I read this the other day, thought it might be of interest.
This is a very relevant thread about fired clay strength (MOR). Please have a read.
For the final project in my Clay and Glaze class this semester, we mixed about 50 clay bodies for testing, including red and white earthenwares, stoneware, porcelain, and sculpture bodies. Besides other tests, we extruded numerous bars of each body and broke them to measure MOR (Modulus Of Rupture, which is a measure of the bending strength). There are other strength tests that can be done (chipping tests, for instance), but MOR is a quick and easy way to predict how well a body will hold up to the bumps of everyday use.
Out of all these tests, there were a number of interesting trends:
1. Any amount of grog weakens clay bodies, especially in sculpture
bodies that are essentially underfired. Some of the sculpture clays were so weak at cone 04 that we couldn't measure them- the bars broke at initial contact before any stress was applied. Any texture in the clay tended to have the same result, though the texture from using 50 mesh fireclay seemed to have only a minimal effect. Really fine grogs- those less than 80 mesh- also had little effect.
2. Glaze made a huge difference in strength. Crazed glazes lowered results 50% or more from the strength of the same bar unglazed. Uncrazed glazes raised the strength of the bars from 50 to 100 %. I had read this before, and assumed that it was mostly related to the lack of surface flaws on a smooth glaze (cracks like to start at a flaw- take away the flaws and it's more difficult for a crack to start). What I found interesting is that the amount of compression also mattered. We glazed the porcelain bars with three different versions of my Pete's Clear glaze, which ranged from mild compression for the original version to a very low expansion version that places the clay in a very high compression. Consistently, the higher compression versions produced higher MOR results.
3. Clays have to be fired to maturity to get good strength. Even firing porcelain bodies to cone 9 rather than 10 lowered strengths a good deal. As an aside, I define maturity as the point at which a body achieves its best strength and glaze fit, and no longer suffers from marked moisture expansion. Absorption, in my opinion, is not a good indicator except within one clay body group (such as "high fire porcelain"). Porcelains may need to have less than 1% absorption to avoid moisture expansion problems, while mature white earthenwares can have upwards of 20% absorption (which is why those cheap white tiles on our shower walls don't develop delayed crazing).
4. "Smooth" counts for more than "glassy", which seems to contradict one bit of standard wisdom I've heard in the past.
5. Quartz seems to be a problem- at least in a minor way. Porcelain
bodies that used a combination of pyrophyllite and quartz were stronger than those which used only quartz as a filler. It's a bit of a mixed bag, though, because glazes on pyrophyllite bodies tended to craze more.
What were the strongest clays? This will surprise you- it certainly did me. The strongest clays, consistently, were (drum roll, please) red earthenware clays fired to a full cone 04.
Yep, that's right. Plain old Redart based, smooth red earthenwares. They were stronger than smooth, brown or gray stonewares, and even stronger (over all) than porcelain, which I had assumed would be best.
Yes, it was very important to fire them to a full cone 04: cone 06
didn't hack it. Surprisingly, taking them to cone 1 did not increase
MOR, though they certainly were denser and felt more solid and chip resistant. Within red earthenwares, we got consistently higher strength from those using wollastonite as a secondary flux (5 to 10%), rather than talc. It seemed best to use red clay in amounts of 50 to 70%, and while Redart alone (for the red clay portion of the body) gave the best strength, we got much better workability (and only a tiny bit less strength) by using a mixture of red clays, such as Redart mixed with Ranger Red (from Texas) and Apache Red (from Colorado).
As with porcelain, the clay was made much stronger with glazes that fit, and higher compression glazes were strongest of all. Our all-time champion (for strength, NOT workability) was the following recipe, glazed with Linda Arbuckle's Majolica and fired to a full cone 04.
KT 1-4 Ball Clay, 30%
I thought you might find this interesting. I only teach a Clay and Glaze class one semester every three years, so while I plan to do some follow up tests (these tests raised as many questions as they answered), don't look for those results any time soon!
University of Nebraska at Lincol