M340S, M340GS


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.


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, we recommend cone 5. It is possible to get success at cone 6 sometimes, but it is too close to the margin to trust, fire lower. Verify your kiln controller is accurate, then manually program it to finish the firing at the desired temperature. If some parts of your kiln fire hotter, do not put these clays there. Overfiring will result in 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 bars shown are cone 8, 7, 6, 5, 4 (downward).


Toxicity: the vast majority of the manganese particles are encapsulated within the clay matrix. The tiny percentage exposed at the body surface are under the glaze. It is not the manganese particles themselves that expose at the glaze surface. Rather particle surfaces that contact the underside of the glaze bleed out into it from below, doing so as a function of the glaze thickness, opacity and melt fluidity. Thus, food contact with a glass surface having isolated manganese-pigmented regions is not at all the same thing as with raw manganese metal (or a glaze having powdered manganese as a colorant, some have up to 5%).

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


M340GS with GA6-B Alberta Slip base glaze. Cone 6.

M340S with G2926B white inside and G2934Y white outside. Fired cone 5 C5DHSC slow-cool schedule.

M340S with G2926B white glaze. Caution: Do not fire to cone 6 unless you are sure your kiln is not over-firing, it is better to fire to cone 5 using a drop-and-hold schedule.

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.

Safety Data Sheet

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Logo Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508