Glazing with glass

This is a little ceramic cup where different textures are intertwined. On the outside, the brownish color with shadows of iron, black and dark yellow gives a sense of roughness and brutality, further accentuated by the roughness under the touch. On the opposite, the interior of the cup is shinny with bright colors and very smooth under the skin. The cracks in the pale green give a sense of fragility. The transition between the two worlds starts at the middle of the cup. The whitish mat zone reminds us of the foam left by the waves on the shore of the ocean.

This cup has been made from stoneware with 10 to 20% of large grog. The grog is composed of millimetre to sub-millimetre fired clay particles and gives an inherent roughness to the piece. The pots made from this stoneware usually do not shrink during the drying and can be easily sculpted.

After a first firing, the bisque firing at 1000 °C, the cup is ready to be glazed. The bisque step is always required to avoid having water in the ceramic that could lead to cracks after applying the glaze. Also at this stage the cup has shrunk a bit and is less porous. 1000 °C or 950 °C are usual temperatures to which the glaze adheres well.

The glazing step followed by high temperature firing, sintering, at 1250 °C gives the color, reduces the porosity further and thus renders the ceramic waterproof. Commercial glazes are available with many colors and special effects. They look like paints that can be diluted to be sprayed or to dip the bisque. Inside this paint are oxide particles mixed with a polymer dissolved in water that gives the adequate viscosity. At around room temperature, the water from the glaze evaporates or is adsorbed in the bisque, leaving a deposit of oxide particles at the surface. Then, starting at 500 °C, the polymer phase, which is very little, roughly 5 % of the deposit, burns out. When the temperature further rises, the particles melt and cover the surface.

Here I used a commercial glaze for the brownish color. The parts that are very rough and with more reddish and darkish intensities appear mat because I did not put enough glaze to cover the rough surface of the clay. When it melted at 1250 °C, it could not form a thick shinny layer. Also I used the glaze as it was, no additional mixing. On purpose I left it non homogeneous, therefore some areas contain more of one chemistry of particles than others. This gives the surprise of unexpected shades and varying colors. I also believe that the composition of the stoneware interacts with the glaze since the same glaze gives different results depending on the surface on which it is painted.

The interior of the cup is covered by a thick layer of glass. For this, I broke a green glass bottle by filling it with water and putting in the freezer. After smashing the bits of glass with a hammer to reduce their size, I filled the cup with those chunks. Glass from wine or beer bottles typically melts around 1200 °C. That’s what happened. Since the interior of the cup was not flat, more melted glass accumulated on one area, giving the intense green color. The cracks formed during the cooling down. The fastest the cooling, the more the cracks. This depends on the oven. It seems the furnace I used cooled quite fast. When you take the pot out when it is still warm, you can also hear the “cling cling” from the glass cracking.

The making of a ceramic involves a lot of magic and surprise: the change of colors, the unexpected effects, the cracks, the roughness.

Touch, sounds and emotions in one object.


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