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Co-efficient of Linear Expansion - COLE

This test procedure was employed in the Foresight Ceramic Database and now is available for those having an account at Insight-Live.com. Accumulating test data using the variables defined in these procedures enables us to create tools that enable you to compare the physical properties of materials and recipes.

Notes

Most solids expand as they are heated. This expansion occurs in microscopically small amounts and can be observed along a line (linear), across an area and volumetrically. Since ceramics are brittle materials uneven changes in expansion across a cross section or incompatible expansions across joints or between body and glaze can cause ware failure.

In ceramics thermal expansion is a measure of how much a thin fired ceramic bar increases in length as it is heated from room temperature until the melting point of the glaze. It is measured by a dilatometer and yields a curve that can be graphed. This graph is typically distilled down to an average value (which is less than representative if the graph line is very curved) in the 10-6 range. Thus a value of 7.5 is 7.5 x 10-6 in/in/degree C (some data sheets would quote this number as 75 x 10-7). Actually, the units of length do not matter. If this specimen was 1 inch long, it would increase in length an average of 7.5x10-6 inches for each degree increase in temperature.

To be most meaningful this value should be presented like this example:
(25-500C) Pre-fired at 1200C
In this example the range of heating over which the average has been taken is shown and the degree of vitrification of the body being measured is also indicated.

Expansion numbers are typically comparative, thus the units are not specified.

Variables

VAL - Value (V)

Simple dilatometric curve produced by a dilatometer

Simple dilatometric curve produced by a dilatometer

Dialometric chart produced by a dilatometer. The curve represents the increase in thermal expansion that occurs as a glass is heated. Changes in the direction of the curve are interpreted as the transformation (or transition) temperature, set point and softening point (often quoted on frit data sheets). When the thermal expansion of a material is quoted as one number (on a data sheet), it is derived from this chart. Since the chart is almost never a straight line one can appreciate that the number is only an approximation of the thermal expansion profile of the material.

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By Tony Hansen




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