For Paving-Blocks

There is, however, one exception to the numerous failures in slag-casting. It is known as Woodward's patent, and although there is absolutely nothing new in the process, still, through the perseverance of Dobbs, the late manager and engineer for the furnaces of T. Vaughan and Co., an amount of success has been arrived at sufficient to enable the company which works the process to pay a fair dividend. The success has been eminently a practical one, and appears to rest mainly upon 2 points:-Firstly, in the quickness with which the castings are removed from the moulds and placed in the annealing ovens, where the temperature is constantly kept up nearly as high as the melting-point of slag, the heat, after the ovens are full, being so gradually lowered that the outside of the casting cools at the same rate as the inside; the contraction is thus equalized throughout, strains upon the outside are avoided, and the fine surface cracks do not penetrate much below the skin. And secondly, upon the fact that only solid rectangular blocks, with a certain amount of bulk in them, are attempted.

At the works of the Tees Scoria Paving-block Co., blocks are made by running the liquid slag into a series of open-topped moulds. The moulds are of cast iron, and are held by one end upon the periphery of a horizontal wheel or table. The wheel is suspended by tie-rods upon a central pillar. The moulds, when being filled up, are brought in succession under the slag-ruuner by the man in attendance, who watches until the mould is full. When the slag has become consolidated in the moulds, a catch-hook is knocked up, the moulds fall to pieces, and the bricks drop to the ground. When they come out of these moulds, although consolidated, they are still in a sort of half-molten state, and are immediately removed into annealing ovens, which are always kept at a high temperature, so that the blocks receive no chill.- The ovens are of small size, -and when full are sealed up, and allowed to cool down by themselves. There are about 70 moulds upon each machine, and the hotter these are kept the better; whilst, to prevent chilling of the molten slag as it runs into the moulds, they receive a thick coating or washing of chalk or lime after each casting, the lime acting as a non-conductor as well as assisting the block more readily to drop out of the mould.

Thus the casting is not allowed to remain in contact with anything which can extract its initial heat, so as to produce unequal cooling; and, as before stated, the whole success has been eminently a practical one, and reflects great credit upon those who have so patiently worked it out. Large quantities of these bricks or paving-blocks are used in the North of England for crossings, stables, yards, and streets, possessing durability, uniformity, and good general appearance when well set. From a series of tests recently made against a crushing strain, some of these blocks carried a weight equal to the hardest granite.

For Bottle-Glass

The next successful process for dealing with molten slag is Bashley Britten's. He converts it, by a kind of compound process, into glass for bottle-making, and for many purposes where a pure white glass is not essential. The slag is taken from the blast furnace in large ladles upon wheels, in quantities of about 500 lb. In this state it can be conveyed a considerable distance to the glass works, where it is poured into a Siemens regenerative gas-furnace, known as the "continuous melting tank furnace," arranged to work with gas made by a Wilson's gas producer, and considered to be a great improvement upon the furnace employed at the slag-glass works at Finedon. The material is fused and amalgamated in a melting-tank. The fluid metal, becoming fused, flows through a bridge into a secondary chamber called the gathering basin. The glass is withdrawn from this basin through a series of holes by the workmen, and fashioned into bottles, or other useful articles-, in- the usual way. By this arrangement the work of charging and withdrawing the liquid glass is continuous and proceeds uninterruptedly. The consumption of coal per ton of slag-glass should not exceed 10 to 12 cwt.

With each charge of molten slag into the melting-tank, alkalies and sand, and colouring or decolorizing material, are added in proportion depending on the quality, colour, and composition of the glass required.

So far, the only slag operated upon is that produced from the Finedon furnaces in Northamptonshire, a very siliceous slag, the analysis of which is as follows:-

To make bottle-glass equal in quality and appearance to French champagne and claret glass, about 50 Per cent. of slag may be used; for plate-glass, the same proportion, or rather less of slag; but for glass for heavier articles, a much larger percentage can be adopted. Bottles made from slag-glass are stronger than those manufactured in the ordinary way from the usual materials, and will stand 320 to 350 lb. per sq. in.; half-bottles (pints), 420 to 450 lb. per sq. in. Slag-glass, owing to its toughness, is especially suitable for manufacturing into tiles, cisterns, plates, slates, etc., for which glass is not now employed. The chief points of merit claimed for the process are the utilization of a waste product, economizing the heat of the molten slag, and converting it, with additional materials, into good glass, quicker, and at less cost, than by the processes generally employed.