Mt Vision Clay
Initial tests with the local clay are presented.
Collection
The clay is collected from decomposed granite found in road cuts or similar excavation points around the property. It's a primary clay meaning it has decomposed where it was originally deposited and has not been weathered from water or wind transport.
Processing
The material is pretty gritty and needs to be sieved to collect the clay particles using a 60M mesh. Here is a picture of the resulting slip.
Once dry enough, a pinch-pot and test bar was formed as shown below. The clay does not pass the coil test, cracking as shown.
The results of drying and bisque firing are provided in the table below. The bisque fired vessels are shown in the cover photo.
| State | Weight (gm) | % Water | Length (mm) | Shrinkage | Absorption % |
|---|---|---|---|---|---|
| Wet | 113 | - | 100 | - | - |
| Dry | 82 | 27% | 91 | 9% | - |
| Bisque 05 | 71 | 13% | 87 | 4% | - |
| Bisque 05 Wet | 82.5 | - | - | - | 16% |
| Cone 6 | 70 | - | 82 | 6% | - |
| Cone 6 Wet | tbd | - | - | - | tbd |
Val Cushing's notebook states for an average plastic clay suitable for wheel throwing, Wet to Dry shrinkage of ~6% is normal and another 6% - 8% is expected from Dry to Bisque, or 12% - 14% total.
Levy, Shibata & Shibata in Wild Clay, give an example for somewhat similar looking yellow clay of 4.4% + 6% = 10.4% shrinkage.
We observed 9% + 4% = 13% shrinkage.
Levy, Shibata & Shibata record 16% water in their clay and we observed 27%
Cushing states absorption of ~5% is expected at any temperature while Levy, Shibata & Shibata recored 22% at Cone 04, but < 2% at Cone 10. We measured 16% at Cone 05.
We will test again when we fire to Cone 6, and we should retest a few times given this is just one data point.
Levy, Shibata & Shibata also note the following general characteristics for clay bodies:
Throwing clay bodies
- Shrinkage: high
- Water absorption: low
- Texture: fine
- Plasticity: high
Hand-building and slab bodies
- Shrinkage: medium
- Water absorption: medium
- Texture: medium
- Plasticity: medium
Without further modification of the clay, I would classify this clay as
- Shrinkage: high
- Water absorption: high?
- Texture: fine
- Plasticity: medium
So perhaps the clay best for hand-building. A small modification could move it toward a throwing clay? We'll see! The good news is the clay seems to be well within the ball park for usage.
Below presents the final results fired to Cone 6 and glazed with the Mt. Vision Granite base glaze 70% granite / 30% Wollastonite / 10% Silica + 10% Red Iron Oxide.
Remarkably, it fired seemingly perfectly.
Next steps will be to take these measurements again from my bigger batch of clay (which seems to be more plastic) and to try to make something on the wheel.