M340S is M340 with 0.25% added 60-80 mesh granular manganese. M340GS has an additional 4% fine grog.
M340S has the same plastic and drying properties as M340. M340GS is not quite as smooth and 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 no gain in drying properties (the most common reason for adding grog to clay bodies). 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 also 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. This body MUST NOT be fired higher than cone 6 or it will blister. Do not trust that your electronic kiln controller is accurate, verify using cones. Overfiring will result is loss of ware.
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|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508