Clay from "le Cher" (a region in the centre of France), for instance, is used mainly for pots with underglaze decoration.
Clay with pyrites : for glazes pots (see picture section "Glazes").
Clay from Dordogne (another region in the centre of France) : mainly for glazes pots.
Dordogne White : used with some glazes, or for raku.
Porcelain clay that vitrifies at 1300 °C.
Earthenware clay : for some jewelry.
We are testing new clays for bonsaï pots : a very dark clay, another with grog between 2 and 6 mm diameter (!)...
Some glazes give very differents results, according to the clay on which they are fired.
Clays contain variable amounts of iron, which can also modify the glaze and we can use this property, using a pyrite rich clay to locally modify the glaze.
One must also take into account the coefficients of expansion between clay and glaze. If the differences are too large it can break the pot.
STEPS OF MAKING :
1- Throwing: on an electric wheel.
2- Turning the foot :
Usually the day after
throwing, depending on the consistency, we must turn the foot.
3- A special case: decorated underglaze pots only :
The design is carved with
a needle, freehand.
4- Drying : for all pots.
This stage must be
carefully controlled. If the drying
is too rapid or not homogeneous, it can create stress in the
clay; source of cracks or breakage, sometimes visible only
after firing.
5- First firing : biscuit firing :
The pots are fired at
900°C. At
this stage, they are still porous, making possible for the glaze to
stay on the pot (next stage).
6- Glazing:
6a- underglaze decorated stoneware pots only:
With a brush, we apply
metallic oxides
to the pots, being careful to respect the carved decoration
and forgetting nothing !
6b1-
Glazed stoneware:The pots are plunged into
the glaze bath.
According to the glaze, which may run, we glaze the
pots more or less low down, to avoid glaze running on the kiln shelves
at the end of the firing, sticking down the pots.
6b2- painting on glazes: (before the glaze firing)
One can decorate pots that
have been dipped in a grass-ash glaze.
One then paints on it with a brush, freehand, with cobalt and iron
oxides.
7- Kiln packing :
We fill the kiln so that
pots are
as close as possible to each other, but without touching.
(or they would stick to each other).
8- Second firing:
Our pots are generally
woodfired
(except for some glazes, such as copper reds, fired in a gas kiln, or
cristalline glazes in an electric kiln).
We fire at 1 300 °C so that the body is vitrified. It is then completely non-porous, even the unglazed parts (unlike earthenware which depends on the glaze, as the body remains porous).
We are able to control the temperature inside the kiln with the help of a pyrometer which measures the temperature of the atmosphere in the kiln, and also pyrometric cones placed near the spy-hole to measure time/temperature towards the end of the firing.
We fire at 1 300 °C so that the body is vitrified. It is then completely non-porous, even the unglazed parts (unlike earthenware which depends on the glaze, as the body remains porous).
We are able to control the temperature inside the kiln with the help of a pyrometer which measures the temperature of the atmosphere in the kiln, and also pyrometric cones placed near the spy-hole to measure time/temperature towards the end of the firing.
On this photograph, taken
during a firing, one can see the three cones, giving us temperature
information.
The first cone has fallen at 1250°C, the second has begun to fall (1280°C) and when the third falls at 1300°C, the firing is finished.
(In the background one can see a glazed candle-holder)
The first cone has fallen at 1250°C, the second has begun to fall (1280°C) and when the third falls at 1300°C, the firing is finished.
(In the background one can see a glazed candle-holder)
THE DECORATION:
Underglaze decoration
We make many pieces we call "grès brut décoré" or Raw stoneware decoration.
Once the pot is thrown and turned and dried to just the right stiffness, called "leather-hard" we cut the design freehand into the raw clay with a thick needle. After biscuit-firing, we brush metallic oxides on the decoration. Cobalt gives blue; Chrome, green; Iron, brown. The decoration is then covered, using a paint-spray gun, with a transparent glaze to bring out the design. It is then ready for the glaze firing.
vase : raw stoneware
decoration
Overglaze decoration
After the biscuit firing the pot is dipped in a grass-ash glaze and we paint the freehand design directly over, using oxides as above. The piece is then ready for the glaze firing. Depending on the firing, grass-ash glaze is white, grey or even brown.
