"The deep covering of clay which is thrown over all should be taken from the side ditches, and may be in lumps of all sizes, including the very finest material. It is spread as evenly as possible over the top in a layer of not less than 6 to 8 inches. Finally the whole is tamped and rounded off so that the heat will be held within the flues as long as possible. When coal slack is available the two top layers of wood may be omitted and the coal slack thoroughly mixed with the clay.
" It is necessary to get the fires in the flues well under way before the first layer of wood is burned through. The first action of the fire is to drive out the water contained in the clay before the actual burn ing and clinkering can begin. In burning the gumbo clays a great advantage is gained from the organic and vegetable matter which is contained in the clay, as that in itself aids combustion. " " After the firing is completed not only the portion of the clay which forms the top of the kiln but the ridges between the flues should be burned thoroughly, so as to form a covering of burnt clay Io to 12 inches in depth, which, when rolled down and compacted, forms a road surface of from 6 to 8 inches in thickness.
If properly burned, the material should be entirely changed in character, and when it is wet it should have no tendency to form mud, "When the material is sufficiently cooled the road bed should be brought to a high crown before rolling, in order to allow for the compacting of the material. This can best be done with a road grader. After this the rolling should be begun and continued until the road-bed is smooth and hard. The finished crown should have a slope of at least one-half inch to the foot." Slag Roads. Blast furnace slag is used in some localities as a material for surfacing roads. In some instances also slack from coal mines is used in the same way. Where these materials are available, they may provide a cheap method of improving the surfaces of roads of light traffic. Usually these materials are rapidly reduced to powder under any considerable traffic; in some instances, however, slag may be ob tained which is hard and tough and forms a desirable road metal.