Fireworks coloured flames

In 1801 the French pyrotechnist Claude-Fortune Ruggieri described the use of metal salts in the production of coloured flames. By the early nineteenth century the firework maker had at his disposal a diverse arsenal of materials, many of which are still in use today. The following substances were documented by Audot and others at around that time ... 

The difficulty in producing a good blue flame stems from several important considerations. Firstly, impurities in the chemicals present in the firework tend to produce yellow flames, which detract from the blue secondly, coloured flames follow similar physico-chemical phenomena but operate in different regions of the spectrum. Consequently the copper salts (that are normally utihsed for the production of blue stars) decompose thermally to produce a variety of emissions that radiate from about 325 to 660 nm i.e. from green, blue and violet to orange-red) simultaneously polluting the pure blue flame which appears in the 400 to 455 nm region. 

LANCE Small, tubular firework designed to emit a coloured flame for about 90 seconds. Used for a visual effect in set-pieces. 

It is thought that fireworks began with the history of potassium nitrate. It has. been used for compositions which produce fire dust or sparks as well as a white smoke which has been called "Wabi (Japanese fire). It could not produce coloured flames, but before the appearance of potassium chlorate, fireworkers made various efforts to create colour with it as far as possible. 

A lance is a firework that is about the size of a small pencil that functions in the manner of a flare (Figure 10.3). Thus, the theory of coloured flame production, as presented in Chapter 8, applies equally to the lance. 

Elements from Group 2 are used in a wide range of applications. For example, Group 2 metals produce coloured flames when heated, leading to their use in flares and fireworks. Magnesium is used in powdered form in flares. The large surface area in a fine powder increases the rate of reaction with oxygen. 

The important emitters of firework flames are molecules with the exception of Na atoms. The molecules are produced in quite different forms from the original colour producing materials mixed into the composition. The chemical combination of the emitters are relatively simple and in general are outside the ordinary valency law. For example, they are written as SrCl, BaCl, CuCl etc. and not SrCl2i BaClj, CuCl etc. 

Where the flame contains neither chlorine nor hydrogen chloride a molecular spectrum band CaOH appears, and the colour of the flame becomes orange. In the presence of chlorine or hydrogen chloride gas another spectrum band CaCl appears in the flame. In this case the colour of the flame is an intensive reddish-orange which is not as good as red, but it is sometimes zsed for cheap toy fireworks. 

The BOjj, spectrum for a green flame might be suggested using boric acid as a colour producing agent, but it is too weak to use practically in firework compositions. 

Various spectra which occur in a firework flame, except the useful spectra described above, generally decrease the purity of the flame colour and make the flame become whitish. According to the chromaticity diagram, the flame colour has an inclination to move from the edge to the centre. 

Quality. This material in commerce passes 80 mesh. Apparently there are variations in quality, especially in the Ba content which disturbs the red colour of a flame the smaller it is, the better. It is not very ofte n used for fireworks because of its hygroscopic nature, especially when good ignition is required. But it gives a very bright red to a flame, and could become more useful, if a greater resistance to water could be developed. At present it is used mainly for distress signals for roads, trains and at sea. 

Na-D lines appear in almost all cases because every inorganic substance in firework composition contains sodium compounds as an impurity. The Na-D lines disturb any flame colour except a yellow flame. It may be difficult to obtain perfectly Na-free ingredients, but we must endeavour to get materials with a minimum sodium content. 

Blue is always a difficult colour to produce pyrotechnically because impurities in the chemicals present in the firework tend to produce yellow flames which detract from the blue. 

---