374
EXPOSITION AT VIENNA.
by the use of ordinary lime, and that prepared by the use of cement. The latter is considerably dearer, but possesses great advantages. Formerly the first alone was used, and lasted very well when not applied in exposed positions, but where rain and frost could attack it, it showed, after several years, a tendency to crack and blister, though rarely to fall from the Avail. Its application here is different from our own. Three mixtures are prepared : the first consists of coarse sand and lime which is throAvn upon the Avails forcibly from the troAvel and allowed to become tolerably dry, the precaution being taken that the bricks be not too dry on its application, since otherAvise they absorb the moisture too rapidly from the mortar and cause it to crack; then folloAvs a second mixture of finer sand and lime, Avhich is planed into the required shape, Avhether of cornice or windoAv capping, by a profile cut in sheet iron and stiffened by a board backing,—this is guided as in stucco-Avork by strips nailed to the Avail. On the second coating becoming nearly dry, an almost liquid mixture of the finest sifted sand and lime is thrown as before upon the Avail and again planed out; and finally, corners of moulding, etc., are finished by hand, which presupposes some skill on the part of the workman. It is important that this mastic should noAvhere have a greater thickness than one half an inch, and thus, Avhere great projections occur, the rough form of the moulding should be built out Avith bricks Avhich are broken Avith the troAvel to the required form. For this latter purpose, bricks two feet in length, are burnt in the vicinity of Vienna. The upper surface of such a projection then receives a coA r ering of zinc as a protection against the elfects of rain. All the conditions necessary to the production of good mortar are equally applicable to this mastic. The lime and sand must be pure and good; the latter must be sharp, and best from the pit; during its application and setting the mastic must be protected from the influences of the Aveather, especially of rain. At present, in Vienna, this variety of mastic is going out of use, and the hydraulic variety produced by intermixture of sand and cement is coming into favor ; partly on account of its greater durability, partly from the resulting dryness of the Avails thus coated, and finally from the greater ease in Avorking. The first variety is iioav