The next great step in advance, and which laid the foundation for several processes hereinafter mentioned, was the reduction of the molten slag, as it flows from the furnace, into a soft spongy kind of sand, by a machine known as Wood's slag-sand machine. In principle it is the reverse of the slag-shingle machine, inasmuch as, instead of the wheel being horizontal and the slag running upon a dry table, the slag flows into a wheel placed upon its edge, and falls into a bath of water, varying in depth from 18 to 24 in. The wheel or drum is of wrought-iron, and about 14 ft. in diameter. It is fixed and carried on curved arms. The arms are curved to allow, in the first place, the slag runner or spout to enter the wheel; and secondly, to make room for the sand-receiving spout on the opposite side at the top. The wheel makes about 5 revolutions per minute, and the water contained inside is partly carried up by the elevators, and in falling causes a constant rush of water to the bottom. Perforated screens, or elevators, are arranged to screen the slag from the water, and lift it to the top of the machine, where it drops upon the sand-receiving spout, and thence slides in a constant stream into wooden waggons.

The spout is also perforated, to allow any water which has been carried over with the sand to return into the machine. The perforated buckets have another important function to perform, viz., that of agitating the water. The water, in rushing to the bottom, meeting these obstructions, rolls over in a violent manner, and into this agitated water the liquid slag flows just as it comes from the furnace. The united action of the agitated water and the formation of steam, scatters, as it were, the molten slag in the water into the material called slag-sand. The wear and tear of this machine is very light, there being no working parts coming in contact with the sand or the heat. The heat, being taken up by the water, is thrown off in the shape of steam, which comes away in large volumes. Grey slag takes up about 20 Per cent. of its own weight in water. The total cost of this sand in railway trucks is about 6d. per ton.

On the Continent, a kind of slag-sand has been made - prior to the adoption of the process just mentioned - by running the slag into tanks full of water, and elevating the sand by chain buckets into waggons; but the apparatus is very imperfect, and will only work slag made from forge iron, known as black slag.

The application of slag-sand, in so cheap a form, to the useful arts, naturally followed the production, and after numerous experiments, extending over many months, it was decided to establish separate works in close proximity to the furnaces, where, under Wood's, own direction, various processes could be developed; and in 1876 the first manufactory of the kind was started. In Georgemarienhutte, in Hanover, under the direction of Luurnan, a process of brickmaking was initiated a few months previously.

The remarkable setting properties of slag in a state of subdivision has attracted the attention of scientific men for years, and many schemes for producing artificial stone, cement, etc, have been tried; but, in consequence chiefly of the cost of disintegration, no results were obtained with commercial success.

John Gjers, of Middlesbrough, about 15 years since, produced a coarse kind of slag-sand, which, after grinding under edge-runners, was used extensively for some little time upon the pig-beds; but it had to be abandoned, because it con-solidated too much, causing violent explosions (technically termed "boils "), from the steam from the damp sand being unable to escape when the metal was run from the furnace in pigs.

For Cement

Up to the time when the Cleveland Slag Works was started, there was not a single instance of slag utilization in this country - otherwise than for road-making or for river-work -commercially carried on. Before proceeding to describe the various manufactures produced at the Cleveland Slag Works at Middlesbrough, it is necessary to draw attention to the chemical nature of the material operated upon. The following analysis gives a good general idea of the chief slags produced in the United Kingdom:-

Cleveland.

Hematite Bessemer.

Dowlais.

Dudley.

Lime. • . .

32.61

50.55

30.47

35.68

Silica. . .

36.50

30.50

43.07

38.76

Alumina . .

22.95

15.00

14.85

14.48

Iron protoxide .

0.06

0.45

2.53

1.18

Manganese pro-toxide. .

0.32

0.10

1.37

0.23

Magnesia . ,

5.83

2.00

5.87

6.84

Potash . .

0.59

0.40

1.84

I.11

Soda ....

0.31

0.20

• •

• •

Sulphur. .

1.73

1.60

0.89

0.98

Phosphoric acid

100.90

100.70

100.89

99.26

Less oxygen of the lime com-bined with sulphur . .

0.86

0.75

0.44

..

100.01

99.95

100.45

• •

A table of comparative analysis is given on the next page, for easy reference. It will be noticed that 3 most important component parts of these slags are silica, alumina, and lime, forming, as they do, about 90 Per cent. of the whole. The latter of these, however, chiefly exist as silicates; if to these, caustic lime be added, they are acted upon, water of combination or crystallization being taken up; and if the material be kept damp and exposed to the air, hardening or induration is carried on for months.

-

Slag.

Portland Cement.

Slag Concrete Bricks.

Slag Cement.

Gypsum.

Puzzolanas.

Hematite Bessemer.

Cleveland.

Dowlais.

Dudley.

Lime..........

50.55

32.68

30.47

35.68

60.88

29.90

22.90

32.32

8.00

Silica.....

30.50

36.50

43.07

38.76

23.16

25.15

21.60

0.35

. .

Alumina.......

15.00

22.95

14.85

14.48

7.68

21.80

19.85

• •

• .

Iron protoxide ....

0.46

0.06

2.53

1.18

• •

1.44

4.00

• •

• •

Manganese protoxide • .

0.10

0.31

1.37

0.23

• •

0-26

0.21

• •

• •

Iron peroxide...........

..

..

..

..

3.00

1.66

8.80

• •

12 to 15

Magnesia........

2-00

5.83

5.87

6.84

1.01

5.10

4.36

• •

• •

Potash ............

0.40

0.59

1.84

1.11

0.72

0.53

0.50

• •

. •

Soda ..........

0.20

0.37

• •

• •

0.31

0.36

0.32

• •

• •

Sulphur............

1.50

1.73

0.89

0.98

0.05

1.00

1.19

• •

• •

Sulphuric acid.............

..

..

..

..

2.60

1-25

1.54

46.18

. .

Phospuric.........

..

..

..

..

0.08

0.01

0.02

• .

• •

Carbonic acid........

...

..

..

..

2.60

3.00

• •

• •

Water (of crystallization) .

..

..

..

..

0.77

9.50

12-00

21.00

• •

If caustic lime be added to slags poor in lime, so as to bring this element up to 55 or 60 per cent., it will be seen at once how closely it will resemble the analysis of Portland cement, the composition of which is as follows : -Lime, 60 per cent.; silica, 24; alumina, 8; iron oxide, 4. German Portland cement is sometimes made with as low as 55 Per cent. of lime, whilst Roman cement has often only 50 Per cent. of lime; but these will generally be found to contain oxides of iron in an increased proportion.

The remarkable hardening effect of oxides of iron in conjunction with lime, silica, and alumina, is well known, and is well exemplified in the Italian puzzo-lanas, where, in several of the best qualities, the lime is actually as low as 8 per cent., whilst the oxides of iron run up to 12 or 15 per cent. The hardening effect of iron oxides induced Wood, prior to the development of the slag industries, to employ the dust from the ironstone clamps in place of sand, when making concrete for heavy foundations and the setting properties and strength of this combination have upon examination been fully confirmed. Again, having to erect a row f columns for a large roof upon the bed of an old ironstone clamp, the floor of which had been accumulating for several years, it was found to be so extremely hard that Wood simply levelled the bed down, and set the columns directly upon it. These, after many years, show not the slightest signs of settlement, although the ground underneath had been made up from ships' ballast.

It appears an absolute necessity for obtaining good results, that the ferruginous material should be calcined or roasted, the effect of which is to drive off the carbonic acid and water; the re-absorption of the water, which unites in chemical combination with the material, afterwards assists in hardening.