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A bellows or pair of bellows is a device constructed to furnish a strong blast of air. The simplest type consists of a flexible bag comprising a pair of rigid boards with handles joined by flexible leather sides enclosing an approximately airtight cavity which can be expanded and contracted by operating the handles, and fitted with a valve allowing air to fill the cavity when expanded, and with a tube through which the air is forced out in a stream when the cavity is compressed. It has many applications, in particular blowing on a fire to supply it with air. The term "bellows" is used by extension for a flexible bag whose volume can be changed by compression or expansion, but not used to deliver air. For example, the light-tight (but not airtight) bag allowing the distance between the lens and film of a folding photographic camera to be varied is called a bellows. "Bellows" is only used in plural. The Old English name for "bellows" was blǽstbęl(i)g, blást-bęl(i)g 'blast-bag', 'blowing-bag'; the prefix was dropped and by the eleventh century the simple bęlg, bylg, bylig ('bag') was used. The word is cognate with "belly". There are similar words in Old Norse, Swedish, and Danish and Dutch (blaasbalg), but the derivation is not certain. 'Bellows' appears not to be cognate with the apparently similar Latin . Several processes, such as metallurgical iron smelting and welding, require so much heat that they could only be developed after the invention, in antiquity, of the bellows. The bellows are used to deliver additional air to the fuel, raising the rate of combustion and therefore the heat output. Various kinds of bellows are used in metallurgy: Box bellows were and are traditionally used in East Asia. Pot bellows were used in ancient Egypt. Tatara foot bellows from Japan. Accordion bellows, with the characteristic pleated sides, have been used in Europe for many centuries. Piston bellows developed in Southeast Asia (probably by the Austronesian peoples) using the principles of the similarly indigenous fire piston.

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Blast furnace

A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. Blast refers to the combustion air being supplied above atmospheric pressure. In a blast furnace, fuel (coke), ores, and flux (limestone) are continuously supplied through the top of the furnace, while a hot blast of air (sometimes with oxygen enrichment) is blown into the lower section of the furnace through a series of pipes called tuyeres, so that the chemical reactions take place throughout the furnace as the material falls downward.

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