CHAPTER II. ON THE BURNING OF LIMESTONE IN THE LARGE WAY. 25. LIMESTONE becomes lime on being deprived of its carbonic acid, and of the water it contained, whether hygrometrically or in combination. agent employed to effect this is heat. The (App. V I I I . ) 26. With the same heat, the calcination is effected with more ease and rapidity in proportion as the stone is of a less compact texture, to the smallness in bulk of the fragments into which it is reduced, and to its being impregnated with a certain degree of humidity. (App. I X . ) 27. The contact of the air is not indispensable, but it exercises a useful influence, especially in regard to argillaceous limestone. Moreover, no limestone can be converted into lime in a vessel so close as to render the escape of the carbonic acid impossible. (App. X . ) 28. Limestone which is pure, or nearly so, supports a a white heat without inconvenience ; the compound limestone, on the other hand, alloyed in the propor tions necessary to form hydraulic or eminently hy draulic lime, fuses easily. certain precautions : a U n d e r the Its calcination demands the heat ought never to be intense heat o f the h y d r o - o x y g e n b l o w p i p e this substance affords the brilliant light, the beautiful application o f which to the m i c r o s c o p e is n o w so well k n o w n . — T R . Downloaded by [ University of New South Wales] on [26/10/17]. Copyright © ICE Publishing, all rights reserved. 14 BURNING [CHAP. OF L I M E S T O N E pushed beyond the common red heat, the intensity being made up for by its duration. 29. The compound limestone, when too much burnt, is heavy, compact, dark-coloured, covered with a kind of enamel, especially about the angular parts; it slakes with great difficulty, and gives a lime carbon ized and without energy; sometimes it will not slake at all, but becomes reduced, after some days exposure to the air, to a harsh powder altogether inert. b 30. The pure and compound limestones when in sufficiently burnt, either refuse to slake, or slake only partially, leaving a solid kernel, a kind of sub-carbo 0 nate with excess of base, which possesses properties of which I shall have to speak elsewhere. b (App. X I . ) This is called dead lime, and should b e carefully distinguished from the sub-carbonates, o r unburnt limes, mentioned in the next paragraph. M r . A n d r e w s ( M e c h a n i c s ' Magazine, N o . 1 2 6 ) notices four cases in which dead lime m a y b e f o r m e d : 1st, when the lime stone contains m u c h alumina, and it has been heated so as to harden the mass; 2nd, when the limestone contains silica, and it has been strongly heated after the expulsion o f the carbonic acid ; 3rd, when the limestone is so strongly heated as to b e in some measure melted, in w h i c h case the carbonic acid is frequently not entirely separated from the central parts o f the large lumps, and they o f course effer vesce with a c i d s ; 4-th, when the limestone is strongly heated after the total separation o f the c a r b o n i c acid, so that the lime neither splits when water is added to it, nor does it effervesce with a c i d s . — T R . c A sub-carbonate is a combination o f one equivalent o f any alkali with less than a full equivalent o f carbonic acid. Thus a combination o f one equivalent o f lime, with half an equivalent o f c a r b o n i c acid ( i f such a c o m p o u n d could b e f o r m e d ) , w o u l d b e a sub-carbonate o f lin*e. T h e meaning o f the term equivalent, is the definite proportion o f acid required to neutralise exactly a given quantity o f alkali, o r o f alkali t o neutralise a given quantity o f a c i d . Downloaded by [ University of New South Wales] on [26/10/17]. Copyright © ICE Publishing, all rights reserved. «•] IN THE LARGE W A Y . 15 31. The calcining of calcareous minerals consti tutes the art of the lime-burner. According to situa tion, either firewood, faggots, brushwood, turf, or coal is used. 32. It would be tedious, as well as useless, to de scribe in this place all the limekilns which have been suggested or tried within the last few years. I shall content myself with saying, that the forms of interior most generally adopted are, 1st, the upright rectangular prism (pi. I. figs. 1 and 2 ) ; 2nd, the cy linder (pi. I. figs. 3 and 4 ) ; 3rd, the cylinder sur mounted by an erect cone slightly truncated (pi. I. fig. 5 ) ; 4th, a truncated inverted cone (pi. I. figs. 7 and 8 ) ; 5th, an ellipsoid of revolution variously curvated, or egg-shaped kiln. (PI. I. figs. 7> 8, 11, and 12.) 33. The rectangular kilns are in use in Nivernais, and in the south of France: they burn in them, at the same time, limestone and bricks. The limestone occupies very nearly the lower half of its capacity. The upper is filled with bricks, or tiles, laid and packed edgeways. 34. The cylindric kilns are principally employed upon works which consume a large quantity of lime in a short time. They are termed " field-kilns" (*' fours de campagne"); their construction is expe ditious and economical, but precarious. Above a pointed ( " ogive") or oven-shaped vault, they raise, in the form of a tower, a high stack of limestone, which they enclose by a curtain of rammed earth, and supT h u s 2 2 grains o f c a r b o n i c acid c o m b i n e with, and exactly neutralise 2 8 grains o f lime ; the equivalents o f carbonic acid and lime, therefore, are always in the proportion to one another o f 2 2 : 2 8 . — T R . Downloaded by [ University of New South Wales] on [26/10/17]. Copyright © ICE Publishing, all rights reserved. 16 BURNING OF LIMESTONE [CHAP. port outwardly by a coarse wattling, in which care is taken to leave an opening to introduce the fire be neath the vault. 35. The kilns of the third kind are constructed in a solid and durable manner, like the four-sided kilns: no bricks are burnt in these; the largest stones occupy the lower part of the cylinder, the smaller pieces and fragments are thrown into the cone which surmounts it. 36. The kilns of the fourth and fifth kind are spe cially intended for burning with coal. 37. The interior wall of the kiln is generally built with bricks, or other material unalterable by heat, cemented throughout a thickness of from thirty-two to forty centimetres ( 1 2 to 15 inches nearly), with a mixture of sand and refractory clay beaten together. 38. In the flare-kilns fed by logs or brushwood, the charge always rests upon one or two vaults built up dry with the materials of the charge itself. Un derneath these vaults they light a small fire, which they gradually increase as they retire, in proportion as the draught establishes itself, and gains force. On reaching the exterior they adjust the aperture at the eye of the kiln suitably, and then keep it constantly filled with the combustible. The air which rushes in, carries the flame to a distance over every point of the vaults : it insinuates itself by the joints, and is not long in extending the incandescence by degrees to the highest parts. 39. There are some kinds of stone which the fire, however well regulated, seizes suddenly, and causes to fly with detonation: we cannot, without running Downloaded by [ University of New South Wales] on [26/10/17]. Copyright © ICE Publishing, all rights reserved. 17 IN T H E LARGE W A Y . II.] the risk of spoiling the charge, use these for the construction of the vaults and piers in loading the kiln. In such a case, we employ for that purpose materials which are free from such an inconvenience. 40. Practice can alone indicate the time proper for the calcination. It varies with a multitude of cir cumstances, such as the more or less green, more or less dry quality of the wood ; the direction of the wind, if it favour the draft, or otherwise, &c. d The master burners usually judge by the general settling of the charge, which varies from ^ to ^. In a kiln 6 of the capacity of from 6 0 to 7 5 cubic metres, the fire lasts from 100 to 150 hours : every cubic metre of lime consumes (on an average) 1.66 steres in fire wood, 22.00 steres in faggots, and 3 0 steres in fascines, furze, or other brushwood/ ( A p p . X I I . ) 4 1 . In the coal kilns by slow heat, the stone and coal are mixed. Of all the methods of burning lime, this is certainly the most precarious and difficult; more especially when applied to the argillaceous lime stone. A mere change in the duration or intensity of the wind, any dilapidation of the interior wall of d General Treussart remarks concerning the burning o f the Obernai lime, that b e i n g calcined with w o o d , it is very difficult t o obtain a homogeneous result; there are always lumps o f lime w h i c h are over-burnt, and others which are n o t sufficiently s o . T h e lime- burners, he adds, ought to keep up the calcination o f their lime for a longer period, but with a less intense h e a t ; this w o u l d n o t c o n sume more w o o d , and a m u c h better result w o u l d b e obtained.— TR. e F r o m 211.8 to 2 6 4 . 7 5 c u b i c feet f A stere is equal t o o n e c u b i c metre, o r 35.3 English c u b i c TR. feet.—TR. c Downloaded by [ University of New South Wales] on [26/10/17]. Copyright © ICE Publishing, all rights reserved. 18 BURNING OF LIMESTONE [CHAP. the kiln, a too great inequality in the size of the frag ments, are so many causes which may retard or acce lerate the draft, and occasion irregular movements in the descent of the materials, which become locked to gether, form a vault, and precipitate at one time the coal, and another the stone, upon the same point; hence an excess or deficiency in the calcination. 4 2 . Sometimes a kiln works perfectly well for many weeks, and then all at once gets out of order without any visible cause. A mere change in the quality of the coal is sufficient to lead the most experienced limeburner into error. In a word, the calcination by means of coal, and the slow heat, is an affair of cau tious investigation and habit. (App. X I I I . ) 4 3 . The capacity of a furnace contributes, no less than does its form, to an equable and proper calcina tion. There are limits beyond which we cannot en large it without serious evils. W e have drawn and marked in plate I., figs. 9 , 1 0 , 1 1 , and 1 2 , the sections of four kilns executed and tried in the Department of the Maine and Loire, by Messrs. Ollivier Brothers, manufacturers of hydraulic limes. Number 9 has been abandoned in consequence of producing lime always too much or too little burnt; number 1 0 answered tolerably; numbers 1 1 and 1 2 answered perfectly. (App. X I V . ) 4 4 . The bulk of coal burnt to produce a cubic metre of lime, necessarily varies with the hardness of the limestone used; but within narrow limits. When not referring to the chalks, or friable marles, we cal culate on an average upon three cubic metres of lime from one of coal. Downloaded by [ University of New South Wales] on [26/10/17]. Copyright © ICE Publishing, all rights reserved. 45. 19 IN T H E LARGE W A Y . II.] The calcination of limestones presents other important problems for solution; but the reasonings of the closet, unaided by experiment, will always be insufficient in a matter of this kind. It is for this rea son, that we here abstain from the discussion of a number of projects more or less ingenious, but wholly theoretical. W e must beg the reader nevertheless to refer to the notes; he will there find some details use ful to be known, when he is called upon to apply him self specially to the burning of lime. (App. X V . ) c 2 Downloaded by [ University of New South Wales] on [26/10/17]. Copyright © ICE Publishing, all rights reserved.