). Consider using a transparent or white liner glaze for food surfaces.
Glazes that work on M340 may not work on this (e.g. bubbles, bubble clouding, pinholes with G2926B, G2934 matte). But Alberta Slip and Ravenscrag Slip glazes work well for us. Of course colorants in the body will bleed into glazes, often making them appear very different than they would on lighter burning bodies.
Crazing: Functional ware must remain craze-free (crazing is unsanitary and drastically reduces ware strength). Because ware is not crazed out of the kiln does not mean it will not do so with time. Do cycles of a boiling water:ice water immersions (BWIW test) on a piece to test glaze fit (by stressing it to bring out any crazing or shivering tendencies).
Coffee Clay works well with the standard white version of our L3954B engobe (visible on some of the pictures here). If G2926B crazes over it, consider G2926S (it utilizes a low expansion frit, either Ferro 3249 or Fusion F-69).
This body is a great candidate for the engobe process, we recommend the L3954B recipe.
Clear glazes often do not work on dark bodies. The center mug is clear-glazed with G2926B (and is full of bubble clouds). This dark body (M390) is exposed inside and out (the other two mugs have a white engobe inside and midway down the outside). G2926B is an early-melter (starting around cone 02) so it is susceptible to dark-burning bodies that generate more gases of decomposition.
Left mug: The outside glaze adds 4% iron to G2926B (the glaze was not screened, so iron particles are agglomerated and acting as a fining agent, removing the bubbles). Right mug: The whole thing is glazed with GA6-B Alberta Slip base glaze. These amber glazes have an added benefit: The color darkens over dark burning bodies (to almost black).
Substitute the 10% Zircopax for 10% Mason 6600 black stain in the L3954B engobe recipe. You can apply this to M340, staining just the sections needed. The color is much blacker than Coffee clay, it produces a better glaze response and glaze will fit as expected.
M340 mugs with L3954B black engobe (it uses Mason 6600 black stain). They are overglazed with GA6-C Alberta Slip Rutile blue on the outsides and Alberta Slip GA6-A amber base on the insides.
Coffee Clay fits L3954B engobe perfectly. These bi-clay sandwich the body and engobe together, they have dried and fired perfectly flat (neither side has pulled or pushed to curve the bar).
Coffee Clay with Ravenscrag base glossy, white and matte glazes. These were fired using the C6DHSC firing schedule. There are lots of small dimples on the double-thickness sections at the tops.
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 (using an IWCT test, 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.
Drying Shrinkage: 6.5-7.0% LOI: 6-7% Water Content: 21.5-22.5% Drying Factor: C120
+48: 0.0-0.1% 48-65: 0.4-0.8 65-100: 1.5-2.5 100-150: 1.5-2.5 150-200: 4.0-6.0
Cone 4: 5.0-6.0% Cone 5: 3.0-4.0 Cone 6: 2.0-3.0 Cone 7: 1.0-2.0
These Coffee clay mugs have been white L3954B engobed at leather hard stage on the insides (the center one partway down the outside). After bisque the left and right ones were white-glazed on the inside (using G2926B+10 Zircopax). The one on the right has GA6-A (Frit 3195 version) on the outside (the center mug inside and out). The GA6-A over the black clay produces a very deep, rich ultra-gloss surface. The mug on the left has Ravenscrag floating blue (GR6-E) on the outside (producing a very right color over the Coffee clay).
Left: Coffee clay with L3954B white engobe inside and partway down the outsides. The liner glaze is Plainsman whiteware clear G2926B. Outside glaze is GA6-C Alberta Slip Floating Blue. Right: M340 with black stain replacing the Zircopax in the engobe recipe. It is glazed inside and out with Alberta Slip base GA6-A (using Frit 3195 as the flux).
Coffee and M340 marbled. Inside is L3954B engobe and G2926B clear glaze. Outside glaze is Alberta Slip base GA6-A (using Frit 3195). The GA6-A is used because it does not develop micro-bubble clouds as do normal clear glazes on this body.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508