All over the world fireworks are a popular way to celebrate important occasions including birthdays, the new year and public holidays, but did you know that many of the red fireworks are based on lithium?
What is the colour of lithium? White? Silver-grey? Purple? Green?
Well, in different forms of lithium all of these colours are possible. In metallic form lithium is a shiny silver-grey colour, similar to most other metals (see page 8). In its carbonate form lithium is a brilliant white crystalline powder which looks very similar to table salt.
The main lithium-containing minerals, spodumene, petalite and lepidolite, show a remarkable variety of pinks, greens, purples, and pale grey-yellows (the inspiration for ILiA’s colour palette), while some of the more exotic minerals, for example tourmaline-rubellite, are truly spectacular.
There is more to the colour of lithium than meets the eye. And wait, we only scratching the surface!
The burning question
Have you have ever sat around a campfire at night watching the wood slowly burn and noticed unusually coloured flames? Perhaps there was some metal litter thrown into the fire? What you witnessed was probably the subject of optical emissions spectroscopy, or the science of different metallic elements burning with different coloured flames.
In chemistry, the ‘flame test’ is an old but reliable way of testing for the presence (or absence) of different elements, especially alkali metals such as lithium, sodium and potassium. Each of these metals burns with a unique colour signature, clearly visible to the human eye.
The reason that different elements appear to burn with different colours is down to their fundamental atomic properties.
When an atom is heated its outer electrons move into a higher state of energy and the atom becomes ‘excited’. If the atom is allowed to cool down again it will return to its usual ‘ground’ state and emit some energy in wavelengths that appear as visible light.
Lithium ions react in the flame of a Bunsen burner. Lithium is unusual as it reacts with both the oxygen to form lithium oxide (Li2O) and nitrogen to form lithium nitride (Li3N). (Image: Charles D. Winters / Science Source)
Because each element has a different atomic structure, the emission spectrum for each type of element is unique and universally consistent. Most emit more than one wavelength of visible light, the dominant wavelength determining the unique flame colour seen with a naked eye.
For example, copper produces a blue flame, calcium an orange flame, sodium a yellow flame, and barium a green flame. For the emission spectrum of lithium, the strongest line is at ~670.8 nanometres (nm) and a second at ~610nm, which together appear as a brilliant crimson-red flame.
This is the colour of lithium.
Seeing lithium atoms for the first time
It was crude flame tests that arguably first prompted and then later confirmed the discovery of lithium.
In the days before the invention of dynamite mining often involved building a fire directly on an ore body, heating the rocks and then rapidly quenching them with water. This would cause the hot rock to crack and splinter into manageable lumps that could be removed and processed. This fire quench (or fire-setting) method of mining was in use at an iron ore mine on Utö island in Sweden in the late 18th century when it was visited by a Brazilian polymath, Joze Bonifacio de Andrada e Silva.
It is said that Andrada was shown a white mineral which gave off an intense red flame when it was thrown into a fire. Fascinated, he collected a sample for analysis and later demonstrated that it was a previously unrecorded mineral. This was the mineral petalite. (However, since petalite was an unwelcome impurity in Utö’s iron ore deposit, it seems plausible that those early miners would have cursed whenever they saw red flames appear in the fires that they built in the mine!)
Two decades later a young Swedish scientist called Johan Arfwedson demonstrated that petalite contained a previously unknown element and named it lithium. The discovery was independently confirmed a year later when a German chemist, Christian Gottlob Gmelin, observed that the new element burned with an intense crimson-red flame, a colour which had never seen before in laboratory flame tests.
Practical applications
Today flame tests are still widely used due to their simplicity and reliability. A basic flame test can play a useful role in analysis, for example field testing crude mineral samples. They are also excellent for inspiring the next generation of young chemists in school science classes.
The principle behind flame testing also forms the basis for two highly technical analytical techniques, as Jeremy Brown, Group Technical Development Manager at Alfred H Knight, explained. “In Flame Atomic Absorption Spectroscopy (FAAS), a flame at ~2700°C (4890°F) excites lithium’s electrons and a specific amount of light is absorbed, allowing for the determination of lithium content of a sample. In Ion Coupled Plasma Optical Emission Spectroscopy (ICP-OES), a plasma at ~10,000°C (18,000°F) excites the electrons to a higher state and then as they return to a ground state, they emit specific wavelengths. Detecting and measuring the intensity of those emissions across multiple wavelengths enables precise quantitative analysis of a sample.”
Having fun and saving lives
Fireworks are undoubtedly the most visible use of the emission spectrum of lithium and probably one which we have all witnessed, although perhaps without realising it. The fact that different elements burn with different colours is fundamental to fireworks; to make yellow fireworks add sodium salts, for blue add copper, for orange add calcium and so on.
Traditionally red fireworks were mostly made using strontium salts – strontium also has a strong emission spectrum in red wavelengths – but in the last decade there has been a growing trend to replace it with lithium due to concerns in the US and elsewhere about the effect of traces of strontium on human health. An added advantage of using lithium salts in pyrotechnics is that by modifying the recipe slightly, pinks and oranges can also be produced.
However, the practical applications of lithium as a red pyrotechnic colorant are not purely for fun, they save lives too.
Red emergency flares have a long history of attracting attention in times of distress. Historically these too were based on strontium salts, but are seeing a move towards lithium, especially in the United States where work was led by the U.S. Army Research Laboratory in partnership with the Ludwig Maximilian University in Germany. While electronic red lights (using lithium batteries) are increasingly popular, pyrotechnical flares are still widely used and have a number of applications, including as location beacons in military exercises and as emergency ‘SOS’ flares for ships and mountaineers.
Lithium minerals, salts and applications come in a multitude of colours, but one stands out as representing a fundamental, inimitable property of lithium: scarlet-red. This unique colour not only helps geologists to identify lithium in minerals, but brings joy to millions of children watching firework displays and even helps to save lives too.
Not many elements can say that!
