Bricks when they are kilned fired, expand and shrink in the firing process. This expansion and contraction are influenced by many factors, such as variation in shale and clay deposits, temperature, variations in pack configurations and their position in the kiln.
Therefore variation in brick sizes is very common and quite natural, particularly if your bricks are a mixture of colour and tones. Thus it is recommended that you allow for this variation in brick sizes in your choice of bricks. Sand stocks and blends tend to have a greater variation than straight coloured bricks
Bricks of distinction made in the traditional way