Page 33 - Curriculum Visions Dynamic Book

To be a useful refractory, suitable materials must not only have the properties just mentioned, but they must also resist wear (because hot liquids may be churning around against them), and they must resist the tendency to crack when temperatures change rapidly.
Refractories get some of these properties from the way they are fired. Refractories are meant to have quite a coarse texture compared, say, to whitewares and even bricks.
Grains are used in refractories that may be millimetres
across. Such large grains would be unacceptable in any
other kind of ceramic. The large filler grains tend to result
in larger pores than in other ceramics. Both of these features make the ceramic less likely to crack when heated or cooled. On the down side, they also make refractories less strong than normal bricks, and so they are not used for building, just for lining furnaces.
One way of preventing the refractory from reacting with the liquid it holds is to make it match the acidity of the liquid. Refractories can be made acid by adding to them sand and fireclay. They can be made alkaline by adding dolomite rock (magnesium carbonate).
(Above) You can
see refractory bricks in place around the entrance to this glass- blower’s furnace.
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