Two things have to happen to the mixture of dairy products and flavourings that make up ice-cream: they must be frozen and they must be stirred, to prevent large ice crystals from forming. Before electrical freezing machines were available, the ice-cream mixture was put into a chum, around which a mixture of salt and ice was packed. Heat from the ice-cream mixture gradually passed into the colder ice, until the cream itself was frozen. Meanwhile, the mixture was stirred by means of a paddle connected to a handle outside the tub. This became harder work as the ice-cream froze!
There is a good summary in The National Trust Book of Sorbets, Flummeries and Fools by Colin Cooper English (published 1985). Time-consuming and costly, the old-fashioned way was to place the ingredients into a thin drum, which was then sunk into a larger container which held a mixture of ice and salt. Although water freezes at 32F (0C), milk and cream will not freeze until they are down to 20F (-6.7C). The salt melts the ice and produces a brine with a temperature around 17F (-8.3C), and it is this freezing brine which provides the refrigeration. The effort needed to produce a serving of ice-cream in an early Victorian household can be seen in this 1856 recipe: ‘Break a pail of ice in pieces, add four pounds of salt and mix well; put a pewter freezing-can in an empty pail and surround it with ice; put the pudding … into the can, and turn it very rapidly with the finger and thumb; when the pudding adheres to the sides of the can, scrape off with a spittle or spoon. When the pudding has become stiff, put it into a mould, cover it up with a lid, having put two plies of paper between; bury the mould in the ice; when wanted, take a basin of cold water and wash off the salt, take off the cover, turn it out on a dish and serve.’ All this assumes that you have a handy supply of ice. Those who could afford it had ice-cellars or ice-houses built underground, in which ice from the winter could be kept, insulated by the air trapped in a layer of straw, reeds, chaff or bundles of thin wood faggots throughout the rest of the year. The idea seems to have been used first by the Chinese. At the time of Confucius (500 BC) there were accounts of ice-cellars. Alexander the Great is said to have employed slaves in relays to carry snow and ice down from the mountains. The ice-cream recipe was brought back to Venice from China by Marco Polo in 1292. By the mid-19th century a number of freezing mixtures had been devised, which did not require snow or ice to start them off. They included such lethal cocktails as a mixture of sal ammoniac, nitre and water, said to reduce the temperature from 50F to 10F; nitrate of ammonia and water (50F down to 4F); and sulphate of soda with dilute sulphuric acid (50F down to 3F).
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