In 1871, the waste land for the deposit of slag at the Tees Iron Works being filled up, and the works of the Tees Conservancy having temporarily been brought to a standstill, it became of serious moment to know what was to be done with the slag.
The cost of cooling it, and putting it on board barges for taking out and tipping it into the sea, was so heavy, that it was suggested the slag should be prepared in such a form that it could be tipped into the barges in the same way as coal is done upon the Tyne and other places. To meet these requirements, several schemes were proposed and tried; amongst the first (and only successful one) is the horizontal rotary slag-cooling table designed by Wood, and which, with little alteration, continues to work up to the present time. The machine upon which the slag falls revolves very slowly, and is about 16 ft. in diameter. The top of this table is formed by a series of slabs; these receiving or cooling plates, or slabs, are about 2 ft. in width, each forming a segment of the circle. These plates are kept cool by having a zig-zag wrought-iron pipe cast in them, through which water circulates, being fed from a centre globe; the water, after passing through 2 plates, flows into the basin under the table. These water plates are bolted down in such a way as to be able freely to expand and contract.
The liquid slag, as it flows from the usual runner, spreads itself upon the moving table into a broad band of slag, varying in thickness from J in. to f in., depending upon the quantity and fluidity of the slag. From the point where the table receives the molten slag a distance is traversed of about 10 or 12 ft., to allow the slag to consolidate; after which, water from a jet is made to flow freely upon the surface of the hot slag until it reaches a set of scrapers, when, having become nearly cool, it is pushed off into iron waggons below. When the slag reaches the scrapers, it has become somewhat brittle, and readily parts from the table, and slides off in large flat pieces. When perfectly cold, it is tipped from the waggons, and falls into small-sized pieces, called "slag-shingle." The produce of this machine has found such ready sale that it has been kept going almost constantly ever since it started, and about 200,000 tons have been sold, chiefly for making concrete. In place of paying 6d. per ton to get rid of the slag, it has realized about 1s. 3d. per ton.
The large concrete blocks, each weighing about 230 tons, constructed by Fowler, for dropping into the sea to form the head of the Tees breakwater, are chiefly composed of this material, and several heavy foundations for engines, drainage work, building, etc, in the district, have been executed with it.