After darbying, or bull-floating, wait until the concrete stiffens before proceeding further. When the concrete can sustain foot pressure with no more than one-fourth of an inch indentation, subsequent operations can be undertaken. The waiting period will range from 2 to 8 hours or more, depending on weather conditions, the temperature of the concrete and the nature of the mix. The utilization of correct technique throughout the finishing operations is vital if the floor surface is to wear well. In general, wear resistance increases with compressive strength. This does not mean, however, that by achieving a certain minimum strength good wear resistance is guaranteed. Various finishing factors also contribute to the performance of the floor. Floating has three purposes: it embeds large aggregates just below the surface; it removes slight imperfections, humps and voids to produce a plane surface; and it compacts the concrete and consolidates surface mortar prior to troweling. Magnesium or aluminum tools are preferred for hand floating. Wood floats tend to stick to the surface and to tear it; they should never be used with air-entrained concrete. Troweling immediately follows floating. Its purpose is to produce a smooth, hard surface. Do no trowel an area that has not been floated; preparation with a darby or bull float alone is not sufficient. For the first troweling, whether by power or by hand, keep the trowel blade as flat as possible. Pitching or tilting of the blade at too great an angle will lead to "chattering," which will produce an undesirable "washboard" effect. Floor concrete must be kept continuously wet throughout the curing period. Wet curing methods are preferable though not always the most practicable. Methods include a wet covering of burlap, hay, straw, or a 1 inch layer of sand. Water retaining films and paper and liquid membrane forming compounds are also effective for curing.