|Monthly Tech-Tip |
It only takes a few minutes to make these. But you would be amazed at how much information they can give you about a clay! These are SHAB test bars, an LDW test for water content and a DFAC test disk about to be put into a drier. The SHAB bars shrink during drying and firing, the length is measured at each stage. The LDW sample is weighed wet, dry and fired. The tin can prevents the inner portion of the DFAC disk from drying and this sets up stresses that cause it to crack. The nature of the cracking pattern and its magnitude are recorded as a Drying Factor. The numbers from all of these measurements are recorded in my account at Insight-live. It can present a complete physical properties report that calculates things like drying shrinkage, firing shrinkage, water content and LOI (from the measured values).
LDW LOI, density and water content test procedure for plastic clay bodies and porcelains
The DFAC Drying Factor test visually displays a plastic clay's response to very uneven drying. The test is extreme enough that almost all plastic clays will crack.
In ceramics, glazes and bodies have a chemistry, a mineralogy and a physical presence. All of these need to be understood to adjust and fix issues.
What is clay? How is it different than dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.