Page 30 - Curriculum Visions Dynamic Book

Particle sizes of the filler used in bricks made from shale are greater than for whitewares, so the final texture is coarser.
Fire clays are those that can be used in high temperatures. They are clays with fewer impurities than shales or surface clays, and they have more uniform chemical and physical properties.
These clays are unlike metals in that they soften slowly and melt or fuse gradually when heated. That is why firing all ceramics in kilns takes many hours.
Firing bricks
Most bricks are fired in continuous production tunnel kilns, except when they are made on a small scale. Firing temperatures for bricks are typically 1,000°C
to 1,100°C lower than for whitewares. Only the edges of the clays turn to glass at these lower temperatures, and the interlocking needles of glass produced hold
the sand in place. However, these temperatures, and
the length of time of firing are critical. Without proper firing, the bricks would not have the strength they need, and they would crumble under the weight of overlying bricks or break up if wet and then subjected to freezing temperatures. Bricks that are fired for too long are very strong; but too much glass has formed in them, and so they are more brittle and likely to crack under load.
Because so little clay turns to glass in brickmaking, there is little opportunity for the glass to fill in the natural pores in the mixture. Bricks are therefore
quite porous (have many small holes) and reasonably permeable (let water seep through). That is why bricks used on the outside of houses are generally protected by the overhangs of roofs (eaves) of buildings. The eaves stick out to keep the walls below mostly dry. Furthermore, a space is usually left between the outer brickwork and the inner building, and a waterproof lining sheet covers the inner framework. In this way water seeping into the brick does not get into the walls of the rooms.
The porosity caused by a mixture of large and small particles does have major advantages. Its
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