Insulating fire brick (IFB)

What are they?

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.

For small quantities,see "Refractories" in the Yellow Pages.

Joining bricks

Conventional building practice is to have wide, 10mm (3/8”) bed and perpendicular joints between bricks, so that the top edges describe neat straight lines and the deep mortar joint can accommodate bricks of different height.

This is not the case with refractory brickwork. There, the mortar joint is kept to the absolute minimum; with no mortar at all being preferred.

This is seen in the walls of American multi sided kilns, where the maker is able to machine each brick to an identical size and hold them together with stainless steel bands without mortar.

Of course, flat areas such as floors and lids do not allow this. Even with multi sided panels with SS clamping bands, there is the risk that individual bricks could work loose; so a mortar join of minimum thickness is used.

Mortar

Mortars used for refractory work are different from those  used in domestic and commercial construction.

Those used in kiln construction are called ‘airset’ mortars. They are premixed, usually in sealed steel pails of about 15kg, are of a darkish colour with a reasonably fine texture. They have a shelf life of up to 9 months if kept sealed.

For information on their use, and instructions for making your own patching mortar, see 61. Mortars and coatings.

Shaping IFB

All 2300 grade IFB are easily sawn or machined using handsaw or sawbench and can be drilled with metal drill bits or cut with wood routers. Saws with toughened tips and other tools with tungsten carbide tips will give longer life. Do NOT use a hammer drill.

The brick dust contains silica and crystobalite, so a face mask or respirator should be worn.

Additional information on using IFB and on making repairs to brick linings is being prepared.  Watch NEWS for more.