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. Flint and steel 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. Metal working Molten metal sparks can be created when metal is heated by processes such as Bessemer conversion of iron to steel or arc welding. Arc welding uses a low voltage and high current electric arc between an electrode and the base material to melt the metals at the welding point, which often creates sparks. To reduce the risk of burns, welders wear heavy leather gloves and long sleeve jackets to avoid exposure to extreme heat, flames, and sparks. In spot welding, metal surfaces that are held in contact are joined by the heat from resistance to electric current flow. It is common for a spray of sparks in the form of molten metal droplets to be ejected from the parts being joined. or the resistance heating of spot welding. Fires