The Salty Potters

Firing Temperatures

Low Fire = Bisque Fire = Cones 06, 05, 04

Mid Fire = Glaze Fire = Cones 5, 6

High Fire = Glaze Fire = Cone 10 (we do not fire at high temperature)

Clay

We use Laguna B-Mix cone 5 white clay. This means our clay must be fired to at least cone 5 to be vitrified (non-porous). To be considered food safe, your pottery must be vitrified; otherwise, it is still quite porous and can absorb moisture and bacteria. Our clay is not vitrified (still porous) at low temperature firings, so any pottery intended to hold or store food, should be fired at mid temperature.

Glaze

All of the glazes we stock are food safe at either low or mid temperature. Arguably, putting a food safe glaze on our clay and firing at low temperature would give you a food safe surface, if food were to only come in contact with the glazed surfaces. This is not recommended. We recommend only firing glazed items at low temperature if the piece is strictly decorative.

See each glaze bottle and other available information to determine the acceptable firing temperatures for the individual glaze.

How is temperature measured?

A kiln contains thermocouples which measure the temperature in the kiln at any given moment. Our kiln has three thermocouples; one near the bottom, one in the middle and one near the top. These thermocouples are used by the controller to follow the assigned firing program, turning the heat elements on and off at specific temperatures.

 Pyrometric cones are pyramid shaped ceramic indicators of heat work. Heat work is the combination of temperature and time which are what matter for maturing clay and glaze. These pyrometric cones (witness cones) are placed in a kiln at various levels to indicate heat work during the firing of a kiln. The melting of the cones are the accurate indicators of what really happened heat-wise in the kiln. Using a combination of temperature and time held at the top temperature, we attempt to reach our desired heat work (cone melting) for the current firing.

The photo below shows the three witness cones after a mid temperature firing. The center cone is the target and that is the perfect bend to indicate the heat work matched our target (in this case, cone 5). The left cone is for cone 6 and the right for cone 4. The bending shows that cone 4 was exceeded, cone 5 was exactly as desired and we did not reach cone 6 in heat work. In a typical firing, three sets of witness cones are placed in the kiln at approximately the same level of each thermocouple. This will indicate any differences in the heat work in different regions of the kiln.