SLATES 839 



Of the last of these, Dr. Percy remarks : 



An immense quantity of iron slag, far richer than many iron ores, is annually 

 thrown away, and it may bo that the presence of phosphoric acid in sensible quantity 

 is one of the causes which prevents the re-smelting of this slag to advantage. The 

 fact has not yet sufficiently attracted the attention of those engaged in the manufac- 

 ture of iron. The discovery of a method of extracting economically good iron from 

 these rich slags would be of great advantage to the country, and could not fail amply 

 to reward its author.' 



Numerous attempts have been made to utilize the slags produced in great quantity 

 from our blast furnaces, but hitherto no process appears to have been attended with 

 success. One of the most recent attempts has been that of Mr. Charles Wood, of 

 Middlesbro'. He employs a machine for caking the slag, which is simply a horizontal 

 rotative table on which the slag flowing from the slag-spout of the blast-furnace is de- 

 posited and slowly borne round in a continuous layer of from half an inch to three- 

 quarters of an inch in thickness. The table is composed of thick slabs of iron with 

 water flowing freely through them to keep them cool. The thin layer of slag is solidi- 

 fied by the cool slabs, then water is allowed to flow freely on it, and scrapers placed 

 athwart the table break the friable material into pieces and gather it right away into 

 the waggons. Mr. "Wood attempts to prove by comparative statements that great 

 economy would be effected by the use of the caked slag and slag-sand as concrete, and 

 of the slag-sand mixed with lime as a mortar. The slag- sand is prepared by allowing 

 the molten slag to flow from the furnace into a rotating drum containing water ; the 

 slag falling into the water is disintegrated to a coarse powder. 



Blast-furnace slags have been much used for road-mending, but they do not answer well 

 on account of their extreme brittleness. This fault can, however, be to a great extent 

 remedied by devitrifying them. This is done by allowing them to cool very slowly. 

 The slag, by a process introduced by Mr. Egleston, is cast in huge blocks, which are 

 then subjected to pressure ; after the blocks are cold they are found to possess much 

 toughness, and are said to furnish an excellent material for road-making. 



For many years past the slags of copper furnaces have been used for building 

 purposes, and to a less extent the slags from blast-furnaces. Processes are now being 

 practised, which though somewhat complicated and troublesome, furnish blocks which 

 are completely impervious to damp, possess the necessary toughness, and are admirably 

 suited for the foundation of buildings. 



In one plan, when the furnace is tapped, the slag is allowed to run into a semi- 

 circular vessel, which being on wheels, is readily brought to and from the furnace. At 

 the bottom of this vessel, is a layer of sand and coke dust three centimeters thick. A 

 bent rake or paddle is then employed to mix thoroughly the slag with the sand and 

 cinders, until the gases cease to be evolved, and the mass is nearly solid. The semi- 

 solid mass is then ladled into moulds, provided with iron lids, which are fixed down as 

 soon as no more bubbles of gas appear. When completely solid, but while still red 

 hot, the block is placed in an annealing oven, and covered with coke-dust, so that the 

 complete cooling shall not take place in less than three or four days. 



When the slag contains 38 per cent, and upwards of silica, a serviceable building 

 stone can be obtained from it by simply taking care that the annealing process is 

 sufficiently long. This is in some works effected by allowing the whole of the slag to 

 run down a shoot into a pit lined with sand and ashes, with which it is also covered 

 up. If proper precautions have been taken to prevent premature chilling, it will be 

 nearly ten days before the slag is sufficiently pasty to allow of its being filled into 

 moulds. The blocks are, subsequently, as carefully cooled as in the former process. 



In some parts of Belgium the slag is met, as it leaves the blast-furnace, by a stream 

 of water, with the effect of breaking it up into a powder even finer than sand. This 

 product the puddlers use for making the moulds for their pig-iron, and greatly prefer 

 it to sand. A kind of glass is also made byrunning the slag on iron plates, which are 

 afterwards cooled by the judicious application of water. The slag-powder is also used 

 for mortar-making. Very rapid hardening is said to be thus secured, a point of great 

 importance in the building of foundation-walls and all subsoil erections. Bricks are, 

 in some parts of Europe, glazed by powdering them with slag before drying, and 

 afterwards burning them out of contact with carbon. The glaze thus produced is very 

 perfect, and as the slags are of different colours a variety of tints are obtained. Tiles, 

 drain-pipes, and earthenware generally may be thus treated. It has been tried how 

 far a mixture of clay and granulated slag may with advantage be used for fire-bricks. 

 The results of its use in a brass furnace arc said to have been exceedingly satisfactory. 



SLATES. (Ardoises, Fr. ; Schicfcr, Gor.) The substances belonging to this 

 class may be distributed into the following species : 1. Mica-schist, occasionally used 

 for covering houses. 2. Eoofing slate. 3. Whet slate. 4. Polishing slate. 5. Draw- 

 ing slate, or black chalk. 6. Adhesive slate. 7. Bituminous shale. 8. Slate-clay. 



