From the
brick primer:
Quote:
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Insulating fire brick. These light-weight refractory bricks are designed to stop heat, and as such have low conduction and low heating holding capacity. They are often used to insulate industrial equipment. A typical insulating fire brick weighs about 2 lbs, compared with an 8 lb light duty fire brick.
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They are very light and very soft. You can dig your fingernail into one.