Before the 1870s, when portland cement became commercially available, most masonry structures — the Egyptian pyramids included — were built using only lime and sand. "It's the best mortar ever developed," says Tim Meek, a leading Scottish authority on repointing historic buildings. The key to its superiority is the lime itself. (The ground-up limestone commonly added to masonry cement is something else entirely.) This is kiln-fired limestone, slaked for up to a year, until it turns into custard-smooth brilliantly white putty. Blended with sand, the putty makes a mortar that's permeable to water vapor and flexes with changes in temperature. If hairline cracks form, rain will wash some of the surrounding lime into the gaps, repairing them. Lime mixes are easy to chisel out when the time comes to repoint although, as Meek points out, that time may be a long way off: "I've seen 600-year-old castles with their original mortar, and they're in fine shape." Lime putty comes in one color — white — but mortar comes in many shades and textures. If only color is important, the putty can be tinted in a range of hues using iron-oxide pigments, above. To match a mortar's texture as well as color perfectly, some companies keep stocks of sand, which comes in as many shades as there are beaches. These companies analyze mortar to determine the type of sand they should add to replicate the recipe for the original mortar. They may not be able to track down the exact pit that supplied the original sand, but they can get close.