New Sericite Clay Samples

I was recently in China doing some research. I have written a little bit about that, intermittently, in the last month or so. While I was there, I arranged to get my hands on some new and different sericite samples. These have now just arrived here last week and I have done my first tests with them.

I now have 4 different Chinese sericites to compare.

Although it isn’t immediately obvious from the image above. If you look closely, there are 4 different colours of rock samples from top to bottom, white, cream, grey and pale buff. They all look more or less white, but they each have various tints or shades of colour to their whiteness. They all do have one big thing in common. They all throw badly, the palest ones being the least plastic and most difficult. They feel a lot like my local Mittagong porcelain stone, only better behaved.

I felt like I’d gone all the way around the world and come home again when I threw these tests. They felt so familiar.

I’m really looking forward to seeing the results fired. I had a bit of trouble with the usual shrinkage and drying cracking problems, but I did get some of them through successfully. But I lost quite a few. Still, nothing that I’m not used to, and I’m getting good at recycling the turnings and failures.

I almost filled the tray on my old wooden kick wheel with turnings after trimming just 12 small bowls. I must be removing at least half of the weight of the original material to get them thin enough to look and feel like porcelain. I aim for 2mm at the rim and 3 mm lower down, graduating 5mm at the foot. This tapered wall thickness allows the best translucency at the rim and higher up the pot, while retaining sufficient strength to hold the pot up against gravity while it sits at 1300oC in the kin to develop enough glass in the body to be come translucent.

If I’ve done it right, the whole finished, fired, ceramic mass, has the correct quantity of primary and secondary mullite crystals to glue it all together, while becoming glassy enough to allow plenty of light to pass through.

Too glassy and/or too thin and it slumps. Too thick or not fired high enough and it stands up straight, but isn’t very translucent. It’s a bit of a fine line to tread.

not too bad.

As I sit and grind away at this damp, ground-up rock dust with my tungsten carbide tools, I realise that I’m truely happy doing just this. There is a gusty wind outside, but I’m in here sitting in the sun and I don’t want for anything more at this moment. This is fun. I can’t wait to get them into the kiln.

It has all the promise of something special about to happen.

I love that.

High hopes are not enough

After I had such a terrible firing a couple of weeks ago. I got stuck in and remade all the work. I’ve filled the shelves and had 2 solar fired biscuit firings and I am almost ready for the third. Then I can pack the wood fired kiln again. Hopefully this time with more success.

I had slaked down all the turnings from the last work session and made half of this work from the recycled sericite bodies. I have made test batches of 5 different sericite this time round. I have 2 Chinese minerals, 1 Korean, one English and one Australian in this batch. Although strictly speaking, the local one is a complex mixture of Illite, kaolinite, quartz and felspar.

I’m always looking for something that I don’t know, so the search is a bit difficult as I’m working semi-blind. I know more or less what I want and how to achieve it. After all, I have been doing this research for the past 40 years and more. But getting what I consider excellent results is always elusive and difficult. It doesn’t help when the kiln shelves break and collapse… Still, I’m over that and all the new work is made and drying. I’m just turning the last of the throwing today.

Sericite is funny stuff. It isn’t plastic like other clays, it’s quite short in most cases and has to be coaxed along to get any height in the form. I learnt to throw the inside of the form as I want it, leaving a thick wall to retain the form without it squatting. I then have to turn quite a bit of material off the outside to realise the finished form from the thick lump at the base.

Although sericite isn’t plastic, it still shrinks quite a lot and has a terrible tendency to crack in drying where it is thick. I have developed a few techniques to cope with theses peculiarities. I’ve learn’t to throw everything on batts, as lifting the pot with my fingers causes a memory distortion in the body that shows up later in drying. I’ve learn to ‘polish’ the out side of the clay after I have finished throwing, to seal any possible defects or weak spots that might start a crack during the early stages of drying and stiffening. I have also learnt to turn the bases in stages. Thinning out the clay as soon as it is stiff enough. This stuff can’t be turned like ‘normal’ clay based bodies. It has to be very firm, almost dry to get a smooth finish. If turned too early, it chips and tears, making an awful mess of the fine surface. It’s a bit like cutting soft goats cheese, it just tears.

Trimming the base of this pot almost bone dry

I’m getting better success rate these days with each variation and improvement of these techniques. It’s a little frustrating to loose so many pots, but they slake down quickly and can be stiffened in plaster tubs out side, then re-thrown within a week.

It would be better to leave the body to ‘age’ in a cool dark place for a month or two, but with so many variations of sericite porcelain bodies in constant testing and development. I’d loose track. As it is I really have to concentrate and keep very good records, marking every bag with a permanent marker, identifying every bucket of turnings in the same way and carrying this over to the slaked recycled material in the buckets and in the plaster tubs. It’s quite a paper trail of provenance, keeping track of it all. I also keep a daily diary in the workshop of what I’m throwing and what I’m throwing it out of. Every pot gets inscribed with the batch of body and later identified with oxide as to which glaze I used. I also keep log books for the glaze and the body ball mills, so that I know what I made, the date it was done, and what it was made from. I always seem to have a dozen different batches on the go at any one time and they all need their own bucket and bag storage space. It’s organised chaos.

I’m nothing if not thorough. I have to be. With over 3,000 glaze tests and hundreds of clay body tests. I need to keep track. I really want to make simple, elegant, beautiful things. But things with a particular character that I admire. Just a little bit ‘damaged’ or altered by the process of their making. Perhaps I should say ‘enhanced’ by its process and journey, plus the unique quality of its material. I like my pots to have a story embedded in their form and surface. I can pick them up years later and ‘read’ that story. I like that.

I’ve just made a few more white tenmoku bowls. They are slightly bigger, fuller and rounder, more generous in feeling, less austere. I haven’t finished turning them yet. They still need more work over the next few days, but I have high hopes for them.

Unfortunately high hopes are not enough. It’s long term, steady, pains taking, thorough and often boring, consistent research that makes progress for me.

That’s really rewarding!