Glaze firing pottery allows for many different techniques and approaches, as well as a wide range of temperatures. The approach most widely used at JoVic Pottery is based on oxidation of mid-range stoneware pottery. With more than 30 years of testing, we’ve developed a number of delightful, labour-intensive, approaches here. We do not use any commercial glazes on our work. As for the firings, we prefer the use of an electric kiln and fire our pottery to what is commonly called cone 6 (approximately 1225º C), basing our final results on the use of pyrometric cones. These pyrometric devices are used to gauge heatwork during the firing. They provide the potter with a visual clue and help determine shut-off since they deform at specific temperatures. The cones are generally arranged in a series of three, with the guard cone deforming first indicating that things are getting close, the middle one deforming at shut-off time, and the final cone indicating an over-firing (believe me, it happens). With kiln temperature varying throughout different locations in the kiln (depending on how well it’s packed and stacked), the cone sets are usually placed in three different areas, each visible via a peephole. Potters sometimes have to arrange one part of a kiln to begin cooling, while another must be kept at full heat to ensure maturity of the glaze.
Glaze detail. Photo Credit: Edward McCrea
Ceramic glazes generally contain silica to form glass, in combination with a mixture of metal oxides such as sodium, potassium and calcium which act as a flux and allow the glaze to melt at a particular temperature. alumina (usually from added clay) to stiffen the glaze and prevent it from running off the piece, colorants such as iron oxide, copper carbonate or cobalt carbonate, and sometimes opacifiers such as tin oxide or zirconium oxide. It definitely requires the potter to become familiar with each of the ingredients being used… we may not have been overly interested in the subject during our school years, but the craft of glazing soon turns some of us into chemists.
Platter Detail. Photo by Edward McCrea.
At JoVic Pottery, we use many approaches to glazing our work. In some cases we start with an engobe fired on during the initial bisque stage. Pottery is not matured (turned into stoneware) during that initial firing; rather it remains porous enough for us to apply glaze in liquid form prior to final firings. In the case of the platter detail shown here, the engobed and carved apron of the platter was waxed over before applying glazes. On this piece there are multiple layers of glaze applied at varying thicknesses to create depth and colour.
All of our functional stoneware pottery is free of lead or other minerals and oxides that run the risk of leaching into food, and it can be safely placed in your dishwasher too. There are, however, some truly decorative glazes used on our artworks. They can contain high amounts of manganese, lithium or cobalt. But don’t worry, we’ll never risk your health, and we minimize ours by wearing masks when we concoct these gorgeous finishes that we know will never come into contact with anything that will leach them. Visitors to our studio are often surprised by the number of containers with glaze ingredients sitting on our shelves. We always welcome their questions, but we don’t share our recipes. The countless hours we invest in glaze development will ensure we keep these secret.
