Most of our tiles and ceramic wares are made using the slip casting technique. Not many people are familiar with this process, so we thought we’d show you how we do it.
We start with a sketch of our idea. Clay shrinks as it dries so we have to make sure our drawing makes allowances for shrinkage by making it about 7-10% larger than we want the final product to be.
Using our design drawing as a guide we sculpt a model in clay. If there are several designs in the series we try to do them all at the same time to keep them consistent in size and pattern. When the model is finished it’s time to make the mold.
The model is placed on a smooth surface inside a hinged wooden form. All the cracks in the form are sealed with strips of clay and the inside is brushed with a mold release product. Plaster is carefully mixed to avoid bubbles and then poured slowly over the model to the desired depth.
When the plaster has hardened we remove the form and pull the clay model out of the plaster leaving a detailed imprint of the design. After cleaning, the mold is placed on wire shelving for 2-3 days until it is completely dry. Larger molds may take as long as a week to lose all their moisture.
Once the mold is thoroughly dry we are ready to pour the first casting. The molds are set out on the table and leveled. Liquid clay slip (about the consistency of melted ice cream) is poured carefully into the mold to the correct thickness. The dry plaster in the mold draws moisture from the liquid clay and the casting sets up in a few hours. When the clay is firm to the touch and shrinks away from the sides of the mold the casting is ready to remove. The mold is flipped over onto a piece of drywall and the tile or bowl drops out.
The casting is left on the drywall and is placed on wire shelving to dry. Small, thin items take 2-3 days to dry — larger, thicker castings may take twice as long. This slow drying period allows the clay to shrink gradually to avoid cracking.
Once dry, the casting (now referred to as greenware) can be cleaned: surfaces are smoothed, rough edges are trimmed and sanded and we make sure the tiles are straight, flat and square. Because the greenware is very fragile (it breaks with about the same pressure as a thin chocolate bar) this step has to be done with considerable care and patience.
The cleaned greenware is ready for its first firing and is loaded into the kiln. This first firing is called bisque firing and takes approximately 16 hours from start to finish. The clay reaches almost 2000 degrees and when finished firing is called bisqueware.
After bisque firing the tiles, bowls and other items are ready to glaze. Single glaze tiles usually require three coats of glaze – bowls and other 2-sided wares need three coats on each side. Handpainted items such as our Birds on a Vine tiles take several steps to complete and are finished with two coats of clear glaze.
This the most exciting part of the process – when you open the kiln and get to see what all the time and effort has produced. The glazed bisqueware is fired a second time to slightly lower temperatures. Sixteen to eighteen hours later the tiles and bowls and ornaments are cool enough to be unloaded from the kiln.