A blackboard (also known as a chalkboard) is a reusable writing surface on which text or drawings are made with sticks of calcium sulfate or calcium carbonate, known, when used for this purpose, as chalk. Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone. A blackboard can simply be a piece of board painted with matte dark paint (usually black, occasionally dark green). Black plastic sign material, using the trade name sintra is also used to create custom chalkboard art. A more modern variation consists of a coiled sheet of plastic drawn across two parallel rollers, which can be scrolled to create additional writing space while saving what has been written. The highest grade blackboards are made of a rougher version porcelain enamelled steel (black, green, blue or sometimes other colours). Porcelain is very hard wearing and blackboards made of porcelain usually last 10–20 years in intensive use. Manufacturing of slate blackboards in the U.S. began by the 1840s. Green chalkboards, generally made of porcelain enamel on a steel base, first appeared in the 1960s. Ubiquitous in American classrooms for three decades, green chalkboards have largely been replaced by whiteboards in U.S. schools.