Volltext Seite (XML)
52 THE CIVIL ENGINEER AND ARCHITECT’S JOURNAL. [February 1, 1S68. spoil the whole surface of the clay and obliterate the finer touches of the modelling, by going over it with a sponge, or who introduce no end of fancy touches and original markings to conceal slovenliness in the moulding. There are, too, those who, at this stage of the manufacture, daub over the work with a wash of some horrid colouring material to make the finished block burn of a pretty pink or other approved tint. Blocks thus finished invariably lose their gay colours after a year’s exposure to the weather, and the artificial surface formed by this painting operation lets in the frost in winter, and splits up into innumerable hair cracks. If terra cotta must have the creamy look which is aimed at by this process, it would be far better to give it a coat of distemper before leaving the manufac tory (similar to the priming of joiner’s work), the first shower would then get rid of the colouring matter without in any damaging the terra cotta. The burning of the blocks is hardly a question of any moment to the architect. The clay stands from ten days to a fortnight (according to the season) to dry. It is then placed in the kiln and burnt with a steadily increas ing heat for five or six days, after which the kiln is slowly cooled, and the finished blocks are taken out ready for use. The final colour of the terra cotta depends to a very great extent upon the intensity of the firing and the moisture or dryness of the clay before it is placed in the kiln. I can give no certain data to determine from the colour the quality of the terra cotta, but I have frequently found that a pale bilious grey or very light yellow colour denotes insufficient firing, while a rich brimstone yellow or pink is a sign that the material has been burned to the utmost it will stand without vitrifying. I cannot here refer to the many mixtures which some manu facturers inflict upon their clay. Powdered terra cotta, called grog, is very commonly made use of in considerable quantities, for the sake partly of economy and partly to prevent excessive shrinkage when drying ; but the use of it is attended with many evils, which I cannot dwell upon now. There can be no doubt that the purer the clay is, the better will be the terra cotta. III.—Terra Cotta considered with reference to the Builder. Wc have now had a hasty glance at the clay in all the prin cipal stages of preparation, and we will suppose the terra cotta to have been delivered upon the works. The blocks should be carefully fitted together on a floor or even surface to check their aggregate lengths, to try them as to squareness, and to ascertain how they fit to one another. Small inaccuracies may be remedied by rubbing down the edges on a stone (no easy matter by the bye with good terra cotta), and for important errors the blocks have to be clipped down with the chisel— this is a dangerous and difficult proceeding, and should be resorted to only as a last alternative. Insufficiency of length may be overcome by widening the joints. It is at this stage of the work that the advantage of keeping the blocks small in size is most manifest. Large blocks twist so much in many cases that it is absolutely necessary to cut and rub away the surface, and this is attended with the unsightly result I referred to in the early part of my paper. Blocks up to loin, in length are, as a general rule, very true, and it is only in the greater lengths where cutting becomes necessary. In fixing the terra cotta, care must be taken to fill in the hollows of those blocks which have to carry heavy weights,and the cement grouting with which this filling in is done must be mixed very poor, as I have known the expansion of the cement in setting to burst the terra cotta all to pieces. Eight parts of sand to one of good cement is strong enough for this purpose. The blocks themselves may be carefully bonded in with the brickwork in cement, in the proportion of three of sand to one of cement, the face joints being raked out, and pointed with a mixture of one part of powdered terra cotta to one part of lime. I have prepared diagrams of some few cases of terra cotta construction, from which it will be seen that the use of iron is almost a matter of necessity. This I consider to be the only drawback to the extensive use of terra cotta in small blocks. I admit that it is quite possible in the cornice to carry the weight by means of York landings and large slabs of stone tied down by bolts in the usual way, but I incline to think that the con struction with iron joists is the more preferable method of the two. In the case of columns, however, there is no choice but to use iron, unless we trust to the terra cotta by itself. Here it will be of interest to bring under notice the only experiment I know of, as to the resistance of terra cotta to compression. The plan of the 1862 Exhibition was so arranged that the exist ing south arcade of the Royal Horticultural Society’s Gardens might serve as the refreshment rooms, and form the ground floor of a lofty building, and it therefore became necessary to ascertain the amount of weight that the terra cotta columns of the arcade would sustain. In order to arrive at a fair con clusion, Captain Fowke resolved to have one of these columns tested, and the experiment was accordingly undertaken by the late Mr. Wakeford, the clerk of the works for the gardens. Unfortunately the ultimate results were not at all conclusive. Still they are, till others are obtained, of the utmost value. The column, which is in six pieces, including the Bath stone abacus, was put together in cement with plugs in the usual way, on a wooden support or base. It was then enclosed in a frame, the head of which consisted of two deals resting on the abacus and bolted edgeways to the uprights. The column was then crushed by lifting the wooden support on which it stood by means of the piston of a hydraulic ram. The fault of this arrangement was that the abacus, instead of bearing against one rigid body, was forced against the edges of two deals, and the weight was thus unfairly thrown on the cap, which splin tered away gradually, and finally caused the fracture of the upper section of the column, with a load, at the moment of failure, of about eighteen tens. The weight of twenty and a half tons had, however, previously been sustained by it, and there can be no doubt that, but for the defect in the apparatus, it would have carried much more. The result I have just given, which is I believe much beneath the actual strength of terra cotta, is nevertheless quite sufficient to prove that this material may be entrusted with all ordinary loads. Indeed, I think that the column IJ have drawn in my diagram, which is 15ft. high, and 1ft. 6in. in diameter, would carry a weight of twenty five tons with greater and more permanent safety than the cast-iron column or core, 8in. diameter and lin. thick, which has been inserted to carry this load. In all my remarks, where I have used the word terra cotta, I must be understood to refer to the yellow or grey variety. The qualities and properties of red terra cotta are so inferior and so different to the white, that I have not devoted much con sideration to it. The clay from which it is made must, in order to produce a fine colour, be a mixture, or it can be but little fired. When pure clay is used, it must be burned with a far less heat than fire-clay, and the resulting terra cotta is consequently soft and wanting in durability. The Fareham brick-earth would, I am convinced, be a splendid material for the production of red terra cotta, and I commend it to the notice of manufacturers. The ordinary red clay is a tertiary forma tion, and becomes, when hard burnt, the disagreeable colour, wc associate with flower pots and drain-pipes; but it is pos sible, by mixing it, to produce terra cotta of a deep crimson hue. As you have already heard my objections to mixed clays, you will understand my reasons for mistrusting red terra cotta. I must now notice the special advantages of the use of terra cotta in London. Mr. Matthews in his able essay on the “ Materials for London Exteriors,"* speaks in very favourable terms of the application of glazes, in various colours, to the surface of bricks, and looks forward to the time when such glazed bricks will be procurable at a cheap rate. Now, in terra cotta, we have a substance eminently adapted to receive a glaze, and enamelled terra cotta, the Della Robbia ware of the old Italian times, is, I hope, the material destined to beautify and decorate the London of the future. As Paris has become beau tiful by its splendid stone, so smoky London will, by its durable (imperishable, I may almost say) and decorative majolica ware. In glazed terra cotta we have, I maintain, the panacea for all the evils which cause the failure or the ill success of our modern London architecture. Victorian architecture relies, to a very great extent, upon colour for its enrichment and for its effect. Up to the present time, the costly city banks, public buildings, and warehouses have, after a few short months of splendour, fallen a prey to soot. It is very humiliating to be obliged to confess tliis, but I am afraid that we must all admit that with the soli tary exception perhaps of polished granite, every material yet * See Civil Engineer and Architect’s Journal, Vol. 29 (1866) pages 3 and 43
