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 relatively low boiling point of 1,180 K (910 °C; 1,660 °F). 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. Alloys containing at least one metal with a low boiling point can be used to control the color of the spark. The lower boiling component evaporates and burns in the vapor phase, while the metal with a high boiling point serves as a carrier. In the vapor phase, bright element-specific light emission takes place. For example, a eutectic ytterbium-copper-alloy forms long green sparks and burning lithium silicide shows long red spark segments. The duration of a spark’s existence is determined by the initial size of the particle, with a larger size leading to a longer-lasting spark.

Metals with low thermal conductivity are especially good at producing sparks. Titanium and zirconium are especially good in this respect and so are now used in fireworks. Copper, on the other hand, has a high conductivity and so is poor at producing sparks. For this reason, alloys of copper such as beryllium bronze are used to make safety tools which will not spark so easily.

Robert Hooke studied the sparks created by striking a piece of flint and steel together. He found that the sparks were usually particles of the steel that had become red hot and so melted into globules. These sparks can be used to ignite tinder and so start a fire.

In colonial America, flint and steel were used to light fires when easier methods failed. Scorched linen was commonly used as tinder to catch the spark and start the fire, but producing a good spark could take much time. A spinning steel wheel provided a good stream of sparks when it engaged the flint, and a tinderbox designed to do this was known as a mill.

In a modern lighter or firesteel, iron is mixed with cerium and other rare earths to form the alloy ferrocerium. This readily produces sparks when scraped and burns hotter than steel would. This higher temperature is needed to ignite the vapour of the lighter fluid.

Molten metal sparks can be created when metal is heated by processes such as Bessemer conversion of iron to steel or arc welding.