(Consult Chapter XIII for method of mixing and making concrete and installing forms.) Be sure to use good clean sand and aggregates, for dirt and dust and other matter may cut down or destroy the effectiveness of the concrete. If your sand and gravel piles seem very dusty and full of dirt, they can be hosed down to wash them. Wash off the drainage section, too, using a fine spray so that when you pour your con- crete on top of it there will be adherence with the drainage material; washing just before pouring concrete will also keep moisture from being drawn quickly out of the concrete by the drainage base, because the drainage material will be wet.

After the concrete is poured and leveled, cover it with boards, building paper, or wet sacking held off it by strips of wood to prevent its drying out too rapidly. In cool weather it may be simpler merely to shade it to keep the moisture from drying out too quickly.

Remove the covering and test it each day. When it has set enough to support your weight but is not yet really dry (usually two days in summer, and two to four days or more in the cooler times of the year in most places), you will be ready to begin your mosaic work. Mix enough mortar to cover a section of about 12 to 18 inches-a bucketful will be enough to cover a square of this size. Spread it on the concrete, trying to make the edge of the mortar coincide with the edge of the pattern or design you are using, so that the next unit can be joined without an obvious joint showing. Some craftsmen use a board or a piece of heavy metal bent into shape to contain the mortar; others merely place the mortar roughly, leaving an edge which can be cut cleanly away before it dries so that fresh mortar can be placed to abut with and bond to the finished section.