Medium temperature, slightly textured, plastic, semi-vitreous, yellow-buff burning, fine grogged, general purpose native stoneware.

M325 is a general purpose material and it is a mix of two of our plastic stoneware clays, a silty stoneware, and a unique low-melting plastic buff burning material similar to the well known Lincoln 60 fireclay. It has 6% added fine grog to open the body up to give it a better drying properties. The body burns a yellowish-buff color that is quite a bit darker than M340 but much lighter than our tan-firing M350.

Process Properties

M325 is quite plastic and its unique working properties are the main reason for its use. Even though it does have some fine grog added, you will not be able to detect it easily for most types of work. The body dries quite well considering its plasticity.

One area of concern is that the grog will tend to produce rough glaze surfaces on areas where the glaze is thin (i.e. the lips of functional ware). Thus occurs as sharp grog particles expose themselves above the glaze surface as the latter thins during firing. Avoid sponging rims on dry or leather hard ware since this tends to remove fines and expose grog particles even more.


Fired test bars of M340 (left) and M325. Fired at cone 8, 7, 6, 5 and 4 (top to bottom). M340 has a more pronounced color shift (from straw to grey) between cone 5 and 6 than M325 does. This indicates vitrification is beginning.

M325 fires to a yellowish buff color from cone 3 to 4, then as temperature reaches cone 5, the effect changes toward grey buff. From cone 6 to 7 it is a stone-grey. At cone 8 it is unstable; sometimes it will produce a vitrified material, other times it will begin to bloat. If you are not using M325 primarily for its fired color, you might try M340 instead. M340 is smoother and thus better for the production of functional ware.

M325 is the material of choice only if you require the dark yellow-buff fired color. Otherwise we recommend you use M340 for functional 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).


M325 with GA6-C Alberta Slip rutile blue glaze fired at cone 6.

Since M325 has a small amount of fine grog added to the recipe you might find that it tends to cause certain glazes to pinhole. If this is the case consider soaking your kiln at the final temperature to give the glaze a chance to level out and heal imperfections left from gases bubbling through. We have found that automatic kiln controllers work extremely well for this. If glaze pinholing is still a serious problem you might consider using our M340.

M325 should work with the same glazes as most of our other middle temperature bodies. However, since it fires to a darker color you might find that certain colored glazes or underglaze decorations might be subdued. This happens because the iron in the M325 muddies the purity of other coloring oxides. If this happens use a white slip on the clay surface or try another light firing clay body.

Glaze Recipes

You can develop a compatible glossy or matte base for this body from our suggested starting point base recipes available on our Internet web site at Information is given on how to fit the glaze to your body and how to customize it it for colors, opacity, speck, variegation, etc. For slip decoration, be careful to match drying and fired shrinkage of the slip with the body since low temperatures generate little glass to adhere the slip.

Thermal Expansion

We do not supply thermal expansion values. If a chart is supplied here, please view it only as a way to compare one body with another. Please note that, although you may calculate the thermal expansion of a glaze, this cannot be done for clay bodies since they do not melt. The best way to fit glazes to clay bodies is by testing, evaluation, adjustment and retesting. For example, if a glaze crazes, adjust its recipe to bring the expansion down, fire a glazed piece and thermal stress it (using an IWCT test, 300F into ice-water). If it still crazes, repeat the process.

Physical Properties

 Drying Shrinkage: 6.0-7.0%
 Water Content: 20.5-21.5%
 Drying Factor: C120-C130
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

     +48: 0.1-0.5%
   48-65: 2.0-3.0
  65-100: 2.5-4.5
 100-150: 2.0-4.0
 150-200: 4.0-6.0
 200-325: 7.0-10.0

Fired Shrinkage:

 Cone 4: 3.0-4.0%
 Cone 5: 4.0-5.0
 Cone 6: 4.5-5.5
 Cone 7: 4.5-5.5

Fired Absorption:

 Cone 4: 5.0-7.0%
 Cone 5: 3.5-4.5
 Cone 6: 2.5-3.5
 Cone 7: 1.5-2.5

Chemical Analysis

 BaO       0.3
 CaO       0.2
 K2O       2.3
 MgO       0.9
 Na2O      0.2
 TiO2      0.6
 Al2O3    18.2
 P2O5      0.0
 SiO2     68.6
 Fe2O3     1.5
 MnO       0.0
 LOI       7.2%

Safety Data Sheet

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We Are Rationalizing Our Product Line

Plainsman manufactures bodies by grinding and pugging clays that we mine (native bodies) and by batch mixing bagged minerals and materials that we import (refined bodies). We stock about 10,000 boxes of 50+ clays (some in multiple stiffnesses) and need to reduce the warehousing and production burden of small-run bodies and remove obsolete and legacy products. For bodies being discontinued: We have migration paths and can assist with issues. Some changes involve increased cost. In certain cases you might consider having us custom-mix a body so you can continue to get it, but please work with us on trying to adapt to alternatives first.

This product has issues

  • This body contains materials that we longer mine or get.
  • This contains grog that our grinding system is pulverizing excessively, it causes splitting.
Logo Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508