All Glossary

42 mesh

This is a bit of a magic number when it comes to particle size reduction of ceramic minerals and clays. 42 mesh represents the practical minimum particle size limit that an industrial hammer mill (coupled with a vibrating screen for particle classification) can produce when grinding clays. While finer sizes are possible, e.g. 60 or even 80 mesh, capacity suffers as an increasing volume of returns is generated. "Returns" are the oversize material rejected by the screen, it recycles back to the mill, reducing efficiency. Homogeneity of the end product is also affected since coarser particles tend to concentrate one clay type (assuming a mix of multiple clays are being processed). When returns can be kept to a minimum a hammer mill can produce product having a wide particle size distribution (from very fine to quite coarse).

A number of factors increase efficiency and product homogeneity:

The clay feed arrives at the mill dry (e.g. below 3% water).
The clay feed arrives evenly and precrushed, with no large lumps.
The clay feed mixture contains sandy materials.
The clay feed is slow enough to keep returns to a minimum.
The clay being ground is inherently fine particled, having as few contaminants as possible (thus secondary clays are assumed).
The hammers and mill lining are in good condition.

For structural applications (e.g. brick, sculptural), hammer mills are ideal. They are rugged and can grind rapidly for sizes in the range 10-20 mesh. For pottery applications, 42 mesh can be tolerated well if the clay mix predominates in naturally fine particled material. 42 mesh iron particulates will produce fired speckle but these are not generally a problem in oxidation firing. Drop and hold firing will generally heal all pinholes to produce a quality glaze surface. The texture of 42 mesh pugged clay bodies can be very pleasant and tactile, being appreciated by many potters.

By Tony Hansen Follow me on

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