# How is rock candy made?

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## Summary

Rock candy is grown, not shaped: sugar is dissolved in hot water to make a supersaturated syrup, and big crystals slowly form on a string or stick.

## Description

Rock candy, also called rock sugar or crystal sugar, is a sweet made of relatively large sugar crystals. Unlike most sweets, it is not cooked into shape. It is grown.
How it is made
Make a strong syrup. Sugar is dissolved in water. Heating the water first lets far more sugar dissolve, and that produces larger crystals.Give the crystals a start. A string, a stick or plain granulated sugar gives the dissolved sugar a surface on which crystals can begin to form.Wait. As the syrup stands, sugar comes out of the solution and builds up as crystals. They form after six to seven days. In one test, the first crystals showed after about a day, but it took at least five days to grow candy worth eating.Colour, if you like. Food colouring can be added to the syrup to make coloured rock candy.
The science
More than twice as much sugar dissolves in boiling water as in water at room temperature. So when a strong, hot syrup cools, it holds more sugar than it normally could. It is supersaturated, and the extra sugar is ready to come out of the solution. It just needs a starting point. A seed crystal, or anything that disturbs the syrup, sets off crystallisation, and the sugar builds up in large, crunchy crystals, each new layer added around the last a little like the rings of an onion.
Fondant starts from the same kind of supersaturated syrup, but it is stirred hard once it has cooled, so it forms a great many tiny crystals instead of a few big ones: see how is fondant made?
Why the syrup has to be so strong
Bethany Brookshire of Science News Explores tested this in 2020. A syrup of three parts sugar to one part water grew an average of 10.5 g of visible candy on each string. Syrups of equal parts sugar and water, or one part sugar to three of water, grew essentially nothing after five days, and some of the cups went mouldy.
An old way to eat sugar
Islamic writers in the first half of the 9th century described making candy sugar by growing crystals from cooling supersaturated sugar solutions. The English words sugar candy date from the late 13th century, from Old French çucre candi, and go back through Arabic qandi to Persian qand, meaning cane sugar. Rock sugar is still widely used. In East Frisia a lump called Kandis goes in the bottom of the teacup; in Iran, rock candy called nabat is taken with tea, and saffron is the most popular flavour; and in India, mishri is eaten as a sweet or used to sweeten milk or tea.
Not British rock
The American term rock candy should not be confused with British rock, the opaque boiled sugar stick sold at the seaside, which is hard at first and turns chewy in the mouth: see how is seaside rock made? and why is rock called rock? For more on the word candy, see what is the difference between sweets and candy?

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