Bricks are made of clay, sand and lime, and are either hand molded or extruded by machines into regularly sized units. They are then air dried and fired in kilns. The chemical content of the clay, lime and other additives affects the brick’s color, as does the temperature and atmosphere of the kiln. The outer surface of a brick, known as the fireskin, is a hard, dense surface that protects the softer inner brick body from deterioration.
Bricks can be made with a wide range of surface textures, including smooth, sanded, velour, wire cut and striated, amongst others. In addition, different colors can be produced by applying minerals to the brick surface or altering the conditions in the kiln. For example, manganese or other minerals can be speckled on bricks to create ‘iron spot’ brick, whereas flashing the brick in the kiln produces various color patches on the surface of the brick. Glazes may be applied to bricks prior to firing to produce a strong, shiny glazed surface on the outside of the brick. Glazed bricks come in a range of colors, but white, brown and green are common. Clinker bricks, which are irregularly shaped dark brown or black bricks, are produced by burning the bricks in high-temperature kilns.
The size of historic brick varies widely from long and thin Roman bricks to tall and fat jumbo bricks. Most bricks produced in the mid- to late-nineteenth century and early-twentieth century are Standard bricks, which are slightly longer than modern Modular bricks. Historically, bricks were solid units but late-nineteenth and early-twentieth century bricks were sometimes imprinted on the sides with a manufacturer’s name or symbol, known as a frog. Solid bricks are heavier than modern hollow or perforated bricks (which have a varying amount of holes in the center), but were sometimes carved into shapes or decorations.