Ceramics clay use in cob

Anyone have any experience in using ceramic art clay with cobbing?

I am a ceramics art teacher at a public school. In short, the students end up with a certain amount of wasted clay that is unusable in their art and we do not have a pug mill to reconstitute (reuse) the clay. So it gets trashed. I have zero experience with cobbing but am interested in finding a good use for this "trash" clay. (White Stoneware clay)

Any advice, experience or suggestions would be welcome. Thanks!

I had to look up what a pug mill was.

I assume that this clay hasn't been fired yet?

When I was in high school, the ceramics teacher just used 5-gallon buckets to recycle the clay with. Any unwanted un-fired clay got dumped in a bucket of water. When the bucket was full, he'd snap a lid on it and stick it in the closet for a month. Once in a while he'd check on it and add water if it seemed dry, or pour off the water if there was so much it was pooling at the top. After about a month, the clay was pretty homogeneous and could be used again.

He had a pretty big collection of these buckets, but it worked.

A pug mill certainly makes fast work of recycling a lot of clay to perfect consistency, but buckets for aging and wedging boards for finishing the clay work too... as long as there is student labor to do the work Do students pay for their clay? If so, there should be incentive for an ambitious few to get clay just by working for it. When I was in a community studio, I never put my waste in the big slop buckets, but saved and rewedged it myself.

White stoneware is usually pretty fine-grained, and would want a lot of sand added to make any sort of cob, probably three times as much sand as clay. This would give a more useful quantity of cob in a given period than you might expect, so might be of interest. If you are actually building something from cob, larger gravel or small stones can be incorporated in mass sections and will somewhat reduce shrinkage. Angular stones are much better than round ones.

I'd say it should work. The stickier the better. Clay is mined where I live for pottery, and it makes cob like re-wettable concrete.

I have used ceramic fireclay powder from a pottery studio to make a cob pizza oven.  It worked well!

You have a goldmine of awesome material! I have built, plastered and sculpted using waste clay. Yes, you can easily make cob by drying the old clay, smashing it up and adding small proportion of this clay to local sandy dirt and straw. But the art material is special and is best suited for artistic finishing work: covering up rough cob or adobe.
The clay has been especially helpful material for plastering over terrones (dried sod blocks). I use a 1:9 ratio of refined clay to sharp sand for plaster.
I have also mixed straw and clay slip to make light straw clay which is wonderful for sculpting. Use a sturdy armature such as scrap metal, rebar or wood with baling wire or sisal twine or rope then cover with clay straw. Tie sisal over the straw clay to achieve strong adhesion on complex shapes. After drying, use old saws or rigid rake heads for subtractive effects. Plaster with the 1:9 clay and sand mix. Add natural fibers to your mix for personalized effects. Change the color with iron oxide, charcoal, ochre, fibers, crushed red lava or smashed garden pot grog or colored sand. Wipe leather-hard surfaces with a damp sponge to reveal the special additives. Experiment: the possibilities are vast.

2 comments.

1.  While I pug mill is a nice toy proper washing, settling and  wedging will get the clay back to usable.  And where you have free labor and it is teaching a skill you should work on anyway.

2.  That said for those doing high temperature cob stuff the clay should be a very good choice as many of the pottery clays have better numbers than the typical fire clays used for such projects.  Be aware your sand will often not take the heat as well as the clay if you are really pushing it.


Quick explanation.  Washing clay is used to get debris out and sort the clay by size.  Raw clay(mud) is mixed with enough water to turn it into a thin soup.  That soup is then poured thru a strainer to get the bigger debris out.  Rocks. sticks, roots etc  Containers of this watery mixture are then settled while the clear water coming to the top is repeated poured off.  Often useful to combine containers at this point and reblend to a thicker soup consistency mixture.  Coarse stuff goes to the bottom, silts next, then the desired clay and finally a layer of top that is organic slime and some fine clay.  Scrape that slime layer off.  Gather the clay layer sorting by touch, scraping it off.  Taking the good clay(likely combine several batches) make it back to soup and settle again.  Gather the desired clay junking what you don't want.  This process is often repeated a number of times to sort the clay by particle size.  This also serves to wash most of the salts and carbonates out of the clay.   Once the clay is sorted and you have what you want those containers are well mixed with enough water to make really thick soup.  The final settling the goal is to avoid air or water pockets in the clay.  By banging, shaking etc most of those can be worked to the top.  Now dry it down to clay working moistures by pouring tiny bits of water off each day.  If you vibrate the clay often you can keep it from cracking and speed this water separation.

Now wedging is the process of blending the clay to consistent moisture while pushing any remaining air pockets out.  It is the act of pushing thin layers repeatedly without folding as folding makes air pocket which you are trying to get completely gone at this stage.  Done correctly wedging cause any air pockets to be squeezed and push out of the clay.  Most common way to screw this up is to end up folding clay while wedging putting air pockets in rather than getting rid of them.  It is a skill like any other and takes practice.