Warning: Highly technical pottery post (picture at the very end)
Alongside my glaze testing, I’ve also been testing two clay bodies for glaze fit, general performance (throw-ability, cracking, etc) and vitrification. The two clays in question are:
Standard Clays 240 :
“A very smooth, plastic throwing body.”
Shrinkage: 13% at C/6.
Absorption: 2.75% at C/6
(information from Standard Clay’s website)
WC‑901 Maccabee Mix 5 Clay
“This is Axner’s response to Laguna’s B-Mix 5, which is one of the most popular clay bodies in the United States. It is white and smooth. It is similar to porcelain but it throws like a dream.”
Approx. Cone: 5
Wet Color: White
Penetrometer Target: 7
Avg. Shrinkage 2±% 13%
Avg. Water Absorption 1±% 2% (C/6)
COE x 10-6: 7.275
(information from lagunaclays.com)
As per Laguna, their clays are formulated and categorized to perform as ^06, ^5 and ^10. However, most clay can be successfully fired at varying temperatures (1 to 2 cones) above and below their indicated firing ranges. I tested both clays at C/6 tip touching, using the firing schedule recommended in MC6 Glazes.
First, a few words about clay usability issues.
I have to special order the Standard 240 from a distributor, who then mails the clay to me. This requires a bit of planning. It also means that all the bags I got were very fresh and quite soft. For me, this was a good thing as I found the 240 was a lot easier to throw big (>3 lbs) when it was fresh. Once it sat around for a bit, I had to soften it before attempting centering over 3 lbs.
I had a few cases of small drying cracks in the inner bottom of bowls (similar to s-cracks, but not all the way through) that appeared pre-bisque and one full on s-crack in a small cup (one of 16) that came out of a bisque. There was no rhyme nor reason to the cracking, and I’ve not had them repeat with my recent batches. I heard from another user that she had similar experiences with the clay. A bit of a worry.
As I noted in my first glaze-test report, the Standard 240 fires to a cream color. The MC6 glazes appear to fit it well.
I order direct from Axner (which means that the clay is in my studio within 3 days!). The clay is moist and perfect for throwing smaller pieces but I struggle with it for throwing anything larger than 3lbs. What this means is that large bowl throwing requires advance planning and careful preparation of the clay (moistening clay, adding very wet clay to the stiffer clay, wedging and slamming and wedging some more). Once the clay is “right”, its a pleasure to throw big (which is what I did yesterday). To date, I’ve had no cracking problems with this clay.
The Maccabee clay fires to a cold white color (slightly grey). Again, MC6 glazes appear to fit it well.
Water Absorption Testing:
I put both clay through a DIY test of clay vitrification. Clay vitrification affects whether or not a fired clay piece will absorb water. This has ramifications for suitability for use in a dishwasher and especially the microwave. It also has ramifications for overall performance of the glaze pieces.
I conducted absorption tests as outlined in Mastering Cone 6 Glazes (test tiles first bisqued, then fired unglazed to glaze temperature, weighed straight out of the kiln, boiled for 2 hours, allowed to cool in the water, dried and quickly weighed again). I repeated the tested three times (using clay from different batches).
The fractional moisture absorption percentage is calculated by [boiled weight-original weight]//[original weight] x 100.
The test results for the Axner Maccabee ranged from 1.02% to 1.77% (average = 1.42%)
The test results for the Standard 240 ranged from 4.00% to 4.52% (average = 4.33%)
In their book, John Hesselberth and Ron Roy recommend confirming that water absorption for one’s fired clay is no higher than 3% and preferably 1-2%. I will not be ordering any more Standard 240. Time to see if the friendly folks at Axner can make sure I get softer clay next time I order! 😉