Why is blue the rarest firework colour?
In this article
Around midnight on 4 November 1605, Sir Thomas Knyvett led a search of the cellars under the House of Lords and found a man who called himself John Johnson, with the gunpowder that was meant to blow up the House at the State Opening of Parliament on 5 November, according to The National Archives. His real name was Guy Fawkes.
More than four hundred years later, people still mark that night with fireworks in every colour, and the one firework makers find hardest to get right is blue, because the copper that makes it only glows blue at exactly the right temperature. John Conkling, then technical director of the American Pyrotechnics Association, told NPR in 2013: “If your temperature gets too high, you lose the colour. You wash out the colour.”
Red
The colours in a firework come from metals, and red comes from strontium, named after the village of Strontian in the Scottish Highlands.
Green
Green comes from barium, the metal in the chalky drink people have before some X-rays.
Orange
Orange comes from calcium, the same metal that is in your bones.
Yellow
Yellow comes from sodium, the metal that made the old orange street lamps glow.
1859
Each metal gives off its own exact colours, and in 1859 two German scientists used them to work out which metals are in the Sun.
Gold
Gold sparks are tiny bits of charcoal or iron, and they glow because they are so hot.
Blue
Blue comes from copper, and it is the hardest colour of all to make.
Blue
Copper only gives a good blue when the heat is just right, and if it burns too hot, the blue washes out towards white.
Purple
For purple you need red and blue at the same time, so it is nearly as hard to make as blue.
Crackle
The crackle comes from tiny pellets called dragon’s eggs, which burn for a moment and then pop.
1830s
Most fireworks were gold and white until the 1830s, when Italian makers found a way to add bright colours.
1605
Guy Fawkes was caught with 36 barrels of gunpowder, and the lantern he is said to have carried is in the Ashmolean Museum in Oxford.
How far?
You see a firework before you hear it, so count the seconds until the bang: every three seconds means it is about a kilometre away.
Finale
Next time you watch a display, look out for a deep, clear blue, the rarest colour in the sky.
The metal for red was first found in a Highland lead mine
Strontian is a village on Loch Sunart, in the west Highlands, built for the miners who came after lead was found in the hills above it in 1722, according to National Museums Scotland. The physician Adair Crawford looked at an unusual rock from one of its mines and found a new mineral in it, and in 1791 the chemist Thomas Charles Hope proved it held a new element and noticed that it made a candle flame burn red, as the Royal Society of Chemistry explains.
In 1808 Humphry Davy isolated the metal at the Royal Institution in London and named it strontium, after the village, and the museum counts Strontian, by population, as the smallest place in the world to give its name to an element. Strontium salts still give fireworks and flares their brilliant reds.
The metal for yellow is in the Sun as well
In 1859, in Heidelberg, the chemist Robert Bunsen and the physicist Gustav Kirchhoff built a spectroscope, an instrument that spreads light out through a prism. They heated tiny samples of one element after another in a flame, and every element they tried gave off its own set of bright coloured lines, always in the same places, so they could tell what was burning in a flame from its lines alone.
Sunlight seen through a prism has dark lines across it, and Kirchhoff found that one of the darkest, known as the D line, sits exactly where sodium’s bright yellow light falls. He worked out that the dark lines are made by the elements in the Sun’s outer layers, and so he showed that the Sun contains sodium, the metal that turns a firework yellow, as the Linda Hall science library describes.
The next year they found a metal nobody knew existed. In mineral water from Dürkheim they saw blue lines that matched no known element, and they named the new metal caesium, from caesius, the Latin for sky blue, according to the Royal Society of Chemistry.
Why blue is so hard to get right
Inside every firework shell are small pellets called stars, which burn in the colour you see. In a blue star, the colour comes from copper compounds that break apart if the star gets too hot, and then the blue is lost, as the chemistry site Compound Interest explains. A star that burns too cool gives out hardly any light at all, which is why Conkling told NPR that “you need a perfect flame temperature.”
So a strong blue in a display means whoever made that firework got the temperature exactly right.
How Bonfire Night began
Once the plot was found, Parliament declared the fifth of November a day of thanksgiving for the King’s survival, and people marked it, much as they do today, with bonfires, fireworks and food.
Fawkes is said to have been carrying a lantern when he was caught. It is made of sheet iron, with a holder for a candle inside, and Robert Heywood, who had been a proctor of the University of Oxford, gave it to the University on 4 April 1641. It was kept in the Bodleian Library until 1887, when it moved to the Ashmolean.
Why this matters at home
You only see the colours that are in the light. The old low-pressure sodium street lamps gave out only yellow-orange light, so under them most colours looked like shades of grey, as the Naked Scientists explain.
By a window in the day, you can tell navy from black and match the sky pieces in your jigsaw. Under many lamps in the evening, navy and black can look the same. At home, the HD Pro+ gives you daylight to see by, with 98% colour accuracy.
Firework colours: your questions answered
What makes fireworks different colours?
Most of the colours come from metal salts packed into the stars, the small pellets inside each shell: strontium for red, barium for green, calcium for orange, sodium for yellow and copper for blue, according to PBS NewsHour. Silver comes from magnesium and aluminium burning white-hot.
Why are purple fireworks so rare?
Purple is hard to make too, because it needs a blue copper compound and a red strontium one burning together in the same star.
What makes fireworks crackle?
Firework makers call the crackling mixture dragon’s eggs: small granules that burn fast and go off with a crack. They were first made with a lead compound, and because lead is poisonous, most are now made with bismuth compounds instead.
How can you tell how far away a firework is?
Count the seconds between seeing the burst and hearing the bang. Sound travels about a kilometre every three seconds, so a six-second gap means the firework is about two kilometres away.
Why do we have fireworks on Bonfire Night?
Bonfire Night marks the discovery of the Gunpowder Plot in November 1605, when the King survived a plan to blow up Parliament. It was celebrated with bonfires and fireworks then, and it still is today.