M340S Old

M340S is our pre-2023 M340 speckled body. It had 0.2% added 60-80 mesh granular manganese. No longer produced.

M340S has the same plastic and drying properties as M340.

The added manganese produces a fine black speckle 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%).

M340S with G2926B transpaent glossy (left) and G2934 matte base.

 Drying Shrinkage: 7.0-7.5%
 Water Content: 21%
 Drying Factor: C130

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 5: 4.5-5.5
 Cone 6: 5.0-6.0
 Cone 7: 5.0-6.0

Fired Absorption:

 Cone 5: 2.0-3.0
 Cone 6: 1.0-2.0
 Cone 7: Bloating