Stoneware

Stoneware is clay fired hot enough — roughly 1200 to 1300 °C — that part of the body melts and fills the spaces between the remaining particles. The word for that is vitrification, and it is the single distinction that separates stoneware from earthenware.

The consequence is that stoneware holds water without a glaze. It absorbs perhaps one per cent of its weight against earthenware's ten or more, so it does not seep, does not harbour bacteria in the body, and survives freezing without the absorbed water splitting it. A glaze on stoneware is decoration and a glaze on earthenware is the waterproofing.

It is also much stronger, and it is the reason the everyday tableware of most of the world is stoneware or porcelain rather than the earthenware that preceded them.

Thrown, jiggered, slip-cast or pressed, dried, and fired once or twice. The clay has to be one that survives the temperature: an earthenware clay put in a stoneware firing slumps and bloats.

Salt glazing, which is the traditional finish, is a genuinely unusual process — common salt thrown into the kiln at peak temperature vaporises, reacts with the silica in the clay surface and forms a glassy skin in place. Its characteristic orange-peel texture is the reaction happening unevenly.

Tableware, cookware and ovenware. Drainage and sewer pipe, historically in enormous quantity, where vitrification and chemical resistance both matter. Chemical and food-industry vessels. Studio pottery, where it is the standard body.

And the storage jars, bottles and tankards of the early modern period, which is what most surviving pre-industrial stoneware is.

Made in China from the Shang dynasty, and in Europe from the Rhineland in the medieval period — the German salt-glazed stoneware of Siegburg and Cologne was traded across the continent and is what the English word 'stone jug' refers to.

England reached it late, and John Dwight's Fulham patent of 1672 is the usual marker. The Staffordshire industry that followed is where Wedgwood and the modern ceramic industry come from.

Fired ceramics cannot be unfired, so a broken piece has no route back to the material it was. It is crushed as aggregate or landfilled, and it is inert in both.

The impact is the firing, which is a high-temperature process held for hours and is the great majority of a ceramic's energy cost. Against that, a stoneware vessel that lasts decades is doing better than most of the alternatives it would displace.