394 Dr. F. H. Hatch — Iron-ores of the United Kingdom. 



Calcining raises the percentage of iron considerably. If the loss on 

 calcination is 25 per cent, the increase is one third of the percentage 

 of iron in the raw stone. Thus, the 32 per cent, which is the average 

 iron content of the raw Northampton stone, becomes 43 in the calcined 

 material ; the 28 per cent of the Cleveland stone becomes 37 ; the 

 24 per cent of the Oxfordshire stone, 32 ; and so on. It follows 

 that the use of calcined stone increases the output of pig-iron 

 per furnace. Furthermore, there is a saving of fuel in the blast- 

 furnace. 



The calcining is either effected in Gjers or Davis Colby kilns, with 

 or without forced draught, or in open heaps — "clamps", as they 

 are termed in Northamptonshire. It is a slow roasting process, 

 and the fuel required is about \\ cwt. of coal to the ton of 

 ironstone. 



The Jurassic ironstones, although poor in iron, are valuable because 

 of their considerable thickness and widespread occurrence at only 

 a slight depth below the surface. "With the exception of the 

 Cleveland district of Yorkshire, where the ironstone is now mined 

 underground, the workings are almost everywhere at the surface, the 

 ironstone being quarried after stripping off an overburden of soil, sand, 

 or clay as the case may be. Since the angle of the dip is usually small 

 or, in other words, the beds are practically horizontal, considerable 

 areas can be worked before the overburden becomes too great for 

 removal at a reasonable cost. As much as 60 feet of soft material 

 (sand or clay) can be removed and, under favourable conditions, 

 probably 100 feet will be removed. 



The different beds of ironstone vary considerably in thickness. 

 The thickest is the Frodingham bed in North Lincolnshire. This 

 ironstone is 25 to 30 feet in thickness, and consequently can be 

 worked very cheaply by mechanical excavation. Before the War the 

 cost of the stone in wagons at the quarries (exclusive of royalty) was 

 not more than 1*. per ton. Probably it is double that now. 



The workable part of the Northampton ironstone varies from 6 to 

 13 feet, averaging about 8 feet. The lower portion of the bed is 

 generally too poor in iron for economic extraction. During the early 

 part of the "War it was. sold at 3s. 3d. per ton of raw stone. 

 Subsequently the price was fixed at 3s. 9^. per ton, plus %d. 

 for every Is. 3d. rise in wages above the rate current on the 

 12th November, 1917. 



The Cleveland Main Seam varies from 5^ to 12 feet. It is mined 

 at depths ranging from 100 to 600 feet, the mines being worked on 

 the bord and pillar system. The cost of the stone at the pit's mouth 

 is now about 10s. per ton, as against 2s. to 4s. for the quarried 

 stone. 



The output per man employed on surface and underground in the 

 Cleveland mines averages 2 - 2 tons per shift as against 4 to 15 tons 

 for the quarried stone. 



In addition to the Main Seam, other ironstone beds both above and 

 below the Main Seam, have from time to time been worked on a 

 small scale, in the Cleveland district. Their geological relation to 

 the Main Seam is shown in the following table : — 



