This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
163. Brick may be called an artificial stone, manufactured in small pieces for convenience in laying. The principal ingredients in brick are clay and protoxide of iron. Other substances that form part of ordinary clay either do no good or are absolutely harmful, carbonate of lime, in any large quantity, rendering the clay absolutely unfit for making brick. Sand or silica should not exist in any excessive quantity, as an excess of sand renders the brick too brittle and destroys cohesion. Twenty-five per cent, of sand is considered a good proportion.
The protoxide of iron in the brick clay causes the red color in the brick after burning, the coloring varying with the proportion of iron. With more intense heat, the brick, if slightly fusible, may be vitrified externally and become a sort of greenish blue. The presence of magnesia or a small percentage of lime in the clay will change the red color, produced by iron, to a cream or buff. The mottled, or Pom-peian brick, now largely used, have the mottled effect produced by the use of coloring matter in the clay, or by mixing clays of a different chemical composition.