|Monthly Tech-Tip |
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.
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.
Bringing Out the Big Guns in Craze Control: MgO (G1215U)
MgO is the secret weapon of craze control. If your application can tolerate it you can create a cone 6 base glaze of very low thermal expansion that is very resistant to crazing.
Adjusting Glaze Expansion by Calculation to Solve Shivering
This page demonstrates how you might use INSIGHT software to do calculations that will help you increase the thermal expansion of a glaze while having minimal impact on other properties.
Understanding Thermal Expansion in Ceramic Glazes
Understanding thermal expansion is the key to dealing with crazing or shivering. There is a rich mans and poor mans way to fit glazes, the latter might be better.
Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
Band-aid solutions to crazing are often recommended by authors, but these do not get at the root cause of the problem, a thermal expansion mismatch between glaze and body.
|Tests||Frit Softening Point|
|Tests||Glass Transition Temperature|
|Tests||300F:Ice Water Crazing Test|
|Tests||Boiling Water:Ice Water Glaze Fit Test|
ASTM-C372 Thermal Expansion
Linear vs. Area vs. Volume expansion
Test conducted primarily on materials use to make bodies or glazes.
Tests conducted on bodies made from materials, as opposed to the materials themselves.
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.