Traditionally draught beer came from wooden barrels, also called casks. In the 1950s these began to be replaced by metal casks of stainless steel or aluminium, mainly for quality reasons as they could be sterilised and the beer was therefore less likely to spoil, but also for economic reasons. An additional benefit of the switch to metal casks was that staling from oxygen in the air could be reduced. Subsequently, in the early 1960s a form of metal cask, known as a keg, was introduced which allowed for more efficient cleaning and filling in the brewery.
The essential differences between a traditional cask and a keg are that the latter has a centrally located downtube and a valve that allows beer in and gas out when filling and vice versa when beer is dispensed. Also kegs have a simple concave bottom whilst the barrel or cask design allowed sediment to be retained in the cask. This aspect of keg design meant that all the beer in the keg was dispensed which therefore required that the beer be processed by filtration, fining or centrifuging, or some combination of these, to prevent sediment formation. Lastly, kegs have straight sides unlike the traditional barrel or cask shape. In order to get the beer out of a keg and into a customer’s glass, it can be forced out with gas pressure, although if air or gas at low pressure is admitted to the top of the keg it can also be dispensed using a traditional hand pump at the bar.
By the early 1970s most beer in Britain was keg beer, filtered, pasteurised and artificially carbonated. This change was largely driven by the customer's dislike of sediment in his beer. However, most British brewers used carbon dioxide for dispensing keg beers. This led to beers containing more dissolved gas in the glass than the traditional ale and to a consumer demand for a return to these ales. By contrast, in Ireland, where stout was dominant, the use of a mixture of carbon dioxide and nitrogen for dispensing prevented the beer from becoming over-carbonated. Some of the last remaining natural beers in the world were about to disappear forever.[citation needed] Though rare examples of natural beers could still be found in the farmhouse beers of Northern Europe and the maize beers of South America for example, in essence the last great stronghold of natural beer was about to be wiped out. In 1973 the Campaign for Real Ale (CAMRA) was founded in Britain to save what they came to term "real ale".