Making Pots From Natural Clay
By Mike on Sunday, February 8, 2009, 21:17 - Permalink
An experimental attempt to dig natural clay and make it into fired ceramics.
Obtaining And Preparing The Clay
The story begins at the Blackwater river near Rhinefield in the New Forest - it's a typical new Forest stream - water stained tea-brown with tannins from woodland leaves and peaty heaths, a bed of shingle and coarse pebbles over clay, steep banks of mixed soils and clays - bridged occasionally by fallen trees, a mix of deep pools, meanders, straight runs and broad, shallow natural fords.
In places, the material of the river bank is very conspicuously clay - and it's this that is of interest to me today. In order to avoid causing damage by digging into the sides, I chose a place where the bank had collapsed naturally, exposing a tumble of big blocks of semi-dried blue clay.
And it's that chunk of clay on the lower left that I collected - well - a few pieces of it.
I collected about a kilo of clay pieces in an old ice cream container - as you can see though, there's a bit of contamination by ordinary soil and plant material.
The proper way to do this would be to dry the clay pieces thoroughly, bash them to dust, then sieve them through a very fine mesh, before mixing the dust with water to make usable clay again.
That's a lot of trouble for a very small amount of clay - so I'm cutting corners - I broke about half of my clay into little pieces in a different container
Next, I covered them with water for half an hour. then poured off the water and left the moistened clay to sit for a while - the water continues to even out through the clay pieces.
After that, I pounded and mashed the clay together with the end of a scrap of wood.
The texture of this material is incredibly fine - and there's no smell at all - indicating (I think) that it's pure clay with no residues of organic matter
The processed clay turned out to be a little on the wet, sticky side - so I formed it into rough cylinder shapes and stacked them to allow air flow - I'm going to allow these to dry out very slowly in a large container with the lid kept slightly ajar.
The clay looks coarse and sandy in this picture, but it's not - that texture appeared as a result of rolling it in my hands while it was so sticky and wet.
Next, I need to work out what I'm going to make. My idea at present is to make a small cauldron-style cooking pot and a simple oil lamp - two really ancient-style clay items.
How Clay Works
Fired clay is really a form of artificial metamorphic rock - the application of heat causes some of the mineral particles in the piece to melt into tiny blobs of glass, which fuse everything together permanently
Articles made from clay must be dried before attempting to fire them - otherwise trapped moisture may cause the item to explode when heated - and this drying process must be carried out fairly slowly to prevent the clay cracking and splitting
In a modern kiln, temperatures are controlled very carefully - too hot and the whole piece may actually melt - too fast and it may crack because of uneven stresses, or because of the sudden expansion of some mineral components more than others
I'm attempting to do this in a very primitive way though, which consists - more or less - of just building a fire around the dried pot and allowing nature to take its course. This certainly means the risk of failure will be greater, but it can't be altogether impossible - after all, this is how it used to be done
The sticks of clay have become more firm, less sticky - I took a few and bashed them flat on a plank of wood - this should help to remove any residual air bubbles that might cause a problem during firing
I'm making a coil-formed pot, so I started by making a thick disc of clay
Next, I rolled out some more clay into a long, thin roll and attached it to the disc
I continued, adding more and more layers in a spiral, building up the pot shape
Using my fingers (and a tiny bit of additional water now and again), I smoothed and pressed the coils together - inside and out. It's important here not to just smooth them over, leaving a hidden void - instead, they should be pressed firmly so that the gaps in between the coils close up.
When the outside was fairly smooth, I added another, thicker roll of clay at the top - to make a rim.
A little more smoothing, then I pressed a toothpick into the rim - not just for decoration - this should make the pot slightly easier to grip when it's finished.
The finished pot - (a bit of an ugly ducking, but no matter) - was half-covered with plastic film and left in a cool, unheated room - it mustn't dry out too fast, or it may crack.
Working With This Clay
Considering that I've not really worked very hard to purify this clay, it's actually surprisingly fine and consistent - the texture is not gritty at all - it's like soap - the hard lumps that existed before have all moistened and the texture is the same throughout. It's very pleasant to work with.
The Plan - Revised
My plan for this batch of clay is:
- Make a large coil pot (this page)
- Make a small oil lamp
- Make a small, simple thumb pot (should be less prone to failure in firing)
- Make a simple decorated tile
Hopefully, at least one of these items will fire successfully - ideally, they all will - and if that happens, the final part of the project will be to cook something in the large pot, over an open fire.
Starting with a golfball-sized piece of clay, I made a thumbpot - the easiest kind of small pot to make - the trick being to work it gradually into shape, rather than plunging your thumb right in and trying to force it.
Next, the pot is pinched at one side, to form a handle - not all ancient oil lamps had one of these - they were typically small enough to be simply held in the hand.
Then the pot is drawn out and pinched at the other side to form a crude spout. A disc of clay is formed for the lid.
A hole is made in the lid - this is where the oil will be poured in when the lamp is finished and fired.
The lid is joined to the top rim of the lamp body and blended in by smoothing over with a finger - as with the pot, the join here is not a superficial layer of smoothed-over clay, but a pressing together of the two pieces.
Finally, with the lid blended in all around, the wick spout is built up a little further and the whole thing smoothed as neatly as possible without crushing it.
The piece will be allowed to dry slowly in an unheated cool room - ready for firing.
Why A Lamp?
I've chosen to make a number of stereotypically ancient-style items - a cooking pot, a lamp, a small bowl or cup and a decorated tile - mainly because these are the sorts of items that would historically have been made for open-pit firing - with the exception of the tile, they're going to be items of utility
How Long To Dry?
The pot I made yesterday has dried somewhat - although the clay is still quite damp and soft, the piece has stiffened and set into shape. I would be able to apply a scratched or pressed design now, but not alter the shape without re-wetting the clay and starting over. I don't know how long it will take to dry the pieces ready for firing, but we're nowhere near that point yet.
With the remaining clay - which is still workable, having been wrapped tightly in plastic for the last week - I made a small thumbpot, which I formed into a shallow cup shape - not unlike a Roman wine cup. I also made a thick circular tile with a rather crudely scraped pattern on it (I might give that a bit more work when the clay has dried to leather-hardness).
The items I made last week are drying well and are starting to feel lighter in weight. The large bowl developed a slight crack on the inside which I have bodged closed by wetting it slightly and forcing some more clay into it - I fear this might portend failure during firing for this item - the proper thing to do would be to break it up, re-dampen the clay and make the pot again from scratch, but I'm not going to start over with it now,
Now, We Wait
Now I've just got to wait for the items to dry out completely - this will probably take at least another week or so
My finished pots are now nearing the bone-dry stage - they've turned a lighter colour and have lost weight a bit. the largest pot still shows evidence of a hairline crack that I suspect will doom it during the firing, but I intend to treat that as just another facet of the experiment.
How to fire the pots
I've not really done anything like this before. In order to properly fire pottery, it's normally necessary to raise it slowly to temperatures in excess of 2,000 Celsius. This - both the slow rise, and the top temperature - is probably just impossible with an open-pit firing - about the best I can hope for here is some kind of low-fired earthenware. Realistically, I may end up with nothing more than potsherds.
I need to find some way of delivering high temperatures to the clay for a sustained period - I don't intend to build anything like a durable kiln - there has to be a simpler way.
So I think it will be something like this:
- Make a series of square stacking frames from thick timber
- Stack them into a makeshift chimney shape on the ground
- Excavate air vents underneath the edges.
- Loose-fill the space inside with dry wood - starting with small pieces at the bottom, working up to larger chunks at about halfway up.
- Nestle the clay items carefully onto the top of the wood fill and pack loosely around them with small pieces of dry wood
- Light the thing from the bottom
Then what should happen is that the wood inside the chimney burns upwards, growing hotter as it goes, collapsing gently into a heap of very hot, aspirated embers. As long as I don't overdo the drop, or the loading of the wood, the items ought not to be crushed or shattered.
If it can be called a kiln... here it is - it's just a couple of tapered, stacking wooden boxes made from reclaimed timber - the cutouts at the bottom are to let air in (they'll be augmented by digging out the ground underneath each of them a little).
What Happens During Firing?
Firing a clay pot is actually a bit like making artificial metamorphic rock - it's more than just baking the material dry - the physical makeup and microscopic structure of the material is transformed by the heat.
It's actually a complex, multi-stage transformation - that I don't feel qualified to describe in detail - but in (sketchy)summary, what happens is that little grains of minerals such a silicates melt and become glassy. In the process of so doing, they stick together permanently, binding the material into one fused mass
I'd been looking forward to this for months - the single-use wooden kiln I made from old scrap timber has been cluttering up my garage for too long, so on one sub-zero January day, I decided a nice hot fire would be just the thing...
04 January 2009 - My pots have been sitting on the windowsill, unfired now, for months. Time to get firing...
Before You Read On
I have no intention of wasting the reader's time, so let me begin by stating: The firing was a total failure. Nothing resembling a pot was produced.
I put my single-use kiln on my now-bare vegetable patch and filled it up with dry pieces of timber and twiggy material.
I dug a small hole underneath each of the air vents, to promote airflow.
I carefully nestled the pots into another layer of twigs on top of the main load, then covered them over with more small pieces of wood.
I crumpled a sheet of newspaper into each of the vents, then lit them.
before long, it was burning quite fiercely with an audibly roaring flow of air and very hot flames leaping six feet or more from the open top. The damp fence panel four feet away began to steam and had to be doused with water to prevent risk of it catching alight,
For half an hour or more, it was impossible to get close to the fire. The embers inside were white hot.
Eventually, the fire began to burn down and it collapsed very slowly, gently and gracefully into a heap of small, very hot embers.
I left it like that for another half hour, then couldn't resist gently raking away some of the top embers to see if I could catch a glimpse of a pot...
There was nothing there.
There was no trace of any of the pots - not even broken pieces.
After a very thorough search, I did find a few chunks of crumbly reddish stuff - which may be part of one of the pots, or might just be pieces of garden soil that got baked hard by the heat.
What Went Wrong?
Difficult to be sure, but I think my fire got too hot, too fast - I think this may have surface-fired the material, then caused the clay to pass too rapidly through one of the critical heating phases (during which it may expand abruptly), shattering it into small fragments.
It's also possible, I suppose, that the clay just wasn't as good as I thought - maybe it had too much organic material in it, or just didn't have the right chemical composition to make it worth firing.
Well, I still have about half of the clay left - I could make some more pots and see if I can find a proper kiln to fire them in, or I could do some research on building a proper wood-fired temporary kiln.
I'm not giving up, but I'm not going to try to muddle through another attempt at this on my own. Clearly there's essential knowledge here that can only be learned by lots of trial and error, or from someone else's wisdom and experience.