Kilns and Firing:
Bisque Firing – Preliminary firing to sufficiently harden the ware for glazing. Usually fired to cone 010 to
cone 06 (1830°F).
Glaze Firing – Firing cycle to the temperature at t which the glaze materials will melt to for a glass-like
surface on the clay. Usually the point of maximum body maturity (Cone 10 - 2380°F)
Changes is the Clay – Between 350°F and 400°F all atmospheric moisture has left the clay body. Between
950F and 1300F all chemically combines water and fuses from organic material leave the clay.
(Considerable shrinkage occurs during this time-called the water smoking period).
Mullite Crystals- Between 1750°F and 1850°F needle-like crystals of alumina-silica called mullite begin to
form. These crystals give strength to the clay. As the temperature increases more free silica forms a
glass around the mullite crystals making the clay ever stringer and fully mature. Different clay bodies
achieve maturity at different temperatures. The change in the silica or quartz during firing is referred to
as quartz inversion.
Oxidation firing – A firing in which the kiln chamber retains an ample supply of oxygen keeping the
atmosphere clear. This means that here has been complete combustion of flue. Electric kilns always give
an oxidizing fire.
Reduction Firing – A firing characterized by incomplete combustion of fuel producing carbon monoxide
which combines with oxygen from the clay body and glaze to produce carbon dioxide. This change
affects the color of both the glaze and the clay, (Example: sang-de-beouf, celadon). The atmosphere in a
reducing kiln is smoky and cloudy.
Pyrometric Cones – Small triangular cones made of ceramic materials that are compounded to bend and
melt at specific temperatures enabling the potter to determine when the firing is complete. Measure
the work-hear ratio of the effects of time and temperature on the clay and glaze.
Updraft Kiln – Heat introduced into the bottom of the kiln chamber allowing the heat to coarse through
the ware and escape through a hole in the top.
Downdraft Kiln – Hear introduced at both sides of the kiln, deflected by the bag wall to the top (arch) of
the kiln, and then drawn back down to the flue, exiting through the chimney. The circulation of heat is
accomplished by a strong draft created by a tall chimney.
Neutral Firing – A theoretical firing which is neither oxidization nor reduction; actually obtained only in
proactive by a slight alteration between oxidation and reduction.
Single Firing – A firing cycle in which the normal bisque and glaze firings are combined. Advantages are a
great saving of fuel and labor. Disadvantages include danger of cracking and fragile nature of the ware.
Pyrometer – And instrument using a thermocouple to measure hear in degrees of Fahrenheit or
Centigrade in a kiln.
Quartz Inversion – Changes in the form of crystalline quartz (silica) due to temperature. Hardness in the
clay body is achieved by the formation of mullite crystals giving strength to the clay.
Heat – Quantity of heat energy released, measured in BTUs or Calories.
Temperature – Intensity of heat measured in degrees: Fahrenheit or Centigrade.
Vitreous – Pertaining to the hard, glassy and nonabsorbent quality of glaze or clay. Heat in necessary to
render either vitreous.
Refractory – The quality of resisting the effects of high temperatures. Also materials, high in alumina and
silica that are used for making kiln insulation muffle and kiln furniture.
Saggar – Round box-like containers of fire-clay used in kilns to protect the glaze from combustion gases
and flying ash or debris.
Water Smoking – Initial phase of the firing cycle during which most of the chemically combines water
and gases from organic material leaves the clay, (950°F – 1300°F)
Firebox – Combustion chamber of a gas, oil or wood fired kiln, usually directly below the Kiln Chamber.
Flue – Passageway through which the heating gases pass from the chamber of the kiln to the chimney.
Kiln Furniture – Refractory shelves and posts upon which ceramic ware is placed while being fired in the
kiln.
Kiln Wash – Protective coating of refractory materials (usually half flint and half kaolin) applied to the
surface of the shelves and kiln floor to prevent excess glaze from fusing the ware to the shelves. There
is different kiln wash for the soda kiln and the regular gas kilns- do not mix the two!
Target Cone – desired cone you want to fire to
Guard Cone – one below target cone
Safety Cone – one above target cone