Lime production is mankind's oldest industrial process, dating back many thousands of years. The people of ancient Mesopotamia and other lands found that burning limestone at high temperatures, letting it cool, and then soaking it in water produced lime putty. This putty was mixed with volcanic ash, sea shells, or river sand to produce the first lime-sand mortars. WHAT IS LIME? Lime is made by heating limestone to at least 1,650 degrees Fahrenheit. The heat drives off carbon dioxide (and any water), leaving calcium magnesium oxide, more commonly known as quicklime. The quicklime was then placed in a pit or large wooden box and mixed with water, a process known as slaking. During slaking, hydroxides of calcium and magnesium are formed by the action of water on the oxides. The resulting lime putty was left to mature for several months or even years. The lime putty was then mixed with sand by proportion (1 part lime putty to 3 parts sand) to produce masonry mortar. Lime putty was used as the primary mortar ingredient until the 1930s, when the advent of pressure hydration technology moved the slaking process from the jobsite to the lime plant. The resulting product is what we know today as Type S (special hydrate) hydrated lime. TYPES OF LIME There are three different types of lime--dolomitic, high calcium, and magnesian--deriving from three different kinds of limestone. Today, hydrated lime used in masonry mortars must meet the requirements specified in ASTM C 207 Standard Specifications for Hydrated Lime for Masonry Purposes for Type S hydrated lime. CHARACTERISTICS OF LIME MORTARS Adding lime to masonry mortars produces several effects: increased water retention; increased sand-carrying capacity; self-healing; improved tensile bond; and flexibility.