Problems with black bodies from some suppliers: We tested existing products from a number of manufacturers and found serious issues (over-maturity producing bloating, warping and glaze bubbling; washed out glazes).
Another option to get a dark burning clay body surface
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.
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.
Coffee contains approximately 0.5% of MnO. Maximum firing is cone 6, 2200F, beyond that manganese fumes will be released as raw umber decomposes, this will cause glaze pinholing (also, the fumes are hazardous, be sure to use proper kiln ventilation). Use cones to verify that your electronic controller or kiln sitter is not firing beyond cone 6.
Bisque firing can be done at the same temperature as other bodies (we bisque around 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).
This body is a great candidate for the engobe process, we recommend the L3954B recipe.
Caution About Clear Glazes
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.
Opacifiers in base glazes on Coffee Clay
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
Sieve Analysis (Tyler mesh):
+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
Coffee clay bowl by Cindy and Greenbarn Pottery Supply.
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.
Safety Data SheetClick here for web view.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508