Gill PITTMAN Ceramics
G A L L E R Y
I make the original of the plaque by modelling and carving, starting from a rolled out slab of clay. I then make a plaster of Paris mould and use slip casting - pouring a liquid clay into the mould - to reproduce the piece. As the originals take so long to make, I usually like to make a small number of casts, which allows me to vary the materials and glazes, and at this stage I can also vary the modelling. After the piece has dried comes the biscuit firing, which converts the clay into a ceramic. Glazes are next hand painted on, either earthenware, which then go back in the kiln for a second firing to 1040C, or stoneware, which are fired to 1240C, to produce a strong, non-porous ceramic. I also use porcelain, which is high fired, and, instead of glazing, I sometimes combine it with fusing glass.
I usually make no more than about five casts of each design, before wanting to make something new. The making process for each piece extends over two or three months - but of course I have several pieces on the go at once.
The finished pieces all turn out very differently, depending on the glazes and materials used. Each piece is therefore unique.
Mould Making
When I've finished modelling the original, I prepare for making a plaster mould. With the piece still in the soft raw clay state, it goes on a piece of plastic table cloth, and I build a wall around it with strips of aluminium held in place with clay.
It's very important to get the plaster mix right. If you don't, you get a bad mould that doesn't accurately replicate the original. Usually the original doesn't survive coming out of the mould, and it can have taken 40 to 80 hours to make!
The plaster is weighed out and sieved, then gently and gradually mixed into the water by hand to avoid introducing air bubbles. It's a dusty job, and you have to work fairly fast because the plaster starts setting in 5 to 8 minutes.
Then it's poured over the piece, smoothly in one go, from the centre outwards to avoid getting flow lines in the cast. It takes about 3 weeks for the plaster and the clay to dry enough for the piece to come out of the mould, which then needs further drying before it's ready to use.
Here are some stored moulds.
Slip Casting
Slip is a liquid suspension of clay with deflocculant to keep it fluid. I buy ready-made earthenware and stoneware slips, and also use a semi-porcelain called Parian.
The slip is poured into the mould, and over a couple hours the plaster sucks out water so that a layer of clay is built up against the mould surface, reproducing its details. Then the excess slip is poured out for re-use.
After another couple of hours the piece to dry enough to work on.
The next stage is building up the hollow back of the cast so that the front doesn't sag when it's turned over. I put in strips or blobs of the same clay, and make lugs for hanging wires.
After a few more hours, when the edges of the cast start to shrink back from the mould, I put a board over the top and turn the whole thing over, when hopefully the piece will flop out nicely. It then usually needs a few hours work to tidy it up.
The piece needs further drying before it's ready for the biscuit firing, and it's important to do this very slowly for about three weeks to avoid warping.
Biscuiting and Glazing
Here's a finished piece, the Christmas Fantasy, and the mould it was cast from. It's in glazed porcelain.
But before getting to that point there comes the biscuit firing, which converts the clay to ceramic.
After the biscuit firing I paint the glazes on and, as you can see below, they look nothing like the finished result, so I make lots of test tiles and constantly refer to them.
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Here's this piece after the glaze firing. It's in stoneware and depicts the remains of the wreck of the Helvetia - which ran aground in 1887 - and which you can see on Rhosilli beach, Gower, with Worm's Head in the background.
Firing
I use a 9kW electric kiln, which is now quite old, and doesn't have the programmed control of modern ones. So I manage the firing using a combination of cones - little spikes of clay formulated to bend over at specified temperatures - and several thermocouples that we have retrofitted.
The incentive for these was to check temperature variations within the kiln, which I suspected from the way things turned out. Interestingly, they show that at the start of firing the bottom of the kiln heats up more slowly. But at about 800C it catches up, and at the final temperature can be as much as 40 degrees hotter than the top - and this does not even out during soaking (that's holding the temperature steady for about 20 minutes to mature the glazes). This can make a difference to how the glazes turn out, so placing the pieces at the right position in the kiln is important.
To stand the high temperatures, the thermocouples are K type with mineral insulation in an Inconel sheath. They're stapled to the kiln walls with pieces of old heating element and connected to a selector switch and reading instrument. It's quite a cheap set up, and provides more information than you get from most standard kilns, which usually have just a single temperature measurement.
Heating rate is controlled by a percentage power dial on the kiln, and I take temperature readings every so often and plot a graph of the firing profile for future reference. It takes one and a half days for the heating and three days for cooling in a stoneware firing.
To the left is the kiln packed ready for an earthenware glaze/biscuit firing.
You can see the cones which you watch through the spy hole, some test tiles, animals the children made and the feet of an angel.
Porcelain with Glass
Porcelain is high fired and vitrifies to a white gloss. So instead of decorating with glazes, I sometimes combine it with glass. You can see the results in several of the pieces in the Gallery.
In the recesses between the outlines I sprinkle a few millimeters depth of frit - a finely granulated fusing glass. The piece is fired again to 830C to fuse the glass, and on cooling it often crazes - goes into a myriad small cracks - because it shrinks more than the porcelain. Some special techniques are needed to ensure adhesion of the glass and stop it drawing back from the porcelaion.
I like the depth of colour and the jewelled, sparking effects that you get.
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Here are some test tiles, where I was experimenting with effects for a stormy sky in the Helvetia piece.
