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.

Related Information

Here is why new grinding equipment is a really good idea

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These two pieces have been fired to cone 6, without glaze, I use this as a way of comparing changes in the character of the fired surface over time. These are made from a mix of A3 and 3B, our two main raw stoneware clays. The mix has been ground to 42 mesh (using our hammer mill). These materials vary in the amount of sand they contain and the amount of iron stone concretion particulates, so we will also see smoother and more speckle-free than this as a matter of normal variation. We have made sandy and speckled clays like this for so long that they seem normal to me. Yes, rustic bodies do have their appeal. However, the limits of our particle size reduction equipment and current quarry materials have resulted in importing American materials to satisfy customers who want smoother, whiter, more plastic and more vitreous bodies. We are now producing tens of thousands of boxes a year made from these imported materials! Transporting expensive clays at great distances begs the question of why we have not better leveraged the clay resources that are right here. That, and associated independence, quality and lower production costs, are coming soon.

Here is What Processing a Clay Can Do

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The clay is Plainsman 3B.
Left: Without processing, other than grinding to 42 mesh (currently the finest we can grind on a practical scale). When fired toward zero porosity it burns like this (at cone 6, 8, 9, 10 and 10R bottom to top). Of course, these are not big issues for non-vitreous rustic bodies fired at cone 6. The speckle and bloating are caused by impurity iron-bearing particles and others having an LOI (they decompose and produce gases that cause the bloats).
Right: The impurity particles make up a small percentage; they can be removed in our lab by sieving to produce a natural porcelain that fully vitrifies by cone 6 (the middle bar). Only about 5% of the material was removed to produce this amazing product (we call it MNP).
Imagine what could be done if we were able to mine raw material further east, where clay quality is much better!

By Tony Hansen
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