A year into our new life here on Route 66, my husband is outside digging a
reservoir next to our house when he comes running in to tell me that he hit a big
vein of clay and that I should come check it out. After doing some quick on-site
testing, I determined this clay was some 'oh my gosh!' really good clay! I
instructed my husband to save every last bit of it and off to the back field it went.
|The Year Long Process of Testing
After cleaning debris from a bucketful of this clay, I then proceeded to really test
it out. First I needed to figure out what exactly I wanted this clay to do. In
keeping with my pottery goals of only creating pieces that are completely
functional, I decided this clay needed to be a high-fire clay that would mature at
cone 6 (my normal firing range). This would make it oven proof. It also needed to
be compatible with non-toxic glazes so it would be food safe and easy to clean. I
also wanted it to be plastic enough to throw on my wheel, but stiff enough to
use on my slab roller and with my extruder. I wasn't asking for too much was I.
|The bar on the far right, #27, is the last test bar I did for my Route 66
Oklahoma Red Clay. It took a total of 27 test bars before I was finally
happy with my results.
|told me the shrinkage rate for drying. Bar #2 I added to my next bisque fire. When cooled I
measured the line on this bar and this gave me the shrinkage rate for bisque. I also noted that
the clay was able to withstand a cone 04 fire so Bar #3 was added to my next glaze fire. When
cooled I measured the line on this bar. I also noted that this clay was able to withstand a cone 6
fire which made me very happy. When studying bar #3 I noticed that the clay was less durable
and broke fairly easy. It was also very gritty so I knew I needed to remove a lot of sand. Before I
did any more testing I knew I had to get rid of as much sand as possible. I was very hopeful
though. This clay actually fired to cone 6 without melting into a blob. It actually held its own
very well. I was anxious to get the sand out and move forward.
|Removing Excess Sand
To remove the excess sand from my raw clay, I poured some raw clay sludge into a 60 mesh
sieve that I purchased specifically for this chore. This sieve fit nicely on my 5 gallon bucket. I
added some water to the sludge and with my metal rib moving back and forth, I gently pushed
the clay through the sieve. I kept adding clean water to my sieve and the clay sludge got
thinner and thinner. Eventually the added clean water didn't turn orange anymore and I knew I
had removed all of the clay from my sludge. What was left behind in the sieve was unwanted
sand, roots, little pebbles, sticks, etc...
As a side note - this removing of excess sand is part of my daily routine now. The cleaning of debris
and removing of sand is such a soothing chore. I become so in touch with my clay that I know when it
has changed and what it will need. I know that sounds corny, but it's true. Since this chore takes
about 15-20 minutes, I let my mind wander and I come up with creative ideas, I plan my day, I think up
all sorts of things. This is the first thing I do when I walk into my studio, it prepares both me and my
clay for the wonderful studio work ahead of me.
Test #1 (Throwing/Plasticity) - The clay was plastic (I was able to throw something from it on
the wheel) was it wasn't plastic enough. It tired easy on the wheel and it remained wet too
long. The removal of excess sand improved the grittiness of the clay, making it not so rough
on my hands when throwing but did not really change the plasticity. My conclusion: The red
clay needed an addition of a more plastic clay which would help with plasticity, drying, and
strength to last longer on the wheel.
Test #2 (Shrinkage Rate/Firing Temp) - The average shrinkage rate is 12%. This clay at cone 6
had a shrinkage rate of 15%. To me shrinkage was not too big of a deal, as long as I knew
what the shrinkage rate was, I could work with it. I just didn't want it to go too high. The firing
temperature to mature the clay was important to me. I needed the clay to fire to maturity at
cone 6. The clay did fire to cone 6 but did it reach maturity? Test #3 will tell me that. So far
though, I knew it survived a cone 6 fire and was very happy with that. My conclusion:
Whatever additions I added to the clay, I needed to make sure it wouldn't increase the
Test #3 (Water Absorbency - Maturity/Fire Temperature) - The water absorbency rate that I
was looking for was 2-3%. Test #3 showed that at cone 6 the water absorbency rate for this
clay was 4%. So at cone 6 this clay almost fired to maturity. My conclusion: I needed to bring
the maturity temperature down a little.
Wow! I was impressed with the results of these test. With just some minor adjustments this
clay was going to give me exactly what I wanted. Over the course of the next 6 months, with
the addition of a little Ball Clay and a tiny bit of Bentonite I was able to perfect a recipe that
suits my purposes perfectly. How cool is that!
|arrow points to layer of red clay
|Now I had a new batch of clay with less sand in it. I made more test bars and completed test
#'s 1 & 2 again and recorded my findings in my notebook.
Test #3 is the water absorbency test. Between the findings of the shrinkage test and the water
absorbency test I was able to determine what cone (temperature) my clay must be fired to, to
reach its maturity. To do a water absorbency test I took my freshly 'glaze fired' test bar,
weighed it on a gram scale, jotted down the weight and then boiled the bar in water for 2
hours. At the end of the 2 hours, I immediately removed the hot bar, quickly dried it with a
towel and while still steaming hot, weighed it again. With a quick calculation the difference of
weight between the dry and wet bar, gave me my absorbency rate.