Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/22

 6 THE BUILDING NEWS. Jan. 5, 1872. quired to be greater as the height increases. No safe reliance can be placed on the resist- ance of the mass of earth behind it, for, whatever the intention of the engineer might be in specifying that a weak abutment should be so backed up as to assist in resisting the thrust of the arch, accidents of weather or some otherunforeseen occurrence would often render such intentions nugatory ; the abut- ment, then, is required to be made sufticiently strong to resist the thrust of the arch. But for piers which carry a series of arches other considerations are to bes had. It is always desirable to divide a long viaduct, or a bridge consisting of more than one opening, into a number of arches of equal span, and when this can be done the piers have little other force to resist than the vertical weight of the arches and their extraneous load, the hori- zontal thrust of one arch counterbalancing that of the adjoining one, and, by the insist- ance of its dead weight, contributing greatly to balance the thrust produced by a passing load over the adjoining arch. It would not, however, be a complete balance if the thickness of the arches of a series were to be calculated on the same data as the thickness of a single arch is calculated upon. The thickness of piers, then, unlike abutments, is ina great measure independent of the height, and depends more upon the sort of foundation that can be given to them, a weak foundation requiring a wider pier than one of firm rock. ABUTMENT “~_ ——=0; The impost of an abutment or pier is that part of either upon which the arch immedi- ately rests; and when stone is used with brickwork for the sake of appearance, the impost is the course of stone on the top of the abutment or pier. The springing is that line in which the curved surface of the intra- dos of the arch intersects the face of the abutment or pier. The intrados is the whole 2. ° ~ ~e internal surface of the arch from one spring- ing to the other. The soffit is the upper part of the intrados, immediately under the crown, and is of no certain extent, but is usually understood as extending for a short distance on each side of the centre. The extrados is the back of the arch from side to side. The spandrels are the spaces between the haunches of adjoining arches. The haunch is the quarter of the arch lying midway between the abutment or pier and the crown. ‘The crown is the very top of the arch ; and as arches are always built with equal progress from the two springings towards the crown, the last course of stone or brick is called the key- stone. eo DECORATIVE PROCESSES. GILDING. By AN EXPERIENCED WORKMAN. (Continued from page 459, Vol. XX.) - our last article* we described the pro- cesses used in gilding on glass, as practised by glasswriters, japanners, &c. We now proceed to describe the processes used in the various branches or methods of gilding in connection with our decoration. It will be at once evident that the variety of objects upon which gold in one form or another is used as a means of decoration, and all of which require a different process in the work- ing in order to accomplish the end desired, SPRINCING would require a separate treatise to describe fully. We shall therefore confine our remarks here strictly to those methods of gilding in which gold or silver leaf, and gold and bronze powder, are used. We have no record of the origin of the goldbeater’s art, yet there can be no doubt but that gold was beaten out thin and used for gilding purposes at a very early or remote period of history. Most if not all the great nations of antiquity used gold leaf, or perhaps we may more correctly say thin gold plates, as a means of decorating their furniture and dwellings, but it is not until comparatively recent times that gold has been beaten out to anything like the thinness we have it at the present time. Juyvenal, in his thirteenth Satire, notes a roguish practice : Some pilfering knaves will try From Neptune's cheek, or great Alcide’s thigh, To scrape the gilding ; or from Castor steal All of his plating that their hands can peel. Gold, before it appears in the form of léaf gold, undergoes certain processes, which we shall here only describe in brief, but with sufficient clearness and detail to enable the reader to comprehend the method of its pre- paration, and give some idea of its nature and qualities. Goldis in its nature capable of being beaten out so thin that it will lose its tenacity, and in consequence of this and other properties requires a certain amount of alloy in order to counteract this tendency. The quantity of alloy will of conrse depend upon the shade or colour of gold required— namely, lemon, pale, middle, and regular deep. ‘lhe latter is the staple quality, being used for almost all purposes, while the three former shades are only used for special pur- poses. The average of alloy is about one- eightieth part of the weight of gold, and con- sists of silver and copper, the silver making the colour lighter, and the copper darker, a judicions mixture of both being used to form the most generally-used colour of gold leaf The gold used for the purpose, after being mixed and melted by the refiner, is cast into ingots of various lengths, according to the quantity. These are then passed through or between steel rollers, until it is pressed and lengthened out into a long roll of metal ribbon, about the thickness and consistency of a good stout bonnet ribbon, but with a stiffness the latter has not. A person un- acquainted with its appearance in this state would pass it by as a dirty-looking strip of brass, it not having anything of the semblance of gold about it. This ribbon-is then cut into small squares, which are beaten or hammered upon an anyil, until each piece becomes about lin. square, and in thickness about one seven-hundred-and-sixtieth of an inch, and in weight about six grains, One hundred and fifty of these small squares are then placed between sheets of vellum, about 4in. square, and the whole is then folded in a parchment wrapper. This bundle is then placed upon an anvil affixed to a block, and beaten with a goldbeater’s hammer, weigh- ing about 141b., having a broad round double- faced head, and a short handle. The work- man, by use and practice, is enabled to use this hammer the day through, keeping up & shower of blows, first with one hand and thenthe other (giving the packet of vellum leaves and gold a slight circular move between every blow), which, if we keep in mind the weight of the hammer, is actually wonderful; how many tons he thus lifts in a day would be an interesting calculation, When these one hundred and fifty pieces of gold are beaten until they are almost the size of the vellum they are taken out, and each leaf is cut into four, making 600 squares of lin. These are now placed between sheets of goldbeaters’ skin a (semi-transparent skin obtained from one particular portion of the interior of the sheep), and it is again beaten as before. By dividing these again into four, 2,400 leaves of gold are produced, each of which is about one-fourth the size of the skins. These 2,400 are divided into parcels of 800 each, and again placed between the skins, and beaten until they are nearly 3#in. square. They are now beaten to the required degree of thick- nes. ‘The leaves are cut to the uniform size of $4in. square, and placed into books hold- ing twenty-five leaves of gold, the leaves of paper haying been previously rubbed over
 * BUILDING News, June 9th, 1871.