Terracotta

Terracotta (Italian for "baked earth") is fired clay, especially the porous, reddish-brown kind left unglazed. In roofing, the word is widely used to describe natural, unglazed clay tiles and their colour, though manufacturers also use it loosely for any red-brown tile, including concrete.

Firing and colour

Terracotta is typically fired at around 1,000 °C (1,830 °F); some historic and archaeological pieces were fired at temperatures as low as about 600 °C (1,112 °F). During firing, iron oxides in the clay react with oxygen to produce the familiar red; depending on the clay and kiln atmosphere, colours range from yellow, buff and orange to pink, grey and brown.

Porosity and frost

Ordinary terracotta is not watertight. Its porosity can be reduced by burnishing the surface before firing or by glazing, but glazing alone does not seal a porous body. Clay products exposed to freeze-thaw cycles and salts, including roof tiles in cold climates, are generally fired to or near vitrification so that they absorb little water. Low-fired terracotta can flake and crumble when absorbed water freezes.

In roofing

• Unglazed clay roof tiles of many profiles (plain, pantile, mission, Spanish and interlocking) are commonly called terracotta tiles.
• Terracotta-coloured concrete tiles imitate the look but obtain their colour from pigment, which can fade.
• Architectural terracotta, the moulded decorative facing used on buildings, had a major revival in the mid-19th century and, after the Great Chicago Fire of 1871 raised demand for fireproof materials, remained popular in North America into the early 1900s.

Weathering

Unglazed terracotta roofs weather gradually, often darkening and gaining lichens and mosses that many owners regard as part of their character. Because the colour runs through the whole tile, chips and wear do not reveal a different-coloured core.