Wedge test piece

Culture & Context

Glass and ceramics have many related material qualities and are processed in similar ways. Chemically they are alike however structurally they are very different, which creates compatibility problems when they are combined in a hot state.

Through controlled processing, material properties can alter when each is partially converted into the other. It is recognised by artists in the field of studio ceramics that porcelain can partially convert into a glassy form when high fired to create a translucent material. Likewise it is recognised in the field of industrial engineering that glass can partially convert into a ceramic form when processed in a controlled way to create a glass-ceramic material; however this material is not used by practitioners and would be difficult to develop in a studio environment. Renowned for being difficult to combine, glass and ceramics are usually regarded as being incompatible due to differences in structure, however through my doctoral research I managed to combine them in a hot state to create a unique series of tests and artworks. A compatible mix was developed between glass and ceramics, through the use of a lead crystal (Gaffer Crystal) and a bone china that had quartz added to it at the slip stage, this was then processed into a high fired ceramic aggregate that can be mixed with glass without creating any stresses or cracks in the final test.

Process & Production

The model for this piece was made using plaster, which was cast into a U-bend former, the plaster was then cut to size and shape and the surface finished off by rubbing it on a sheet of wet glass to smooth of the surface, the edges were shaped using sandpaper and were smoothed with a slight bevel. The plaster model was then laid onto the table and set up for casting, gelflex (a type of silicon rubber) was poured over the form and allowed to cool down and set. This is the former for the wax which was poured into the cavity left behind by the original plaster model. Hot wax was poured into the rubber mould and allowed to cool. The wax is taken out of the mould and any over casts or flaws removed by hand. The wax model is used to measure the volume of the glass that is needed for the cast. The wax model is immersed in cold water and the amount of water it displaces is noted down, it tells me how much glass to use. The wax model is then set on a table and the mould investment is covered over the wax model; the wax model is covered in mould material. The wax model is then steamed out of the mould using a wallpaper steamer, the hose is placed into the mould and the wax heats up and runs out of the mould. Once all of the water has been taken out the mould cavity is cleaned by pouring boiling hot water into the cavity until all of the wax is completely removed. The cavity left by the steamed out model is then ready to be fired with glass.

Recipe Details

During the kiln-casting the glass flows into the cavity and fills up the mould. The cast is slowly cooled following an annealing schedule which is pre-programmed into the kiln. Once the kiln has cooled to below 50 degree the door can be opened and the glass filled mould taken out.

Once fully cooled for a further 24hrs the glass can be removed from the mould, by carefully breaking off the fired mould. The glass is then cleaned up with water and can then be cold worked, the over cast at the base which is where the glass flows into the mould needs to be cut off on the diamond saw, the base and sides of the piece is then ground flat on the flat bed grinding wheel and the edges softened and bevelled on the linisher. The surface is then sand blasted all over to remove any flaws.

In terms of materials a bone china slip was prepared, to which quartz was added to the mix, which was prepared in a 1:1 ratio (500g of quartz to 500ml of slip). This was then slip cast into flat sheets of ceramic, dried out and then high fired, once cooled it was crushed into ceramic aggregate chips. This was then mixed with the gaffer glass frit and packed into the mould ready for kiln casting. Gaffer casting glass colour Steel Blue #253 was used. The mould was high fired to 825°C in one casting in a single investment mould made from plaster and malachite, mixed in a 1:1 ratio (250g plaster, 250g malachite & 250ml water)

Measure the glass needed by displacing the wax model in water, a mix of lead crystal and ceramic aggregate is used. Use this displacement technique to measure out the glass and ceramic that is needed for the model size. Wash and place the measured glass and ceramic aggregate into the prepared mould (made from a mixture of plaster and malachite). Put the mould and the measured glass into the kiln and fire to around 825°C.

Prepare the mould, measure equal parts of plaster and malachite with water to create a mould mix which can then be used to cover the model. Carefully apply a thin spray of hairspray to the wax model, then apply a thin face coat (first coat) of the mould mix to the model, allow to dry. Continue to add layers of mould mix in a slightly thicker consistency to the model, continue into the mould has an equal thickness of around 6-8cm all over. Allow the mould to dry, flatten off the base and put into the drying cupboard overnight. Once the mould has dried the wax can be steamed out and the mould is ready for casting.