|Monthly Tech-Tip |
We have not included procedural records for this definition. See the CDRY and SAWL test for examples.
In industry, where articles are handled by machines, it is important to have high green strength. Potters likewise benefit from high green strength by being able to glaze in the dry state and make thinner ware that will withstand handling. In addition, high green strength is a side effect of high plasticity that is desirable from a working point of view, but undesireable from a drying properties point of view.
As stated, dry strength is most closely related to plasticity. Highly plastic clays are strong. The distribution of particle sizes and shapes, their orientation, and the density of dry ware are also factors. The existense of stresses and cracks within test bars can make it impossible to measure an accurate breaking force. These stresses occur in very plastic materials (i.e. ball clays). In these situations, the clay must be blended with an extender like flint to cut the plasticity. These strength measurements are not absolute, but relative to similarly treated materials.
It is also very important to prepare the test bars using a method than does not build in laminiations or particle orientations that weaken the test bar (characteristic of hand-worked samples). It is typical to extrude round bars from a deairing lab pugmill to produce samples that will yield a consistent strength figure. 25 mm dia x 10 cm long extruded bars can be broken on a 7.5 cm span in standard device that records the force necessary to do so. Six to eight bars should be done, and the resulting data can be averaged and substituted into a formula in order to derive the pounds per square inch of dry strength.
Using the above conventions, a typical raw stoneware clay will have a dry strength of 800-1500 for round extruded bars (square bars measure less). Bodies made from refined materials (i.e. whiteware bodies) will measure 500-900.
Round or square fired bars are subject to a force that tests their tensile strength. The strength can be calculated from the force and the dimensions of the cross section where the break occurred.
Enter in inches the span at which the strength testing device is set, that is, the distance between the two points on which the bar rests.FOR - Force (V)
Enter the force necessary to break the bar.DIA - Diameter (V)
Enter the diameter of the bar at the point of the break in inches.STRV - Calculated Value (V)
This is a calculated item to derive dry strength. The formula is:
M = 8 x P x L / Pi x (dxdxd)
Where P = Force in lbs
L = Span in inches
d = Diameter of bar at break
Enter the number of bars which were broken and averaged to yeild the figures entered in the other fields.
Simple Physical Testing of Clays
Learn to test your clay bodies and clay materials and record the results in an organized way, understanding the purpose of each test and how to relate its results to changes that need to be made in process, recipe and materials.
Dry Strength (kgf/cm2)
Drying Factor/Water Content/Solubles
SHAB Shrinkage and absorption test procedure for plastic clay bodies and materials
Dry Strength (Square Bars)
Tests conducted on bodies made from materials, as opposed to the materials themselves.
|By Tony Hansen|
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