Monday, January 3, 2011

Clay - Jonathan Keep

While it is early days in my ventures with ceramic printing the impression I am getting is that the clay qualities desirable in more traditions pottery techniques hold true for printing with clay too. So far I have tried porcelain clay, then a blended buff stoneware clay, a ball clay, a red terracotta clay and a black firing clay. The general rule of thumb with clay is that the whiter it is the less plastic and malleable it is. What is known as ‘short’ clay. The darker the clay, the more sticky it is and often the more plastic and able to bend before breaking. The converse is, the whiter the clay the higher the temperature it can be fired to before distorting. So porcelain is higher fired and stronger than say red earthenware red clay. There is always a payoff and that is why most clay bodies are a blend of a number of materials.

Another factor I expect will influence our choice of clay for printing will be particle size. Short clay tends to have large particles, sticky clay small particles. But then fine sticky clay dries more slowly than white less plastic clay. To support the weight of the clay as it builds up the print must dry quickly but to unsure the layers stick together and bend both qualities are desirable. To further complicate the particle size issue a range of larger particles give structure to clay wall and gives rigidity to a soft clay structure. So often crushed, already fired clay, or what potters call ‘grog’ is added to a clay body. As the ceramic printing head/syringe I am using has a 1.5 mm nozzle I have used a 80# sieved grog, adding about 10% in dry weight. Whether grog is helpful in printing clay only more experience will tell. My gut feeling has been to include grog from the start to help give a bit more structure to the soft printed clay but also as clay powder is very fine the grog gives a bit of ‘tooth’ when mixing the powder with water. Different clays naturally contain slightly different proportions of clay to water for the same consistency. As a guide I have been printing with mixes of 2.5 – 3 parts dry clay mix to 1 part water.

‘Volcanoes’ – ave size 9 x 9 cm (Experiments in modelling with ‘sculpt mode’ in the 3d program Blender)

• Porcelain clay printed well but if it is too dry it goes ‘cheese’ and the extrusion breaks apart, and too wet it cannot support itself and collapses, so consistency is vital. Porcelain prints are the only objects I have glazed so far and the print layering texture looks ok through the glaze.
• The two buff clay were best to print with. The stoneware clay is gritty anyway so I did not add grog. The other buff clay is a ball clay that offers plasticity without being too sticky. The ball clay print quality is finer and more pleasant than the stoneware clay.
• The red clay printed ok as well but depending on what type of glazes you want to use and what temperature you are going to fire to a red earthenware clay might not be desirable.
• The black clay, although it looks great fired did not print well and was very difficult to work with. In preparation the surface dries easily and it is difficult to avoid small lumps in the mix that block the printing head. It was difficult to get the slurry consistency correct. It went from being to dry for the compressor to push it through the syringe to being too wet to stand up during printing with very little water added. I did get a reasonable dark brown ‘ant hill’ print out of a 50/50 red clay, black clay mix.

‘Ant hill’ forms – ave height 13 cm (Experiments in modelling with ‘metaballs’ in the 3d program Blender)

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