M340S, M340GS
Description
M340S is M340 with 0.25% added 60-80 mesh granular manganese. M340GS has an additional 4% fine grog.
Process Properties
M340S has the same plastic and drying properties as M340. M340GS sacrifices a little drying strength to get the added texture. Since there is only 4% grog, and only a portion of that is actually coarse enough to feel, there is little gain in drying properties (many customers buy it on the misimpression that it dries better, however M340 by itself is an excellent drier, mainly because of its high dry strength). The presence the grog is an issue on functional ware because it introduces grit on rims and edges. It is also problematic in the forming process since it produces water splitting problems, most noticeable when pulling handles or throwing bellies and lips (the grog provides entrance points for the water). We recommend that you use the M340S.
Firing
The added manganese produces a fine black fired speck that usually bleeds up through glazes. These bodies MUST NOT be fired higher than cone 6 or they will blister and/or bloat. Do not trust that your electronic kiln controller is accurate, verify using cones, then manually program it to finish the firing at the temperature at which cone 6 reaches 4 oclock. If some parts of your kiln fire hotter, do not put these clays there. Overfiring will result is loss of ware.
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).
Fired test bars of M340GS from cone 8 (top) to cone 4.
Glazing
Toxicity: Since manganese specks bleed up through the glaze, you may be wary about leaching. We balance our toxicity concerns with the knowledge that, although these bodies contain 0.2% manganese (most of which is encapsulated inside the body), many potters use glazes that contain twenty times more! And that in-glaze manganese often crystallizes to a much less stable form at the glaze surface during cooling. The tiny percentage of body manganese here, by contrast, will be dispersed in the glaze on its way to the food surface, becoming greatly diluted and stabilized (assuming of course that the glaze is properly melted and durable).
M340GS with G2926B transpaent glossy (left) and G2934 matte base.
Physical Properties
These properties are for M340GS. M340S has the same properties as M340.
Drying Shrinkage: 5.5-6.5% Water Content: 19.5-20.5% Drying Factor: C120
Sieve Analysis (Tyler mesh):
+48: 0.0-0.1% 48-65: 0.5-1.5 65-100: 1.5-2.5 100-150: 2.5-3.5
Fired Shrinkage:
Cone 4: 3.5-4.5% Cone 5: 4.0-5.0 Cone 6: 4.5-5.5 Cone 7: 5.0-6.0
Fired Absorption:
Cone 4: 6.0-7.0% Cone 5: 4.0-5.0 Cone 6: 2.5-3.5 Cone 7: 1.0-2.0
Gallery
M340GS with GA6-B Alberta Slip base glaze. Cone 6.
M340S with G2926B white glaze
The body is Plainsman M340S. Cone 6. Left to right: G1214Z calcium matte base glaze with 6% titanium dioxide added. GR6-A Ravenscrag base with 10% zircopax (zircon). G2926B glossy transparent base with 10% zircon. G2934Y silky magnesia matte base with 10% zircon.
