Improving a silty clay body

[Jose]

Hi, I  collected some dry local clays (Grand Canyon) for testing. Field tests showed fine grained particles, low plasticity, good amount of silt. Water jar tests showed about 30-35 percent clay,  40 percent silt, and the rest is rocks. The clay body is gritty for sure, but very fine grit. Coil and pinch pot test don't go well. So, from my knowledge my options are,

1. find better clay,

2. Reduce silt percentage with sieve/s 

3. Add plasticity with ball clay???

Would love feedback. Just moved, taking a break from work and dedicating on pottery. Thoughts and suggestions? Thank you and blessings to you and your loved ones.

 

 

[Kellykopp]

Dear Jose, many many years ago when I was in college, I did a test of the local clay here (southeast Indiana, dug out of the bank of a creek) for an independent project. After slaking, sieving, and drying the clay to a workable consistency, I formed several small pinch pots to further test. With the process of pit/barrel firing that you are currently doing, I don't think this will have any bearing with the firing process you are doing , but exercise caution if you fire the clay to a hotter temp than low-fire. My professor advised that I put a "safety plate" of the high fire^9 clay we always used under my little pinch pot before putting it through a high fire and wow, good thing I did as the clay melted into a puddle! So my particular clay from my creek was only good for low fire. I unfortunately ended my experiments there, as at that time I had access to ^9 processes/firing and that was the direction I was headed at the time. One of the notable findings during this experiment was that the clay looked exactly like pottery sherds found from local early Native American campsites...color (red) and white flecks of shell tempering. I am sorry I can't be more help as it was only a small experiment for me. I am sure that others in the forum have more experience than I. Keep notes of your experiments and blessings on your clay journey. 

 

[glazenerd]

Jose

Silt is a non- plastic material; so at this point you do not know how plastic the remaining clay is. Silt is either eroded soil in farmland mainly, or its eroded minerals. Given your location: eroded minerals which is most likely quartz in the 5-20 micron range. You will need a 100-120 mesh sieve to get rid of most (not all) the silt. Another method is separation using a water slurry- akin to making Terra Sig on a very large scale. Once you separate the clay: then check for plasticity and make adjustments with ball clay. Remember, when adding ball clay by hand:it takes plasticity 5-7 days to develop- it is not an instant reaction.

The alumina content determines the cone value it will reach before slumping or melting completely. Common for wild clay to be below 20% alumina; which also means it will not survive much above cone 04. The easiest way to test that is to make a piece that loosely resembles a pyrometric cone, and fire it in a small dish. There is no need to form pieces to check pyroplasticity, unless you just want to. 

Below is a shape I used often to test local clay. It is rolled thin; about 1/4". I then press a pattern on one side. Half that pattern is clear glaze, the other part clear glaze with 9% zircopax. It is singled fired to cone 04 on a washer slab.When I pull it out, I hold it to the end of the roller to check deformation. You can tell by the shape and thickness: it would deform easily if that was an issue. The test piece is roughly 1/3lb of clay. So I check pyroplasticity, glaze, color and pattern retention in one shot. 

Tom

[Russ]

Northern Arizona I presume. Painted desert is a vast volcanic clay field with varying temp clays from earthenware to stoneware . Outside the park along the right of way are roadcuts thru huge clay deposits... hint hint.  might be worth a try and I doubt theyll make a fuss about a bucket or two. check the local regs though. We have these volcanic deposits in nm also. This is up by Georgia O'Keeffe's old stomping grounds.

[Jose]

Thank you all so much. I'm loving how helpful people are here. Nice community. You guys are awesome!