This is a new version of M340S (starting in 2022). This is an all new and better recipe, we are calling it M340 REV.


A cone 6 stoneware, the speckled version of M340. Regular M340 is boring beside this!

  • Less vitreous than M340 to enable bloat-free ware at cone 6 (with a margin to cone 7).
  • More plastic than M340 enabling thinner and larger pieces.
  • Smoother than M340.

New M340S mug fired to cone 6. G2926B white inside and G2934:G2926B 85:15 white outside. Both have 3% tin oxide and 5% Zircopax added.

Process Properties

M340S REV works well with our L3954B engobe. We are not planning a grogged version. In the past the grogged version was not resistant to bloating at cone 6 but this new mix does not require that, because it does not bloat.


Old M340S vs new 2022 M340S (cone 8 down to 4)

M340S REV fired at cone 6 with no glaze leaks (the one on the right took 24 hours for that to happen). However the piece on the left is glazed (with G2926B), the glaze is not crazing, so it is water-tight.

An all-new recipe, this fires easily to cone 6 (and even cone 7) while burning to the same color as M340.


M340S REV with colored glazes fired at cone 6 using the PLC6DS schedule. The white and colored mugs are made using the G2934 base, the clear and purple one using the G2926B base.

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%).

There should be no concerns with glazes, use the same ones you have always used.

Physical Properties

 Drying Shrinkage: 6.0-7.0%
 Water Content: 20.0-21.5%
 Drying Factor: C120

Sieve Analysis (Tyler mesh):

     +48: 0.0-0.1%
   48-65: 0.5-1.5
  65-100: 3.0-4.0

Fired Shrinkage:

 Cone 4: 4.0-5.0%
 Cone 5: 4.5-5.5
 Cone 6: 5.0-6.0
 Cone 7: 5.5-6.5

Fired Absorption:

 Cone 4: 4.5-5.5%
 Cone 5: 3.0-4.0
 Cone 6: 2.0-3.0
 Cone 7: 1.0-2.0

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