Earthenware

Earthenware is clay fired below vitrification, typically between 1000 and 1150 °C. The particles sinter enough to hold together permanently and not enough to close the pores, so the body absorbs water — commonly a tenth of its own weight and sometimes more.

That porosity is a limitation and, in several uses, the point. An unglazed water jar cools its contents by evaporation through the wall. A flowerpot lets a root ball breathe. A roof tile that has absorbed water and then frozen, on the other hand, is a roof tile that has spalled, which is why frost resistance is the property that decides whether a given earthenware is suitable outdoors.

It is softer, weaker and much easier to fire than stoneware, which is why it is the ceramic of every early culture that made pottery at all.

Formed by hand, on a wheel, in a press or by extrusion — brick and roof tile are extruded and wire-cut — dried carefully, and fired once or twice. The low firing temperature is what made it available before anybody could build a kiln that reached stoneware temperatures.

A glaze must be fitted to the body: because earthenware keeps absorbing moisture through its life, a glaze under compression stays intact and one under tension develops the fine cracks called crazing, which is a defect in tableware and a decorative effect in studio work.

Flowerpots, roof tiles, floor and wall tiles, drainage ware, and the brick that is a fired earthenware body by any other name. Tableware in the maiolica, faience and delftware traditions, where an opaque tin glaze hides the coloured body underneath. Sanitary ware historically, before vitreous china.

Architectural terracotta — moulded, fired facade elements — was a major nineteenth-century building material and is why a great many Victorian buildings have ornament that would have been impossible to carve.

The oldest fired ceramic there is. Figurines from Dolní Věstonice are around 29,000 years old and predate pottery vessels by roughly fifteen thousand years — people fired clay into shapes long before anybody used it to hold anything.

Vessel pottery appears independently in several places from around 20,000 years ago in East Asia and later elsewhere, and it is one of the standard markers archaeologists use precisely because it survives, breaks into recognisable pieces, and changes style faster than almost anything else people made.

Fired at lower temperatures than stoneware and porcelain, so its energy cost per piece is the lowest of the three. Inert, unrecyclable as ceramic, and crushed for aggregate or landfilled at end of life.

Brick is the case where the numbers matter most, because of the volumes involved: a brick has a substantial embodied energy and a service life measured in centuries, and reclaimed brick is one of the more successful construction reuse streams there is.