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.
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).
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.
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 http://digitalfire.com/education/glaze/cone6.htm. 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.
We do not supply a thermal expansion value. The reason is that such numbers often mislead users. First, a body has different thermal expansion characteristics when fired at different temperatures, schedules and atmospheres. Dilatometers are only useful when manufacturers can measure bodies and glazes over time and in the same firing conditions. If a chart is supplied here, please view only as a way to compare one body with another.
Another significant issue is that many customers compare measured thermal expansion numbers with calculated values of glazes in efforts to fits those glazes to a body. This does not work. Calculated values are relative only and have limitations that must be understood. The best way to fit glazes to your clay bodies is by testing, evaluation, adjustment and retesting. For example, if a glaze crazes, adjust its recipe to bring the expansion down (using your account at insight-live), fire a glazed piece and thermal stress it (using an IWCT test, 300F into ice-water). If it still crazes, repeat the process.
If we recommend a base clear or glossy glaze, try calculating the expansion of that as a rough guide to know whether your glazes will fit.
Drying Shrinkage: 6.0-7.0% Water Content: 20.5-21.5% Drying Factor: C120-C130 Dry Density: n/a
+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
Cone 4: 5.0-7.0% Cone 5: 3.5-4.5 Cone 6: 2.5-3.5 Cone 7: 1.5-2.5
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508