Gold film has been used in the glass windows of modern buildings to reflect heat. Gold salts are used for toning in photography, and in colouring glass. The gold contained in hard rock has to be extracted by chemical leaching after the ore has been crushed and finely ground. After grinding, the ore is leached in large agitator tanks,nearly always with cyanide in solution. The material in the tanks at this stage is known as 'pulp'.
Most producers employ the carbon in pulp process to recover the gold.
Some types of ore can be treated successfully by the heap leaching technique. The solution percolates through the ore and carries the gold with it into channels, and finally into a storage pond. The carbon particles are derived from burnt coconut or macadamia nut shells, or peat. Desorption involves treating the gold loaded carbon particles with chemicals at high temperature and pressure to produce a concentrated solution rich in gold. The carbon is recovered for reuse.