|Monthly Tech-Tip |
The stated particle size of a material and fired appearance can both be misleading. For example, these are Volclay 325 bentonite particles fired to cone 8 oxidation. They are from a water washed sieve analysis test, the oversize particles from a 325 mesh screen (left) make up 2% of the total and 1% are from the 200 mesh screen (right). Although the 325 particles appear ominously dark, individually they are likely to small to produce apparent fired specks in a porcelain. However 200 mesh sizes can produce visible fired specks, but that fraction of oversize does not have nearly as high iron or flux content. Still, the finer darker particles could agglomerate, it might be better to use a cleaner bentonite to plasticize a porcelain.
Bentonite can make a clay body instantly plastic, only 2-3% can have a big effect. It also suspends slurries so they don't settle out and slows down drying.
Particle Size Distribution
Knowing the distribution of particle sizes in a ceramic material is often very important in assessing its function and suitability for an application.
Specking, or speckling, can be both a fault or feature in fired ceramic ware - caused or produced by metal-bearing contaminants to metallic additives
Sieve Analysis 35-325 Wet
SIEV particle size distribution test to evaluate and compare the particle size profiles of ceramic material powders