<P> The hacks are fixed 12 feet (3.7 m) behind each button; a hack gives the thrower something to push against when making the throw . On indoor rinks, there are usually two fixed hacks, rubber - lined holes, one on each side of the centre line, with the inside edge no more than 3 inches (76 mm) from the centre line and the front edge on the hack line . A single moveable hack may also be used . </P> <P> The ice may be natural but is usually frozen by a refrigeration plant pumping a brine solution through numerous pipes fixed lengthwise at the bottom of a shallow pan of water . Most curling clubs have an ice maker whose main job is to care for the ice . At the major curling championships, ice maintenance is extremely important . Large events, such as the Brier or other national / international championships, are typically held in an arena that presents a challenge to the ice maker, who must constantly monitor and adjust the ice and air temperatures as well as air humidity levels to ensure a consistent playing surface . It is common for each sheet of ice to have multiple sensors embedded in order to monitor surface temperature, as well as probes set up in the seating area (to monitor humidity) and in the compressor room (to monitor brine supply and return temperatures). The surface of the ice is maintained at a temperature of around 23 ° F (− 5 ° C). </P> <P> A key part of the preparation of the playing surface is the spraying of water droplets onto the ice, which form pebble on freezing . The pebbled ice surface resembles an orange peel, and the stone moves on top of the pebbled ice . As the stone moves over the pebble, any rotation of the stone causes it to curl to the inside or outside . The amount of curl (commonly referred to as the feet of curl) can change during a game as the pebble wears; the ice maker must monitor this and be prepared to scrape and re-pebble the surface prior to each game . </P> <P> The curling stone (also sometimes called a rock in North America) is made of granite and is specified by the World Curling Federation, which requires a weight between 38 and 44 pounds (17.24 and 19.96 kg), a maximum circumference of 36 inches (914.4 mm) and a minimum height of 4.5 inches (114.3 mm). The only part of the stone in contact with the ice is the running surface, a narrow, flat annulus or ring, 0.25 to 0.50 inches (6.4 to 12.7 mm) wide and about 5 inches (130 mm) in diameter; the sides of the stone bulge convex down to the ring and the inside of the ring is hollowed concave to clear the ice . This concave bottom was first proposed by J.S. Russell of Toronto, Ontario, Canada sometime after 1870, and was subsequently adopted by Scottish stone manufacturer Andrew Kay . </P>

Where did the term hammer come from in curling