What is rock made of?

For such a colourful sweet, seaside rock has a very short list of ingredients. Almost all of it is sugar. The skill is in what the sugar boiler does with it, from boiling it hard to building the letters by hand.

The ingredients

Traditional seaside rock is made from:

• Granulated sugar, the main ingredient
• Glucose syrup, roughly one part to every three parts of sugar
• Water, just enough to dissolve the sugar
• Food colourings, for the casing and the letters
• Flavouring, most usually peppermint or spearmint

That is essentially it. Like other hard boiled sweets, rock ends up nearly 100 per cent sugar by weight, with only a tiny amount of colour and flavour and almost no water.

What each part does

Sugar gives rock its sweetness and its hardness. Boiled to about 147 degrees Celsius, the hard crack stage, it loses nearly all its water and sets into a hard, glassy solid as it cools: see why is seaside rock so hard?

Glucose syrup helps keep the sugar from forming crystals. In sugar glass, the brittle sugar used for fake windows in films, glucose or corn syrup is added for exactly that reason, so the sugar stays clear instead of turning grainy.

Water is only there to dissolve the sugar at the start. Too little and the sugar can burn, or the finished rock can turn sugary and grain off. Nearly all of it is boiled away.

Colourings are mixed into small portions of the hot mixture on the cooling table, by hand, with a spoon or palette knife, to make the casing and the letters.

Flavourings are poured in measured amounts into the mixture while it is being pulled.

One mixture, three parts

Every part of a stick of rock is made from the same boiled mixture, handled in different ways.

• The centre is the mixture pulled on a machine with one fixed arm and two moving arms. Pulling works air into it and turns it from a solid golden mass into soft white.
• The letters are built from thin strips of coloured and white mixture. Each letter starts about the size of a thumbprint and is shrunk as the rock is stretched: see how do they get the letters in rock?
• The casing is coloured mixture, kneaded and rolled out with a rolling pin, then wrapped around the outside.

The whole assembly, called a boiling, is too thick to get your hands around. It is kept warm in a batch roller and then spun out, stretched into long, thin strings that are cut into sticks and wrapped: see how is seaside rock made?

Keeping it hard

Because rock is so sugary, it pulls moisture out of the air if it is left unwrapped, and goes sticky and soft: see why does seaside rock go soft?

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