|Monthly Tech-Tip |
This property is vital for many types of fired ware (i.e. functional tableware). This test can be difficult and time consuming, however it is quite helpful in determining a clays optimal firing temperature.
Special equipment is required to break the clay bars and record the force needed to do so, and many repetitions are needed to average a reliable figure. A body's fired strength curve, like its fired shrinkage, increases with temperature, levels off for a time, and then drops as expansion occurs. A body's maximum strength can often, but not always, occur some time before the absorption or shrinkage indicate that ideal maturity has been reached. In these cases, it is often better to fire the body with less maturity to stay out of its potentially volatile fired range. For this reason, the temperature of maximum strength could be considered to be the bodies' "functional" temperature. The side effects, if any, of an above-zero absorption should simply be eliminated by a fitted, mature glaze.
Potters making ware that will receive heavy duty use must consider the fired strength of the clay/glaze combination they use. Mug and caserole handles are easily broken if they are made from a weak clay or ware with a poorly fitted glaze. Bowls with thin lips, plates which will be stacked, and lidded pieces also must be made from strong clay with a glaze that complements it well to survive functional use for any length of time.
Poorly fitted glazes can profoundly weaken bodies because fractures tend to start in a little flaw and then propagate. A crazing glaze provides an abundant source of flaws, every one of which could initiate a body crack (especially in higher temperature bodies where there is a good clay-glaze interface). Glazes which are under compression not only smooth the surface providing no clear place for a failure to start, but because fired ceramic is much stronger under compression than tension, they increase the pieces capacity to resist breakage.
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.
M = 8 x P x L / Pi x (dxdxd)
Where P = Force in lbs
L = Span in inches
d = Diameter of bar at break
Cone number of degrees at which bars were fired.SPAN - Span in Inches (V)
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.FORC - Force in Pounds (V)
Force necessary to break bar.DIA - Diameter in Inches (V)
Enter the diameter of the bar at the point of the break in inches.GLAZ - Glazed (V)
This test can be used for testing clay-glaze combination strength to assess
the glaze fit. In these instances, you should be a code# in this field to
indicate the glaze used.
If one set of averaged numbers are entered, then key in the number of samples from which they are taken.
Fired Strength Square Bars
Modulus of Rupture - Fired
Tests conducted on bodies made from materials, as opposed to the materials themselves.
Ceramics, by their brittle nature, have high compressive strength. But in functional ceramics we are more concerned about the tensile strength as this relates better to serviceability.
|By Tony Hansen|
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