coarser particles in the body at their points of contact with each other. In the case of porcelain, where higher firing-temperatures are used, the physical, chemical and mineralogical properties of the constituents in the body are greatly altered. In all cases, the reason for firing is to permanently harden the wares, and the firing regime must be appropriate to the materials used. Temperature As a rough guide, modern earthenwares are normally fired at temperatures in the range of about 1,000°C (1,830 °F) to ; stonewares at between about to ; and porcelains at between about to . Historically, reaching high temperatures was a long-lasting challenge, and earthenware can be fired effectively as low as 600°C, achievable in primitive pit firing. Atmosphere The atmosphere within a kiln during firing can affect the appearance of the body and glaze. Key to this is the differing colours of the various oxides of iron, such as iron(III) oxide (also known as ferric oxide or Fe2O3) which is associated with brown-red colours, whilst iron(II) oxide (also known as ferrous oxide or FeO) is associated with much darker colours, including black. The oxygen concentration in the kiln influences the type, and relative proportions, of these iron oxides in fired the body and glaze: for example, where there is a lack of oxygen during firing the associated carbon monoxide (CO) will readily react with oxygen in Fe2O3 in the raw materials and cause it to be reduced to FeO. An oxygen deficient condition, called a reducing atmosphere, is generated by preventing the complete combustion of the kiln fuel; this is achieved by deliberately restricting the supply of air or by supplying an excess of fuel. Methods Firing pottery can be done using a variety of methods, with a kiln being the usual firing method. Both the maximum temperature and the duration of firing influences the final characteristics of the ceramic. Thus, the maximum temperature within a kiln is often held constant for a period of time to soak the wares to produce the maturity required in the body of the wares. Kilns may be heated by burning combustible materials, such as wood, coal and gas, or by electricity. The use of microwave energy has been investigated. When used as fuels, coal and wood can introduce smoke, soot and ash into the kiln which can affect the appearance of unprotected wares. For this reason, wares fired in wood-