After finishing my porcelain clay study, I have moved on to stoneware bodies. At this point, all I am doing is studying PSD (particle size distribution). Typically commercial stoneware bodies run between 10-20% large particle size ( mesh 30-70). I will not bore you with a photo gallery, but just the difference between 20% hawthorne 35m and 50% hawthorne bond 35m is very revealing. I have seen numerous clay recipes with 40-50% large clay particles in their recipe.

At 20%, the particles are fairly uniform in disbursement: and only minor fissures in the body.

But at 50% recipe blend: larger particles are resting against each other and the open fissures and pores are much more numerous. The open pore/fissures in this sample would obviously lead to weeping.

It is common to make up to 10% silica additions to a stoneware body pending the application. Bodies used for salt or raku often do not have silica additions because it hinders the flashing effect. However, the majority of bodies do: the silica (fines) should help to fill in the pores and fissures. I used 1200m SiC as a marker for silica in the sample below:

The white, to light grey areas attest to the fact that 10% silica is not sufficient enough to fill in all the voids. Perhaps one reason the lowest absorption rating for stoneware is still 2%.

Let the tests continue......

Tom Anderson

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Comment by Tom Anderson on December 7, 2016 at 11:08pm

The Cream Test:

I am sure every thrower out there has noticed the amount of "sloppiness" changes from clay to clay.Perhaps noticed that some stoneware bodies are more sloppy than others? Perhaps a difference between stoneware and porcelain.

The difference is actually based on formulation. Sub-micron ball clays, (ultra fine mesh), bentonites, hectorites, and macaloid are added to clay bodies to give them plasticity. The particle size and the amount of the addition determines how sloppy the clay will get when you throw it.

The Cream Test:

oozes through your fingers and builds up over time: low plasticity/ low amount of fine particles

oozes through your fingers, have to clean your hands a few times: medium plasticity, medium fine additions

oozes quickly, have to clean hands repeatedly: high levels of fine particles; high plasticity.

If you reclaim your clay, either by slurry of pugmill: make this addition to replace the fine particle loss:

80% fine particle ball clay (Om4 ex.), 10% feldspar, and 10% silica

Add one cup of blended dry recipe per gallon of slurry to replace fines,silica, and feldspar lost in throwing.

Tom Anderson

Comment by George Lewter on December 4, 2016 at 4:32pm

Tom, we can add web pages that are static, blog posts are unchangeble but commentable, discussion topics can be created and closed to replies, and reopened for editing or amending and then reclosed. Your choice.

Comment by Tom Anderson on December 4, 2016 at 9:48am


Got a question?  I have been working on a clay formulation hypothesis for the last several years. A series of articles are coming out in Clay Monthly, and Tony Hansen wants to include part of it on Insight. However, Tony is wanting to link the bulk of it to another website. Is it possible to put up the entire formulation criteria, related parameters, limits: and the pin it so it cannot be posted to? I have put up bits and pieces of it on another forum.


Comment by Tom Anderson on November 15, 2016 at 11:14pm

Been dropping off some of my porcelain and stoneware clay blends to my supplier in St. Louis over the past month. They have been testing them in their community studio. Been collaborating with them for awhile now: hopefully by next spring they will be in production. Still have some more testing and tweaking to do over the winter.

Comment by Tom Anderson on November 2, 2016 at 7:40pm

QUESTION: Has anyone used IMCO 400 stoneware clay? Any experience with Tennessee Bell? KT? Not used any of these three, was wondering about the plasticity.


Comment by Tom Anderson on October 25, 2016 at 2:12pm

Christy Mineral Co., the makers of Hawthorne Bond stoneware clay, also makes a red plastic clay. This clay is 30-70 mesh, with 6.2% iron, and high plasticity. I do not find it on any suppliers list, although the mine itself is a little over and hour from me. Was thinking this might be a good clay to use in lieu of grog: and the iron should give some nice spots on the surface.

Anyone familiar with, or have used this clay before?



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