Limestone is the natural stone most frequently used in combination with other masonry materials. It also is the most likely to be installed by a mason. Limestone is a sedimentary rock composed principally of calcite (calcium carbonate) or dolomite (the double carbonate of calcium and magnesium) or a mixture of the two. The Indiana Limestone Institute (ILI) has developed a grading system that can help specifiers determine the type of stone appropriate to their particular needs. It divides limestones into four grades based on the fineness and consistency of the grain. The grades are: select, standard, rustic, and variegated. Limestone is available in a wide selection of standard machine finishes. DESIGNING FOR ECONOMY AND PERFORMANCE Understanding how limestone is graded and fabricated can help designers use the stone efficiently and reduce initial costs. Consulting with a stone supplier early in the design process can help you develop details that reduce fabrication and erection costs. Designing with limestone requires all the usual performance considerations involved in masonry. HANDLING STONE ONSITE For the most part, setting cut limestone in mortar requires the same careful workmanship as any other masonry. Field procedures differ mainly in handling larger, heavier units, and protecting costly stone from stains and damage. MORTAR SETTING Recommended procedures for mortar-setting cut limestone differ from those used with other masonry units. The first involves the need to support the often greater weight of limestone units. CLEANING LIMESTONE Stone for exterior use usually need not be cleaned before erection. After the stone is installed and all joints are finished, high-pressure washing with plain water is an effective cleaning technique.