Barley sugar is a transparent hard candy. It comes in strips, lozenges, twists, and toys.
It is yellowish-brown, unless some red food colouring has been added to make it orangey-brown.
Traditionally, Barley Sugar is made from water hulled barley that has been simmered for a long time, up to 5 hours. The barley both flavours and colours the water. You then strain the water, discarding the barley . The water will be thick and gelatinous. White sugar is then added to the water, and boiled until it reaches the hard-crack stage. Sometimes lemon juice is added for flavouring just after the mixture is removed from the heat.
Commercially it is usually made without the barley water. The flavour is simulated through the use of cream of tartar (aka tartaric acid.)
To make strips, the hot mixture is poured on a surface, ideally marble, allowed to cool a bit, then cut into strips and individually twisted.
 Mrs Beeton advised that the boiled barley could be fed to poultry, particularly young chicks around 1 week old being weaned off soaked bread.
Barley Sugar was first made in the 1600s, as white sugar started to become affordable for more people. In the 1700s, people started poured the hot mixture into metal moulds, to make what became “Barley Sugar Clear Toys.” These went on to become particularly popular with the Victorians at Christmas.
By the 1850s, recipes were already leaving out the barley. In 1861, Mrs Beeton’s Barley Sugar recipe didn’t call for any barley.
Barley sugar is soon to reach the point in history where, despite its name, it has been made longer without its key ingredient than it has been made with it.
The United States Department of Agriculture (USDA) doesn’t see any point in calling it Barley Sugar any longer, and recommends that the term be dropped:
Sec. 515.500 Barley Sugar – Definition, and Barley Sugar Candy (CPG 7105.07)
BACKGROUND: Available information indicates that when sugar is heated to 160 C. (320 F.) it melts without loss in weight, and congeals on cooling, to a transparent amorphous yellowish mass which becomes gradually opaque on the surface from the formation of minute crystals, and that the resulting product is sometimes referred to as barley sugar. A second source of information states that when rock candy is heated to 185oC. (365oF.), it melts into a viscid, liquid, which on being suddenly cooled forms a transparent mass called barley sugar. It is apparent that the term barley sugar is being loosely applied to a product which is not now prepared even in part from barley. We doubt that the term has a derived meaning which would make it understandable to the purchaser, although it may be understood by sugar technologists.
POLICY: We are therefore inclined to discourage the use of the terms barley sugar and barley sugar candy as it is proposed to use them, since they appear to be ambiguous. However, we have made no investigation of consumer understanding of these terms and can advise interested persons only that the responsibility for their use rests upon the manufacturer of the products.
Issued: 10/1/80. Retrieved May 2006 from: http://www.fda.gov/ora/compliance_ref/cpg/cpgfod/cpg515-500.html.
Literature & Lore
TO MAKE BARLEY-SUGAR. 1524.
INGREDIENTS.– To every lb. of sugar allow ½ pint of water, ½ the white of an egg.
Mode.– Put the sugar into a well-tinned saucepan, with the water, and, when the former is dissolved, set it over a moderate fire, adding the well-beaten egg before the mixture gets warm, and stir it well together. When it boils, remove the scum as it rises, and keep it boiling until no more appears, and the syrup looks perfectly clear; then strain it through a fine sieve or muslin bag, and put it back into the saucepan. Boil it again like caramel, until it is brittle, when a little is dropped in a basin of cold water: it is then sufficiently boiled. Add a little lemon-juice and a few drops of essence of lemon, and let it stand for a minute or two. Have ready a marble slab or large dish, rubbed over with salad-oil; pour on it the sugar, and cut it into strips with a pair of scissors: these strips should then be twisted, and the barley-sugar stored away in a very dry place. It may be formed into lozenges or drops, by dropping the sugar in a very small quantity at a time on to the oiled slab or dish.
— Isabella Beeton. The Book of Household Management, 1861.