REFRACTORY BRICKS

Refractory bricks are made out of a refractory ceramic. Its main properties are that it has a low thermal conductivity to save energy. These can support high temperatures(1650 °C) (3000 °F) so that is the main reason why it is often used in fireplace. They are weaker, but they are much lighter, easier to form, hard and insulate far better than dense bricks. People don´t usualy put the normal bricks because they cannot support the variation of temperatures easily.
They usually contain about 30-40% aluminium oxide, 50% silicon dioxide. For bricks of extreme refractory character, the aluminium oxide content can be as high as 50-80%.
This brick was invented in 1817 by William Harry, in Wales. Harry used the possibility of combining different materials with silica, this is how he invented the brick know has fire-proof brick. In 1822 its use in construction spread worldwide.