The Colour of Sparks

As well as emitting at characteristic frequencies due to specific energy transitions, an emitting species such as a hot metal oxide will also possess a component due to black body radiation, which occurs at all frequencies and is a function of temperature only. 

Although it is known that the colour of black body radiation is only dependent upon temperature, sparks have colours that are also dependent upon the type of emitting material. However, the form of the radiance curves does not relate exactly with known molecular energy transitions. This suggests that the mechanism of emission in excess of black body radiation is not yet fully established. It is possible that some emission bands only become active when the metal oxide particle is molten, or that the energy is dissipated simply via collisions with other molecules rather than the emission of photons. 

However, on the grounds of cost, availability, reactivity and safety, the list of metal powders used by the firework maker reduces to Al, Ti, Fe and Mg/Al alloy (magnalium). 

Material Iron Aluminium Titanium Potassium sulfide 

although the colour of sparks is dependent upon flame temperature and may be similar to that of black body radiation, the overall colour effect can include contributions from atomic line emissions, from metals (seen in the UV and visible regions of the electromagnetic spectrum), from band emissions from excited oxide molecules (seen in the UV, visible and IR regions) and from continuum hot body radiation and other luminescence effects. So far as black body radiation is concerned, the colour is known to change from red (500 °C glowing cooker 

Origin Metal or chemi- Metal Metal Chemical 

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Colloidal iridium may also be prepared by Bredig s method, which consists in sparking between iridium electrodes immersed in ice-cooled water. A current of 20 to 25 amperes at 220 volts gives satisfactory results.2 The colour of the hydrosol ranges from red to black, according to the method of preparation. A small current favours the formation of the black solution. 

The fire dust or sparks which are produced by iron or carbon create the colour which resembles that of the black body. The colour of the sparks of Senko-Hanabiwhich is caused not only by carbon but also other materials, also resembles the colour of the black body. We feel that the. colour of the aluminium fire dust is a little different from the black body. The colour changes from red-orange to yellow, white yellow and silver as the temperature increases, and the locus may be written as the line... 

Characteristic colours of fumes (flames), as described by Agricola in 1550, were used to "control" the process of smelting of ores. Talbot (1826) and Wheatstone (1835) reported that the colours of flame and spark induced... 

Priestley frequently mentions the colour of the electric spark in various gases in fixed air (CO2) it is racceedingly white , in inflammable air (Hg) it is red or purple (which he regarded as characteristic). He found that water impregnated with fixed air is by no means so good a conductor of electricity as water impregnated with any of the mineral acids . ... 

In enamels for chemical plant such as autoclaves it is not only the degree of acid resistance which is important but also the freedom of the finish from minute flaws detectable by high frequency spark testing or chemical methods. The chemical methods depend upon a colour change when the reagent such as ammonium thiocyanate reacts with the iron exposed at the bottom of the pinhole or flaw in the finish. Alternatively, an electric cell can be formed via the exposed iron in the flaw and detected chemically. 

A further factor that contributes to the overall appearance of a firework fountain is the brightness of the sparks. As with colour, the brightness is dependent upon the temperature and characteristics of the material used. The brightness of black body radiation varies with temperature as shown in Figure 5.5. 

Platinum Hydrosol or Colloidal Platinum.—A solution of platinum hydrosol or colloidal platinum in water is easily prepared by sparking between platinum electrodes immersed in ice-cooled water,8 a current of about 10 amperes and 40 volts being employed. The electrodes consist of thick platinum wire, and, when placed from 1 to 2 mm. apart, sparking takes place, particles of the metal being tom off and suspended in the water. The liquid thus obtained is allowed to stand overnight, and decanted from any sediment. It has a dark colour, but the individual metallic particles cannot be distinguished even with the aid of a microscope. 

Dry Tests.—Iron salts, when moistened with hydrochloric acid and heated on a loop of platinum wire in a Bunsen flame, emit a shower of sparks. When heated on charcoal with sodium carbonate in the blowpipe flame, the compound is converted into a dark-coloured residue. If potassium cyanide is added to the sodium carbonate and iron compound, and the whole heated on charcoal in the inner flame of the blow-pipe, metallic iron is obtained as a grey, magnetic powder. 

Spectrum.—Compounds of chromium impart no distinctive colour to the non-luminous flame. The spectrum, however, is somewhat complicated, especially the spark spectrum. Careful and complete measurements have been made, for which the reader is referred to the literature. Exner and Haschek be. cit.) state that the most intense lines ( hauptlinien ) in the arc and spark spectra are as follows ... 

Fig.33) Anyhow we can produce various colours of the fire dust or sparks except blue and green by adjusting the temperature and selecting substances which make up the component material in firework compositions according to the principle of the colour temperature. 

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