Ceramics class at Caltech is held in the evening at the Polytechnic High School across the street. Aside from various pottery materials and tools, they have 2 kilns: one for firing them and one for setting the glaze. These are high fire kilns, which means that the finished pieces will be food safe, microwave safe, and even oven safe!
Glazing is the last step after our clay pieces have went through the first firing. After this first firing, the clay will have turned slightly pink and make a tinny sound when hit. Unfired clay pieces that have just dried are more of a light brown and make a thudding sound when tapped. Today's class is devoted to learning how to glaze ceramics.
Step 1: It's important to paint on a layer of wax on any part of the piece that'll touch the surface of the kiln rack, as well as a 1/4 in rim around it. The instructor also noted that wax brushes should be rinsed with hot water right away to prevent ruining the brushes. Glaze basically creates a layer of thin glass on the pot, and gives it additional structural integrity. The wax prevents glaze from dripping onto the kiln and melding the pot to the tray.
Step 2: Choose which glaze you want. We have over 20 different tubs of glazes to choose from, and different combinations of glazes can give drastically different results. The instructor recommends around 2 - 3 layers of glaze at the maximum to prevent it from being too thick and runny in the kiln. In addition, the order in which you do the different colors also makes a difference.
Step 3: Before you actually dip the piece in the glaze, you have to first mix it with a giant mixer. That's because the glaze is an aqueaous suspension of metal oxides (which give it color), silica (which forms the glass layer), as well as other compounds.
Step 5: Metal oxides are available for painting accents because they are more expensive. We have rutile (mineral with mainly titanium oxide), copper carbonate, and iron oxide. These oxides react differently with different glazes to produce different colors. The instructor mentions that it's best to dab the oxides on carefully, since parts that have too much will become metallic and black.
Results and Discussion
Many of the glazes look very pale, but came out extremely vibrant after firing. The goal for this mug was to achieve a speckled turquoise, but it ended up being very dark. It's likely that it was dipped in the wrong order.