Curing techniques fall into two categories: those designed to prevent loss of water, such as application of vaporproof paper or impermeable membranes; and those that supply or retain water throughout the early stages of the hydration process, such as ponding, sprinkling, application of wet burlap, cotton mats, earth, sawdust, hay or straw. Because water has a high specific heat, curing techniques which involve the addition of water also provide good insulation against sudden temperature changes except, of course, during cold weather. Liquid membrane compounds fall under the water retaining category. Their growing popularity is due to the fact that they are easy to use. When an area of concrete is coated with a compound which forms an impermeable skin, the water in the concrete is sealed in and good curing conditions are automatically established. Membrane curing compounds come in three general categories: those having synthetic resin (plastic) bases, those with wax bases, and those with a combination wax and resin base. Curing compounds that also seal and harden a concrete surface generally have a sodium silicate or chlorinated rubber base. Ordinary waterproof building papers are frequently used for curing, but since most of these are usually vapor-permeable they can at best only effect some reduction in the rate of water evaporation from the concrete surface. If paper is to be used for curing, it should be both vaporproof and nonstaining. Paper curing covers usually include a sandwich of asphalt or bitumen, but paper backed plastic and copper sheet types are also available. All are excellent for curing.