Glazing the tesserae: I spray on the glaze, rather than brushing or dipping, which led to very inconsistent results. The spray nozzle attaches to a standard one pint jar; I have dozens of jars with a different color glaze in each. The jars make it easy to change color. The tesserae are sprayed in large groups, sitting on a lineoleum tile (this makes it easier to handle them.) They get a total of three to five coats from different spray angles to a thickness of about 0.2 to 0.4 mm). Glaze thickness can be tested in the overspray area. Viscosity is critical; the glaze must have just enough water to go through the spray orifice easily. This determination has been more a matter of experience; viscosity measurements have not helped much. Between colors I wash off the sprayer with a garden hose, shooting some water through the orifice.
I use cone 5 and cone 10 clays; this results in tesserae which can last outdoors for decades.
Glazes: I started out with premixed Spectrum products at cone 5 or cone 10. The colors I chose are similar to what you find in a watercolor palette. These glazes are opaque; they don't change color radically as they turn into glass. it is possible to mix these colors to achieve intermediate shades. By now I use a lot premixed glaze in powder form as well.
Making clay tesserae (tiles)
The picture shows clay (Laguna B mix with grog) rolled out to a thickness of about 0.19 inch (5 mm), similar to the thickness of bathroom tile. The cookie cutter elements are 3/4 inch copper pipe couplers which I have flattened slightly to be semi-square. The clay sits on a batt, in this case a linoleum floor tile. ( I wouldn't want to handle the clay slab without a batt.) As you can see, the idea is to make tesserae by the hundred, not just one at a time.
Other cookie cutters, in this case made from recycled metal strapping. These make a variety of interesting shapes. .
CERAMICS + MOSAICS
I tried out my color palette by doing the same sparrow over and over.
I began to experiment with black and white, with the idea of tesserae as pixels (as you would see in an over-magnified digital image),and with two colors of grout in the same mosaic.
Kiln: an electric Skutt KM818. It has five shelves with 5 cm spacers. The total shelf area is 0.75 square meters, enough for several thousand small tiles. In making tesserae, I needed lots of shelf space. To load the kiln, I slide them off the linoleum tile onto the kiln shelf.
As the sheets of Hardibacker get larger, I use steel for reinforcement. This perimeter of this framework is Lshaped steel, welded together. The horizontal braces are "hat channel', a stucco worker's product. Hardibacker is attached using construction adhesive and pop rivets.
Substrates: I generally use 1/4 inch thick Hardibacker as a substrate for flat 2D mosaics. Tile adhesives adhere well to this material. Hardibacker is not structural and needs reinforcement. I glue smaller pieces of Hardibacker.
The picture at right shows a framework of 1x2 pine, attached to the Hardibacker with construction adhesive and rust-resistance screws. The wood is lap-jointed; lighter than a solid sheet of plywood. This substrate already has a picture frame and French hanger attached.
On the left is a plaster of Paris mold. It replicates 18 mm metal beads, giving the clay a lot of detail. By now I have dozens of plaster molds.
Below are some finished bead replicas.
Mosaics Early on, I created mosaics by taking whole tesserae from my 'library" of 3 sizes and 30 colors and gluing them down one tile at a time. I use a standard thinset, which comes in powder form, and mix up small batches as I need them. This technique required very little cutting and fitting. Composition/assembly were very fast. The large tesserae forced me to create large mosaics in order to have any detail. I started creating more shapes, like the linear tiles in this pomegranate.