Q.: Specifications for a concrete deck for a swimming pool call for a keystone finish. What is it?

A.: A keystone finish is sometimes also called a travertine finish. It's created by applying a dash coat of mortar over freshly leveled concrete. The dash coat is mixed to the consistency of thick paint and usually contains a yellow pigment. It is applied in a splotchy manner with a dash brush so that ridges and depressions are formed. After allowing the coating to harden slightly, the surface is flat-troweled to flatten the ridges and spread the mortar. The resulting finish is smooth on the high areas and coarse grained in the depressed areas, resembling the appearance of travertine marble.

A similar texture can be produced by scattering rock salt over the surface after floating or troweling a concrete surface. The salt is rolled or pressed into the surface so that only the tops of the grains are exposed. After the concrete has hardened, the surface is washed and brushed, dislodging and dissolving the salt grains and leaving pits or holes in the surface. The salt distribution and grain size should be such that holes of about 1/4-inch diameter are created.

Neither the keystone nor the rock salt finish is recommended for use in areas subject to freezing weather. Water trapped in the recesses of these finishes tends, when frozen, to spall the surface.

The above description is taken from Cement Mason's Guide to Building Concrete Walks, Drives, Patios, and Steps, published by the Portland Cement Association. The booklet also includes photographs of the finishes. Price is $4.75 plus $1.00 for postage and handling. To order it, contact the Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077; telephone 312-966-6200.