Hand-made brick with 'frog' from Lindsey, Lincolnshire, early 19th century.
© Science Museum / Science & Society Picture Library
Between 1801 and 1911, the population of Britain trebled to 45 million. In addition, the proportion of people living in towns increased from 20 to 80%. This rapid urbanisation made huge demands on the brick-making industry; by the middle of the 19th century, 500 million bricks were being produced every year in the London area alone. By 1900 virtually all bricks were machine-made. The indentation into the surface, or 'frog', on a brick served as a key for mortar and to reduce the brick's weight.