Indian Science and Technology in the Eighteenth Century: Some Contemporary European Accounts c23.s5.t3: Furnaces
Indian Science and Technology in the Eighteenth Century: Some Contemporary European Accounts c23.s5.t3
- Language
- English
Furnaces
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Their smelting furnaces, though crude in appearance, are nevertheless very exact in their interior proportions, and it has often surprised me to see men who are unquestionably ignorant of their principle, construct them with precision, in so simple a manner; their unit of measure is the breadth of a middle sized man's finger; 24 of which constitute their large and 20 their small cubit; thus there is a constant ratio of 6 to 5 prevailing throughout these furnaces, nor is it of the least consequence, that their dimensions are larger or smaller, so long as all the parts are in the same proportion; the length of these measures is on an average 19.20 English inches for the large cubit, and 16 English inches for the small one.
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As they have no standard measure their fingers, their span, and their arm are substituted by which a piece of stick is measured which they use in practice; neither is the division of the cubit necessary though the large one is supposed to be divided into six parts and the small one into five, of four fingers each—as the measurement is invariably ascertained by their fingers; the length of these parts is on an average 3.20 English inches.
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Geometrical Construction of the Furnace
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To construct the outlines of the furnace geometrically (Diagram I: fig 1 and 2) rule an indefinite line A.B. which suppose equal to a large cubit of 24 digits or 19.20 English inches, and divide it into 6 parts; at C erect a perpendicular, then from C to E set off 6 parts and it will mark the central point of the greatest bulge, and consequently the point of greatest heat; next, from E to F set off 6 more points, and it will mark the point of cremation; then again from F to G, 6 parts more, will mark the line, where it is necessary to recharge the furnace, after the burden has sunk thus low, and from G to D—two parts more; will give the perpendicular height of the furnace, in 20 parts equal to 5 feet 4 inches of English measure.
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To complete the figure, rule lines parallel to the base, through the points E, F, G, and D, and from D, fig 1, set off three parts to the left hand for the top; bisect it at J, bisect also the bottom at H, draw H, J, right angled at K, and it will be the oblique axis of the furnace (fig 1. K—J) bisecting all the parallels corresponding with CD (fig 2)—then make the parallels AB six parts,—E six parts, F five parts, and D three parts; rule lines through all these points, and the geometrical outline will be completed, the sum of the parallels in parts, corresponding with those of the perpendicular.
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*Practical Construction of the Furnace*
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To construct it practically—dig a fosse 3 feet deep in the annexed form (Diagram 3, fig 2) the semicircular part of which contains the furnace B, the walls CCC being composed of unburnt bricks of large dimensions; the first structure is crude, preserving only an approximation to the required form, the interior being afterwards cut away; a large stone capable of containing heat is placed at the bottom; and in this state it is suffered to remain until thoroughly dry; the next operation is performed by a more skilful artist who cuts away the interior, and plasters it with clay, using the measures above described to adjust its dimensions; he first finishes the top, and from the centre of the back part of it, he drops a plummet, to ascertain the spot where the centre of the front part of the stone is to rest; this plummet line corresponds with the perpendicular CD of the geometrical figures 1 and 2—and thus he obtains not only the required obliquity of the furnace, but the points most essential for the adjustment of all the rest.
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When the furnace is thus far prepared it is again suffered to dry, and in the mean-time, other appendages called by the Indian smelters *Gudaira*, *Pachar*, *Garrairi*, and *Akaira* (names which have no synonyms in the English language), are constructed; the Akaira in particular is a most extraordinary implement, (Diagram I, figs 4 and 5; and Diagram 2, fig 1+); externally viewed it is a clumsy mass of clay enveloping the wind tubes (Diagram I, fig 9) but when it is considered that the complete fusion of this mass, and the perfect completion of the smelting process must be simultaneous results, the implement becomes the most important of all the appendages; thus for instance if it is too small, or too large, its effect will immediately be perceived; in the former case the masset of crude iron will be full of impurity, and in the latter the iron will be consumed, and if it cracks during the operation of smelting, there is no remedy for such an accident—short of dismantling the furnace and commencing the work again.
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I found after numerous experiments that its mean length should be 4½ parts, its mean breadth 3 parts, and its mean thickness 1½ parts and it is somewhat remarkable that the product of these dimensions, should exactly equal a twentieth part of the cubic contents of the furnace when fitted for use; this coincidence may arise from the peculiar nature of the clay of Tendukaira,³ the ingredients of which are well assorted, but the rule will nevertheless apply generally—because clay by admixture is susceptible of being rendered amenable to rule; and therefore this implement will be found in all Indian furnaces to have (or by tempering the clay may be made to have) the same corresponding dimensions.
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The Guddaira is a wedge of clay used to adjust the vertical position of the Akaira when placed in the furnace; and the Pachar is an oblong plate of clay, used in walling up the orifice after the Akaira is placed, and adjusted; these figures and dimensions are given in Diagram I—fig 7 and 8; the Gurairy (Diagram I, fig 6) is a convex plate of clay; perforated with holes and used as a grate—through which the scoria are drawn off.
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When the appendages are ready, and the furnace thoroughly dry, it is prepared for use in the following manner.
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The front part is walled up from the top to the line SS—Akaira to the top (Diagram I, fig 1 and 3) which line is ascertained by the small cubit; one end being placed at C, the other will measure CB and CS (fig 1)—the grate is next put in, its lower edge resting upon the edge of the stone; and the space is filled—with a mixture of pulverised cow dung and Kodo straw—up to the dotted line (Diagram I, fig 1) upon which is placed the Akaira; its sides being every where 1½ parts distant from the walls of the furnace—as represented in Diagram I—fig 4 and Diagram II—fig 1+; where a, b, c, d, are the walls of the furnace, fig. 5 and 1+ the Akaira; the Gudaira⁴ or wedge, is next introduced in order to adjust its vertical angle (Diagram I, fig 1) and this being placed satisfactorily, the Pachar is inserted and the whole has then the appearance represented in Diagram I, fig 3, where No.5, 6, 7 and 8, are the Akaira, Gudaira, Pachar, and Garrairi; nothing now remains but to lute the whole with clay, leaving the ends of the wind tubes open to receive the bellows.
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*Bellows*
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The bellows are as singular in their construction as the Akaira, and are worked by the hand; they are made of a single goat skin, the dimensions of which ought to be 7 parts in breadth when doubled, and 8 parts in length; such proportions being required for circular bellows of 5 parts diameter, and which when worked by a man of ordinary strength will rise 6 parts in height—having 11¼ circular folds; the wooden nozzle through which the blast is conveyed into the furnace surfaces the Akaira in its complex nature, and so little is its principle understood—that the art of making it was once lost at Tendukaira, and was again restored by the smelters of Katola.
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*Construction of the Nozzle of the Bellows*
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To construct its figure geometrically, rule a line AB equal 3 parts (Diagram II, fig 2) divide it into four, giving one of those divisions to each of the legs, and two for the space in the centre; set off a perpendicular from C to D equal 3 parts; bisect it and the middle point will mark the apex of the central angle; then through the point D rule a line parallel to AB and from it as a centre set off each way ¾ of a part making together 1½ parts; divide it also into four, giving one of each to the legs, and two for the space in the centre as before; and then by ruling lines to connect all these points, the outline will be complete; the exterior of the implement is plain but the interior is complex and cannot be described except by a reference to Diagram II: fig 3 which represents it, divided in the middle, to show its internal structure.
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This curious appendage is fastened to the bellows by leathern thongs, and the blast is forced through it at an angle of 24 degrees but when it is luted to the wind tubes of the Akaira, the blast enters the furnace at an angle of 12 degrees, both vertically and horizontally—because those tubes are placed so as to reduce that angle; Diagram II, fig 1+ represents the whole apparatus luted together and placed in the furnace the walls of which are marked a, b, c, d, and it exhibits at one view, the whole of the mechanism of this complex machinery; the furnace when closed up with clay, and the bellows luted in, is represented entire in Diagram III and IV; the dotted lines showing the chimney—A the outer walls, B, a mound of earth to strengthen the walls, C an upper chimney of moveable bricks, D planks laid across the trench to support the bellows and the man who works them, E a stone supporting one end of the plank, F forked branches supporting an iron bar on which the other end of the planks rests, and G a simple apparatus for preventing the bellows from rising from the planks when they are worked.
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The above description is not founded on theoretical conclusions; the measurements given are derived from taking the mean of several and the results were proved in furnaces under my own superintendence; the coincidences of the several parts are very striking, thus for instance, the perpendicular and parallel lines of the geometrical outline are equal in quantity (Diagram I, fig 2); and the top, bulge and bottom being 3, 6, and 4½ parts respectively, show that the furnace is exactly constructed, and that it corresponds well enough with the most regular furnaces of Europe (Diagram I, fig 1); it is also curious, though perhaps of no importance to observe, that the mean of those numbers, being squared and multiplied by the terms of the perpendicular or axis, give the cubic area of the furnace, and show that it is twenty times larger than the cubic content, of the Akaira; the angle of the blast is also worthy of notice, as well as the simplicity by which both it and the obliquity of the furnace is obtained; all these serve to show that the original plan of this singular furnace must have been the work of advanced intelligence, and that its geometrical proportions have been preserved by simple measures; hence though its original form may be changed by caprice or ignorance, its principle never can be lost so long as hands and fingers remain.
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DIAGRAM IV