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Within its thick-walled cardboard tube, a Roman candle that emits a red and a green star contains blue touch paper, delay composition, red star, gunpowder, green star and a clay or plastic base.
Blue touchpaper
This is paper treated with potassium nitrate (KNO3) solution. When lit, a smouldering chemical reaction between the cellulose of the paper and the nitrate transfers heat to a layer of finely milled gunpowder at the top of the firework. As it burns down it ignites the tube's various layers in turn.
Delay composition
Made up of 62 per cent potassium nitrate, 18 per cent sulphur and 20 per cent carbon, this produces a small display of sparks before igniting a charge of gunpowder around the sides and below the next layer.
Gunpowder
More potassium nitrate, sulphur and carbon. The Chinese invented "black powder" around 1000 AD, and it was described by English friar Roger Bacon in the 13th century. Back then, potassium nitrate had to be scraped off the walls of cesspits where bacteria had acted on urine and dung. Today it is made from nitric acid - also used to give wooden furniture an "antique" finish.
Red star
Strontium nitrate, strontium carbonate and aluminium powder. Ignited by the burning aluminium, strontium produces a bright red flame. It is also the active desensitising ingredient (in the form of strontium chloride) in Sensodyne Original toothpaste, blocking the tubules leading to the tooth nerves.
Green star
Barium chlorate, potassium chlorate and charcoal. The burning barium chlorate emits the green colour. In its hydroxide form, barium is used in x-ray examination of the bowel, as it is a good contrast medium.
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This article was originally published by WIRED UK
