Insulating Fire Brick (IFB) are low density brick capable of withstanding high temperatures. They are made in a range of grades, varying in maximum use temperature, insulating efficiency and density; the former affecting heat loss from the kiln and the latter affecting heat storage.

They have a porous structure. One method of manufacture is to mix wood sawdust with refractory clay and then fire it so that the wood burns out, leaving voids. Another is to add calcium carbonate (chalk, whiting) which turns to gas and creates voids. These techniques lower the mass of the brick and reduces the amount of heat it will absorb during a firing.  

Makers and grades are denoted by alpha symbols and the maximum recommended use temperature in hundreds of degrees F. For example K23, where the K denotes Thermal Ceramics and 23 the recommended maximum use temperature of 2300°F.  

Grades of IFB

The lowest grade normally stocked, and that used for glass and pottery kilns, are rated at 1260°C (2300°F).  They are the lightest and the best insulators with the lowest heat storage.

It matters little that potters can fire to above 1300°C (2370°F) as the 2300°F is an arbitrary figure; just as is the 1260°C. In this context, neither are deserving of strict observance.

No IFB are made in Australia, but they are imported from a number of overseas makers.

For some years there has been a worldwide shortage of IFB, so Australian users have had to take what they can get.

The size of most IFB available in Australia is 230mm x 115mm x 76mm (9 x 4 ½ x 3 inch). They are solid and smooth on all faces; having no lightening holes nor ‘frogs’ for mortar retention as do conventional building bricks.

A size common in USA is 9” x 4 ½” x 2 ½”. It is seen in Australia only in the walls of American made top loading glass kilns.  

The bricks are sometimes packed in small cartons of 8 or 10 pieces. Those in a carton will usually be uniform in size but there can be variability between cartons.

At other times they will be shipped in bulk, so some damage to edges and corners can occur.