Marcellus Shale as a Studio Material

The process of making the clay is documented here.
Here are some of the early ceramic cup results:

The shape was based on the form of both a coffee cup and a ceramic planter. The shrinkage in the clay was significant, and some of the cups were unreasonably small for drinking out of. I altered the mold and the firing method and was able to produce a larger cup - much closer to a standard coffee cup size..

Here's a pic of the range of experiments done while playing with the size and shape. There are also a number of examples of attempts to make the shale into a glaze. Some were successful...others not so much!

Below are a few cups with multiple glaze tests:

Here are a few successful glazes:

...and a more preferable form with Marcellus glazes:

There were some spectacular failures:


 Ceramicists call this 'crawling'. It's often considered a defect.

There were surprises even in firing some unglazed clay items. Here, two cups were nestled into each other in the kiln. As they fired, the one on the inside inflated with (what I assume to be) carbon. They were stuck together, so I broke the edges of the inside cup off, exposing the black inner material:

After the clay had been in the studio for a while, I thought it might be possible to try to throw something on the wheel. Over time, the elasticity of clay will increase, so even though initially there was no choice but to cast slip, it had aged enough to give it a try. I was able to throw a bowl:

a rather clunky bowl!

There was a change when it came out of the kiln:

It had fallen on its side after having grown a large wart on its foot! On the inside of the bowl, a bubble pushed through the surface, cracking it slightly. This, I later learned, is what ceramicists call 'bloat', usually a very bad flaw. Basically, if a clay has excess organic material, it can get trapped inside the walls of the clay when it fires. It off-gasses with no place to go, so it causes bulges and cracks. For me it was quite a happy accident. Gas being released and pushing through a surface to change a form...


After having some success in making clay from the shale, I began to wonder if the rock itself, unrefined, might turn into a ceramic material when fired in a kiln.
Working with small thin shards, I conducted a number of experiments to see how the material would be effected by heat. 
 Sample of uncooked shale shards. The porcelain container is about 4" in diameter.

When bisque fired to cone 04 (1945-1971 degrees f), the shards turned orange and bone white. In ceramics, when clay is bisque fired it can no longer revert to clay when exposed to water, and it will absorb fluids. Most terracotta flower pots are bisque fired. Pink, salmon, and orange are common bisque colors for clays that are red, brown or black to begin with. Ceramicists bisque fire their work in order to apply glazes without effecting the original form. These shards, like bisqued clay, still absorb water (as does the original shale rock). 

The above sample was fired to cone 4 (2142 - 2161 degrees f). The reds were darker and clearly distinct from the bone-white areas, with the whites on the interior and reds on outside areas. The shards fused together. A few pieces inflated, showing internal bubbles. The rock had vitrified; like glass, it was no longer absorbent.  

At cone 7 (2262-2295 degrees f), more gases were produced within the shards, now greatly effecting the form of the rock itself. Many inflated, which caused a de-lamination of the layers of shale within certain shards. Some shards were nearly pure bone white.

Cone 10 (2345 - 2381 degrees f) is as high as my kiln (like most electric kilns) will go. The red and white elements separated almost completely from each other. The red darkened to a chocolate brown that was hard, but filled with air pockets - much like a foam. The white elements were thin, much like the original shards, but much harder and less brittle.
I decided to try kiln firing larger Marcellus rocks to see what would happen.  The danger in heating up a rock in a kiln is in ramping the heat up too fast. If the temperature of the rock varies greatly from the outer areas to the core, the rock could explode. With digitally controlled kilns, the temperature could be programmed to ramp up and down extremely slowly. Nick Kripal, the Chair of the Ceramics Area at Tyler School of Art, was kind enough to let me try this, and their tech, John Williams, guided and advised me throughout the process. 
Working with rocks of different sizes, shapes, and s rocks, between 4-7" in width and depth, and less than an inch in height. Each had a different structure: one with flat unfractured layers, another with flat fractured layers, and one that was comprised of thin elongated layers.  These were bisque fired to cone 06 (1828-1855 f)

Encouraged by the lack of explosions, we moved forward. The next firing ramped up to cone 4, and we included a much larger rock. Here were the results:

A ceramic container was built to accommodate the larger rock. It would protect the kiln from melting or an explosion. It was covered with a kiln shelf (the white flakes are from the white-wash used on the shelf).
Here's that same rock, fired to cone 6. It was brittle, and parts of it broke off, leaving the most solid section
I showed the results to the geologists at Temple. They recommended a professor emeritus who they thought could explain what was causing the rock's transformation.
The heat caused the iron in the rock (the red)  to migrate to the outside. What was left inside (bone white) was essentially clay. At a certain temperature, the clay turned into a mineral, mullite, which the professor said " desired and gives the China-like properties to porcelain." Mullite is a refractory material, and is even used at times for making kiln shelves and stilts. 
The bubbles in the shards are caused by off-gasing. I wanted to see if they were possible to eliminate. Through experimenting with prolonged temperature ramps, it became clear that any moisture or gas left in the stone before heating must have found its way out of the rock. The bubbles were from new gases being formed as the heat broke down elements within the stone. I have yet to find a way to eliminate them, and have come to enjoy the unpredictable transformations they create.

Fusing Forms 
After the temperature tests showed that the rocks can fuse together, I became interested in finding out if there might be a way to build fused rock forms. 

There are a few ways that the rock breaks: in elongated flat sheets, in thin chips and chunks, and in 1/4" diam splinters, usually less than 2". The latter fragments are sometimes referred to as having a 'pencil' formation. The first form attempt was made with pencil shards. 

I painted kiln wash on an unglazed wheel-thrown bowl from the studio. Kiln wash is used to protect kiln shelves - it makes it harder for something that's melted to stick. The pencil shards were stacked against the sides of the bowl, held in place by gravity only. 

Here's what popped out of the kiln:

The pieces had fused, but they were very brittle, making the entire piece quite fragile. I decided to re-fire it after spraying it with a flux and water solution, and it came out of the kiln far more solid.

An experiment using flat chips:

This one was super fragile, and fluxing and re-firing didn’t have the same effect.

In a third attempt, using thicker flat shards and firing to a higher temperature, there was greater success in fusing, but the piece broke as I tried to extract it from the bowl it was fired in. Here’s a chunk that was left:

I’ll be running this experiment again (in a different bowl!)

One of the problems in making a form solely with shale shards was that gravity is the only thing which can hold each piece in place. It was tempting to grab some clay to use as a material for connecting parts. Very tempting.  

Some tests were done with different clays to see if a bond with the stone would work. In the image above, a terracotta coil held tightly to the rock.  A test with porcelain proved equally viable. The green strip running down the middle of this sample was a glaze – something I’ve just started to play with.
The only problem in working with clay and shale was the absorbency of the rock. While working with the wet clay, the shale pulled the moisture out if it, which didn’t allow a bond to occur. This was easily remedied by soaking the shale in water first.

Shale shards with a terracotta mortar unfired

and fired.

Clay Slip as a Drawing Medium
There must be a tremendous lost history of art made with mud. What a great material. You can scratch into it, form it, use it on another surface to make images, etc. Even before ceramic was fired, I imagine it had been in use for paint, etc. in the form of mud. 

When watered down, the Marcellus clay slip turned into an interesting drawing medium. I used it in a manner much like ink wash, but the grit in the slip had a mind of its own, generating unpredictable results:

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