Clays From Around Cobb Mt
By HP Bloomer
05.14.2017
Originally published on his blog: Bloomerpottery.wordpress.com
These are a few clays that I have tested for shrinkage and temperature range from around the Cobb Mountain Arts & Ecology Project area. This area is rich in volcanic activity and many of the clays are derived from rhyolites, which are silica rich volcanic rocks, formed during those actives. It would seem that several of them are quite similar when fired but will often have slight nuances in texture and particle size or inclusion. These are all tests that I ran a few months back and am just getting to publishing now. All tests were run in electric/oxidation except for the final cone 10 tiles which were fired in gas reduction. There are a few more clays that I have not shared in this post but may add later on. I hope that this can become a resource for future makers in the Cobb area and a inspiration for those of you who are interested in finding local clays.
Adams Springs Bluff Clay
GPS: 38.857996, -122.719689
Very coarse grained clay that begins to vitrify at cone 6 and starts sticking to surfaces at cone 10. It may need some amending to be workable but has a lot of potential. I did not make a shrinkage bar for this clay.
Big Creek/175 Wash Out Clay
GPS: 38.857517, -122.722183
This is a very fine but slightly sandy clay found in the banks of Big Creek as it intersects CA Highway 175 less than a mile from Cobb Mountain Arts & Ecology Project. Because it is from active alluvial deposits its reliably is questionable. It begins to become vitrified around cone 6 and starts sticking to the shelf at cone 8. By cone 10 the clay has become glossy and is more actively fluxing. This was not tested for shrinkage.
Bottle Rock Road Dark Brown Clay (mile marker 3.6 East Side of road)
GPS: 38.899102, -122.790808
A very sticky dark chocolate brown when raw and full of obsidian inclusions from the east side of the pull out at Bottle Rock road near mile marker 3.60. In tests the clay started vitrifying after cone 6 and started sticking to the shelf/trays around cone 8 although this might have been due to the melting of the pervasive obsidian inclusions common to clays from this source. This clay was not tested for shrinkage.
Bottle Rock Road Light Brown Clay (West side of road near mile marker 3.60)
GPS: 38.898739, -122.791109
This is a very sticky clay very similar to the dark brown clay. It is slightly lighter in color when raw but fires to the same statistics as the dark brown. It is undoubtedly the same vein of material as the previous clay although it may be easier to source as it is more readily available at the surface. It still has quite a bit of obsidian in it and would need to be screened. No shrinkage information is available for this clay.
Bottle Rock Road Clay 1
GPS: 38.898559, -122.791181
This clay comes from a feeder creek that runs parallel to the west side of Bottle Rock road and empties into the source for the dark and light brown clays previously mentioned. Although it fires to a much lower temperature it is much cleaner and has much less obsidian in it. It vitrifies around cone 3-4 and bloats and melts after cone 5. It would make a nice clay body for anyone working at cone 2-3 and with some amendments could certainly be useful at cone 5. The source is easily dug and just off the road. No shrinkage tests were done for this clay.
Bottle Rock Road White Inclusion Clay (mile marker 3.60)
GPS: 38.899108, -122.791128
This very white, sticky, and plastic clay is found in small inclusion in the sandy obsidian riddled cliff at the mm 3.60 pull out on Bottle Rock road. It is a very sandy but very nice clay but does not come in any great quantity and using it as a source material may add to cliff deterioration. It begins to vitrify at cone 8 and is nice even up to cone 10. No shrinkage data was collected for this clay.
Bottle Rock Road North (mile marker 1.9)
GPS: 38.920129, -122.804331
This is a very nice, smooth, fine clay from just north of the previous Bottle Rock samples. It is on the west side of Bottle Rock road and just before another sandy white exposure. The clay is underneath a grassy outcropping and is a yellowish tan color when raw. With a little testing and work it would be a good low to midrange clay. Although the test samples we took were relatively free of any obsidian inclusions it might need a little bit of screening to insure that all debris is removed. This clay seems to loose its porosoty around cone 4-5 and by cone 6 it is starting to bloat and become brittle. All samples fired to cone 8 & 10 were unidentifiable puddles. so it might work well as a glaze component.
Shrinkage: Cone 01: 5%, Cone 2: 10%, Cone 4: 11.5%, Cone 6: 9% (at cone 6 it starts bloating and expanding.)
Glenbrook Clay
GPS: 38.852581, -122.757539
This is another clay from Bottle Rock Rd here in the Cobb area. Its less than 3 miles from the Cobb Mountain Arts & Ecology Project and accessible from a road cut. The Glenbrook clay is on the east side of Bottle Rock near the Glenbrook intersection. It fires very similarly to the previous Bottle Rock sample from mile marker 1.9 but is slightly redder and melts at a lower temperature. It has a very nice and slightly sandy quality and would be nice for working with in the cone 1-2 temperature range. It was porous in tests up to cone 4 and then bloated and slumped but cone 6. So it is a very open clay body that could use some amendments if one wanted to use it in their work. The cone 8-10 tiles melted completely and might be suitable as glaze components for those working at that temperature.
Shrinkage: Cone 01: 5%, Cone 2: 6%, Cone 4: 6%, Cone 6:8%
Running Trail Clay 1
GPS: 38.857720, -122.731303
The Running Trail Clay is from a property that abuts the Cobb Mountain property to the south and may be part of the parent material for the Big Creek Clay. This clay is pretty irregular in particle size and has a 10-12% shrinkage rate. Although it holds together quite well up to cone 6 it does crack quite a bit making it impractical for functional items but potentially useful for sculptural use. In these tests the clay had completely vitrified by cone 6 and started sticking to the test trays by cone 8. It may need some kaolin additions to make it a workable cone 10 clay.
Shrinkage: Cone 01: 10%, Cone 2: 12%, Cone 4: 12%, Cone 6: 10%, shrinkage bars warped and were unreadable beyond cone 6.
South Side Parady Property Clay 1
GPS: 38.859771, -122.727677
This is a extremely thixotropic clay from the south most side of the Cobb Mountain property. It would need to be blended with a more utilitarian clay to make it it workable but might provide a source of clay to use for amending shorter, lower firing clays. It has some inclusions in it and would need to bee screened before use. It seems to be relitively refractory as far as clays in this area go and only starts to vitrify at cone 8. It has a relitively low shrinkage rate coming in under 10% up to cone 8.
Shrinkage: Cone 01: 7.5%, Cone 2: 8%, Cone 4: 8%, Cone 6: 9.3%, Cone 8: 9.5%
South Side Parady Clay 2
GPS: 38.859771, -122.727677
This is a clean (obsidian free) but slightly short clay just a few meters away from the previous sample. It is exposed in a road cut along the firebreak that defines the south side of the property. It has a slightly higher shrinkage rate than the previous sample but is much more workable. This clay starts vitrifying at cone 6 and is vitreous by cone 8. Both this sample and the prior are slightly less accessible and would require bagging and a hike up a steep hill to access.
Shrinkage: Cone 01: 10%, Cone 2: 11%, Cone 4: 11%, Cone 6: 12%, Cone 8: 12%, Cone 10: 12.5%
Well that is all the clays I have tested from the area thus far. I hope you enjoyed the results.