Jan. 26, 1872.
- Treatment—No doubt Mr. Crace’s view of
_ the treatment in decoration of granite is cor- rect and the most effective. He says: ‘‘ The earliest and grandest sculptures of Egypt are worked in granite, the beautiful and durable stone of Assouan (Syena), Egyptian sculpture partaking to some extent of the characteristics of execution most suited to that material. Broad, even surfaces, crudely-rounded con- tours, and obtuse or rectangular sections prevail. A thin ‘arris’ or an acute angle is almost unknown. Where greater detail is required it is either cut deeply at right angles to the surface, or slightly through the polish ; and as the forms must be of the simplest and most conyentional, so the detail must be very limited and very expressive.” Disadvantages.—The weight, the expense, the difficulty of working it. Many sorts will not stand the atmosphere of London and other towns. The most celebrated case I know I am tempted to extract extensively, because it is very seldom such a test has been applied, and the result is not that which one might have expected. Tn the manufacture of sulphuric acid from the roast- ing of ores it is essential to produce as little admix- ture of other bod‘es as possible. Arsenious acid invariably accompanies the sulphurous and sulphuric acids generated in the furnaces. In order to con- dense them before they enter the lead chambers these gases are first led through a channel. At the Mulden Vitriol Works, near Freiberg, where a structure was erected, it was found after a few years that a work of destruction was going on in the interior of the channel which necessitated constant repairs, and ultimately compelled the owners to build the whole afresh on different principle, based upon experience gained in this case. The forma- tion was in rock and built in 1861, and it was 3k yards high, 1} yard wide in the clear, walls con- structed in lime bond with quarry stones cone) it was built in a cavity cut into ‘the rock, which eavity was 44 yards wide, and 6 to 7 yards deep. Space between walls and sides of rock after comple- tion of channel was filled with clay tightly rammed in. The whole ultimately covered with rubbish. After two years’ standing a rising took place along crown of arch. In 1865 this increased to an alarm- ing extent, and examination revealed that quarry stones as well as mortar had undergone some serious change. Atlastit wasresolved to take down structure. When this was done it was found that the increase in height amounted to 1 yard 4 inches, whilst at various places the width had increased 12 inches, 8 inches, 4 inches. The yault was split above, and by combined pressure of upper part of masonry and covering upon lower walls, which had become soft and yielding through the action of the vapours, they were forced upwards. The free intermediate space was but a few inches wide. Payement and floor had become a distorted mixture, the members of the former being pushed “ pell mell” into the latter. Closer investigation showed that while the decom- position of the mortar seemed the same all along, that of the gneiss was different in different parts. The degree of humidity in various places, while it accelerated not the decomposition of the lime (magnesium limestone, or dolomite), had favoured the attacks of the acids upon the quarry stones. The mica of the gneiss played a most conspicuous part in the transaction. At first it became dis- coloured, then its adhesion relaxed, and at last the single silvery scales yanished altogether from the stones. During this process the gneiss ultimately gained the appearance of normal granulite, since one portion of the gneiss character had disappeared, and the remaining components, quartz and felsite, presented a slate-like, although crystalline or granular structure. There was a little change also observable in the felsite; it was partly discoloured, and in that state showed less hard- ness than in ordinary processes. It need hardly ‘be mentioned that quartz did not suffer in any way, but the stability of this component alone could not prevent the gneiss from becoming soft and brittle. The mortar, which was mixed up with river sand, suffered eyen more than the quarry stones. All its carbonates were turned into sul- phates, and wherever sufficient space presented itself crystals of gypsum were formed. In the narrower crevices groups of minute crystals sur- rounded grains of sand. These processes had bad effect on building. The crystallisation required room, and nothing is so heavy as effectually to resist that operation. In places where crystallisation was prevented by a lack of humidity the walls were disfigured by pressure from abeve, and in places favourable to that process this pressare was overcome, and cracks and clefts were the result. It is difficult to ascribe these mischievous effects any other agency than that of sulphuric acid. Coming from kilns mixed with other vapours and
THE BUILDING NEWS. .
63
ture of 400° Centigrade. This high degree of heat prevents its absorbing water out of walls of channel. The conditions for its condensation are a low temperature and presence of humidity. Where these conditions are fulfilled the acid will be pre- cipitated, and hence we find that at certain places in the channel the walls and flooring had absorbed quantities of acid. Moreover, the vapours of sul- phurous acid coming in contact with the alkalies of the walls are thereby enabled to take oxygen from the air and transforming into sulphates, aid in the work of destruction. A chemical analysis in the Sheerer Labora- tory, made by Dr. Rube, corroborates almost fully the above conclusions. Shortly to -sum up results obtained, the causes of the abnormal condition of the channel at Freiberg are to be ascribed 1. To the sulphuric acid acting as a decom- posing agent. 2. To the attacks of that acid upon interior of the walls being most effective on the mortar; next on the mica of the gneiss; and lastly on the felsite contained in the latter. 3. To the crystallisation of the products of decomposition. It was also found that the distortion was the greatest where the products of decompo- sition were kept in solution by an abundance of moisture. Objection to present mode of using granite. I desire to keep from touching on matters of taste, but there is one important point I do not remember to have seen dis- cussed. It is this: Should polished granite or any polished substances be used in external decoration or building? I think I remember hearing Mr. George Godwin remark, in a dis- cussion at the Royal Institute of British Architects, that he objected to the admixture for the reason that in time the work naturally becomes more dull and sombre, while the polished work does not show the effect of time, and, therefore, polished granite, encaustic tiles, and such substances, stand out more unpleasantly as time rolls on, and destroy all harmony. I thoroughly agree with this view, and would strongly urge that no polished material should be introduced unless the whole of the work be polished. I would ask those who differ from my view to walk along Pall Mall, and consider whether the large polished columns of a certain facade in that street are not destructive of unity and harmony. If still in doubt, compare that front with the less expensive front of the Travellers’ Club, and I think then I am certain tohave all agree with me in condemn- ing the use of polished materials with un- polished. The only building I remember where the eye is not offended is a small shop in Lombard-street. In this case, how- ever, the entire ground floor is of red polished granite. It comprises two piers, and seg- mental arch, the spandrils filled in with same material, and is finished square with string- ing ; from this starts the upper stories. Itis difficult to give the price of granite except for polished columns, as the granite merchants only quote their prices for columns ; but the cost of Aberdeen granite, including cartage, labour, and setting plain-faced work per foot cube, may be taken in London at about £1. The price of columns polished dark Shap, light Shap, from the quarries in Westmore- land, or Tonmore Ross of Mull, 10s. a foot run if diameter 3in. up to 4ft. in length, £1 1s. afoot run if diameter 14in.; £3 a foot run if diameter 2ft. Sin.; this latter is a diameter of some dark Shap columns supplied at S. Pancras Railway Station. It will be seen that the small columns are relatively dearer, the reason being that they are more liable to break. BE. ————+.— NOTES ON BRICKWORK.—X. HE thrust of an arch against its abut- ments may be determined on the principle of the resolution of forces into their component parts. Thus, in the simplest case, and con- atmospheric air, it has, in first instance, a tempera- | Sidering for a moment that an arch is but an
assemblage of jointed beams of indefinitely short length, the thrust against the abutments produced by two inclined beams, as in Fig. 7, loaded with a weight W, either suspended or insisted upon the apex, is measured by the length of the lines A B and A C, that thrust being in those directions respectively. The thrust on either side may be resolved into two component parts—viz., that which is sustained vertically by the foundations of the abutments, and that which is sustained horizontally by the resistance of the mass of the abutments to their being overturned by the horizontal thrust against them. This horizontal thrust is the same against each abutment, however the inclinations of the two beams may vary from each other, and this may be found to be true by constructing the lines according to any given angle, and measuring the horizontal distance between the vertical of the point of suspension or insistance, ‘and the point in each beam which determines the thrust upon it in its own direction.
Fic. 7
BRICK FLAT
ee ——__ > Sy TIE RODA 1y01AM A=
In Fig. 7, if A W measure the weight on the joint of the two beams, A B and A C will measure the respective thrusts along them in their own directions, providing the lines W C and W B be drawn parallel to the respective sides. The thrust measured by A B may be resolved into its vertical weight A D, and, its horizontal thrust B b, and the thrust measured by A C may be resolved into its vertical weight A c, and its horizontal thrust C ¢. Each of these horizontal thrusts will be of the same intensity whatever the | relative inclinations of the beams may be, but the vertical weights sustained by the abut- ments will vary with the inclination; thus the vertical weight measured by the line A 6, plus that measured by Ac, are together equal to the whole vertical weight sustained by the abutments. This being found to be true of two members of a structure, inclined at any given angle to the horizon the horizontal thrust of an assemblage of any number of members or equal parts of the length of an- arch may be found to be of the same intensity in any part of it, and therefore this results, that the horizontal thrust against an abut- ment may be measured by the line B@ in Fig. 8, by drawing the line A B tangential to the curve at the springing, and making A 0, measured by the same seale with which the horizontal thrust may be measured, equal to the weight of the half arch ana its load in- cluded within the points A, D, F, W.
