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What is different about fire bricks?

Although looking similar to standard bricks, vermiculite fire bricks or refractory bricks are different in composition. Fire bricks are manufactured to handle high temperatures and differing atmospheres.

These types of bricks are used to construct ceramic studio kilns and furnaces as standard bricks would crack and crumble, with potentially disastrous results. Due to their composition also has exceptional insulating properties, meaning a lower cost of operating over time, despite the higher initial cost.

Even in a domestic environment, such as the rear of a fireplace should only be lined with fire bricks. Fire bricks are extremely difficult to cut. They form a major structural part of kilns and furnaces, and they come in many different shapes to enable a perfect fit.

Hard or Soft Brick

There are two types of fire brick, hard and soft. Hard bricks are very dense and are used in areas for structural integrity and where there is a direct flame.

Soft bricks are lighter and made from refractory clay that contains combustible materials. The combustible material burns out as these are fired, leaving air pockets in a matrix. These air pockets provide the insulating properties of the brick. Soft bricks are sometimes known as Insulating Fire Bricks (IBFs). They can take temperatures of around 1000°c to 1600°c, making them ideal for constructing electric kilns.

Fire Brick Grades

The main ingredient of fire bricks is fire clay. Fire clay contains mainly aluminium oxide and silica, both of which can withstand very high temperatures. Hard bricks come in several grades based on their composition. High aluminium oxide content starts at around 50% and can rise to 98% for bricks used in specialist processes. The higher the amount of aluminium oxide, the higher the price.

The data below will explain in more detail to help you understand how the aluminium oxide (alumina) affects the fire bricks.

Low-duty Fire Bricks:

Typically rated to 1000°c maximum service temperature. Primarily used for fireplace chimneys and contains 24–26% alumina.

Medium Duty Fire Bricks:

Temperature rating to 1500°c maximum service temperature. Uses include backup linings, lower-temperature ceramic kilns, chimneys, and 34–38% alumina.

High Duty Fire Bricks:

Temperature rating to 1500°c maximum in purely heat service. Certain atmospheres can reduce this temperature rating by several hundred degrees and contains 36–40% alumina. Uses include boilers, ceramic kilns, chimneys and backup linings.

Super Duty:

Temperature rating to 1600°c in pure heat service. Certain atmospheres can reduce this temperature rating by several hundred degrees. These are used in similar uses as high Duty but where higher temperatures are involved. Super Duty bricks contain 40–44% alumina.

High-Fired Super Duty:

Temperature rating around 1700°c. A higher burned version of super duty firebrick is designed to lower the porosity. The higher burning increases physical strength and improves resistance to alkali attack and carbon monoxide disintegration. High-Fired Super Duty bricks contain 40–44% alumina.

In conclusion, fire bricks are significantly different to standard building bricks. Therefore standard bricks should never be used in a high-temperature environment. If you are unsure, speak to a specialist company for advice on which bricks would be most suitable.

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