830 HYDRAULIC CEMENT 



or extra heat, which would do little harm to Dorking lime, greatly injures lias by 

 forming a glass between the silica and the lime in the stone, instead of only driving 

 off the water and carbonic acid. The combination between the silica and lime, to 

 which lias owes its hydraulic properties, ought only to take place in the humid way 

 i.e. with the assistance of water, after the application of the lime as mortar or 

 concrete. Lias conies from Lymo Regis in two different forms, the one with a clean 

 conchoidal fracture, and the other of a shaley nature, approaching in appearance even 

 to clay-state, but quite soft. The shaley lias, which contains so much clay as to 

 have the properties of a cement, is not so desirable as the hard clean stone, because 

 it carries less sand, and is therefore more expensive. The stone cost 4s. 3d. a. ton 

 when shipped at Lyme, but 10s. 9c?. before it was stacked round the kiln at London, 

 which is as much as the same stone cost delivered at the works of the New G raving 

 Dock at Leith. Freight to London is always heavy, for there is no steady return 

 freight, like coal, to be had. Notwithstanding the high price of the stone delivered 

 at Shadwell, and having to pay freight on thousands of tons of water and carbonic 

 acid, to be afterwards driven off by the heat of the kiln, the engineer-in-chief of the 

 Dock Company, the late Mr. Rendel, determined to burn the limestone in London, 

 as the extra cost would be a comparatively small item in such extensive works. It 

 was very desirable to have the best possible lime where concrete was to play so im- 

 portant a part. 



Two egg-shaped draw-kilns of brick were therefore erected, of a total height of 

 43 feet from the floor to the top of the dome, and an extreme internal diameter of 

 14 feet, contracting to 5 feet at the fire-bars, and 11' 6" at 32 feet above the floor. 

 The practical objection to having a less diameter than this at the top, is the difficulty 

 that would arise in charging the kiln with evenly-distributed layers of coal and stone 

 tipped in from barrows at the top through the windows in the dome. It has likewise 

 been objected that a contracted top to a kiln prevents the escape of carbonic acid, 

 although it has a counterbalancing advantage in confining the heat, and throwing it 

 down with a reverberatory effect on the charge. This theoretical objection appears to 

 be fanciful. It was found necessary, after these kilns had been working for a short 

 time, to dome the mouth over with a brick arch, to prevent stones from flying into the 

 neighbouring streets. The chimney at the top of this was only three feet in diameter, 

 and yet no difference was afterwards perceptible, either in the time required for 

 burning, or in the percentage of imperfectly-burned stone. On the other hand, there 

 is little doubt that the dome acted economically on the coal required, for the average 

 of 7,000 tons of limestone was little more than one ton of coal to 11 J of stone, the 

 limits being 1 to 10, and 1 to 13. Carbonic acid came away freely after the kiln had 

 been lighted for three hours, and it appeared to have ample room for escape. An 

 average of 11| tons of stone burned by one ton of coal is very high ; but the coal was 

 Welsh, and cost ll. Is. a ton. Newcastle coal, or bituminous coal in general, was 

 inadmissible ; for it was essential to have little or no smoke from kilns in the heart 

 of London. At Leith, one ton of the coal of the neighbourhood appears to burn only 

 six or seven tons of lias, but the coal is only one-third of the price of Welsh coal. 

 The barrows of coal and properly broken stone were lifted to the top of the kiln by a 

 hoist worked by the mortar mill engine, and were tipped in through three openings 

 in each dome, as evenly as possible. After the kiln was lighted, these windows were 

 kept closed with boiler-plate shutters. The cost of charging, including breaking up 

 the stone and coal, was Is. 6d. per ton of the two when mixed in the kiln. Unless 

 the demand for quick-lime was very irregular, the kilns were always kept lighted ; 

 but whenever they were allowed to go out, the charge of quick-lime was left in the 

 kiln, as the place most free from moisture. Whenever the fire is let out in draw- 

 kilns, the next charge is nearly sure to burn irregularly, and there is a considerable 

 loss of heat in re-warming the kiln. Draw-kilns are liable to irregularity from ap- 

 parently slight causes, such as the direction of the wind, &c. ; and in^ the Shadwell 

 kilns there was also a permanent tendency to burn quickest down the side warmed by 

 the adjacent kiln, for they were both in one block of building. But draw kilns are 

 better suited for burning lias than flare kilns, as the heat is more uniformly distributed 

 through the charge ; there is, therefore, less danger of over-burning the lower half and 

 under-burning the upper. 



Each kiln held 100 tons of stone, and burnt 21 tons per diem. The two together 

 produced 25 tons of quick-lime every day, a quantity sufficient for about 97 cubic 

 yards of mortar, or 170 cubic yards of concrete : 9 tons of coal will burn 100 tons of 

 stone, which produce 59'37 tons of quick-lime, or 1,583 bushels of ground lime, 

 enough for 400 cubic yards of concrete, when the ballast is moderately dry. I)IM\V- 

 ing the lime from the kiln cost \\d. per ton of quick-lime. The total mst. of the 

 burnt lime amounted to 24s. per ton. When quite hot from the kiln, 2G.} bushels of 

 ground lime went to the ton ; but after keeping some time, a ton swelled to 30 



