Spark (fire)

A spark is an incandescent particle. Sparks may be produced by pyrotechnics, by metalworking or as a by-product of fires, especially when burning wood. In pyrotechnics, charcoal, iron filings, aluminum, titanium and metal alloys such as magnalium may be used to create sparks. The quantity and style of sparks produced depends on the composition and pyrophoricity of the metal and can be used to identify the type of metal by spark testing. In the case of iron, the presence of carbon is required, as in carbon steel — about 0.7% is best for large sparks. The carbon burns explosively in the hot iron and this produces pretty, branching sparks. The color of sparks used in pyrotechnics is determined by the material that the sparks are made from, with the possibility of adding different chemical compounds to certain materials to further influence the color of the sparks. The basic color of sparks is limited to red/orange, gold (yellow) and silver (white). This is explained by light emission from a solid particle. Light emitted from solid particles is defined by black-body radiation. The temperature of the spark is controlled by the reactivity of the metal. Higher reactive metals lead to hotter sparks. The electronegativity has found to be a helpful indicator to estimate the temperature and consequently the color of a spark. To achieve colors differing from black body emitters, vapor phase combustion of the metal is necessary. A typical example is zinc, with a low boiling of 1180 K. Zinc sparks show an unusual bluish/greenish white appearance. Exotic sparks can be obtained from erbium powder. These sparks switch between surface and vapor phase combustion and accordingly between orange (black body) and green (element specific) emission. The color-change is based on the medium-ranged boiling point of erbium, which burns only partly in the vapor phase. The adjacent rare earth elements thulium, lutetium and yttrium can form color-changing sparks, too, although the visibility of both phases of the same spark is less pronounced due to a lower (Tm) or higher (Y, Lu) boiling point of the metal.