Most people first encounter chalk in school where it refers to
blackboard chalk, which was originally made of mineral chalk, since it easily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacturers now may use mineral chalk, other
mineral sources of calcium carbonate, or the mineral
gypsum (
calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in the
developing world, use of carbonate-based chalk produces larger particles and thus less dust, and it is marketed as "dustless chalk". blows surplus chalk from his hand.
Magnesium carbonate chalk is commonly used as a drying agent to obtain better grip by gymnasts and rock climbers. Glazing
putty mainly contains chalk as a filler in
linseed oil. Chalk and other forms of limestone may be used for their properties as a
base. Chalk is a source of
quicklime by
thermal decomposition, or
slaked lime following
quenching of quicklime with water. In
agriculture, chalk is used for raising
pH in soils with high
acidity. Small doses of chalk can also be used as an
antacid. Additionally, the small particles of chalk make it a substance ideal for cleaning and polishing. For example, toothpaste commonly contains small amounts of chalk, which serves as a mild
abrasive. Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals. 's chalk French chalk (also known as tailor's chalk) is traditionally a hard chalk used to make temporary markings on cloth, mainly by
tailors. It is now usually made of
talc (magnesium silicate). Chalk beds form important
petroleum reservoirs in the
North Sea and along the
Gulf Coast of North America. , a hill figure created by exposing the natural whiteness of the chalk bedrock. The high contrast between white chalk and green turf has led to the creation of
hill figures throughout the chalk downlands of
England. By "scouring"—the process of removing the overlying vegetation to reveal the porous white limestone beneath—monumental geoglyphs are created. A notable 20th-century example is the
Litlington White Horse located in
East Sussex. Managed by the
National Trust, the figure is maintained by periodically clearing away encroaching weeds and refreshing the exposed chalk surfaces to preserve its visibility against the hillside.
Previous uses In southeast England,
deneholes are a notable example of ancient chalk pits. Such
bell pits may also mark the sites of ancient
flint mines, where the prime object was to remove flint
nodules for
stone tool manufacture. The surface remains at
Cissbury are one such example, but perhaps the most famous is the extensive complex at
Grimes Graves in
Norfolk. Chalk was traditionally used in recreation. In field sports, such as tennis played on grass, powdered chalk was used to mark the boundary lines of the playing field or court. If a ball hits the line, a cloud of chalk or pigment dust will be visible. In recent years, powdered chalk has been replaced with
titanium dioxide. In gymnastics, rock-climbing,
weightlifting and
tug of war, chalk — now usually
magnesium carbonate — is applied to the hands and feet to remove perspiration and reduce slipping. Chalk may also be used as a house construction material instead of
brick or
wattle and daub: quarried chalk was cut into blocks and used as
ashlar, or loose chalk was rammed into blocks and laid in mortar. There are still houses standing which have been constructed using chalk as the main building material. Most are pre-
Victorian though a few are more recent. A mixture of chalk and
mercury can be used as
fingerprint powder. However, because of the toxicity of the mercury, the use of such mixtures for fingerprinting was abandoned in 1967. ==See also==