144. Classification of Dressed Stones. Stone masonry is classified according to the shape of the stones, and also according to the quality and accuracy of the dressing of the joints so that the joints may be close. The definitions of these various kinds of stone work have already been given in the previous section, and therefore will not be repeated] here; but the classification will be repeated in the order of the quality and usual relative cost of the work. The term ashlar refers to the rectangular shape of the stone and the accuracy of dressing the joints, and may be applied to coursed ashlar, range, and even random. The next grade in quality is squared-stone masonry, which likewise refers only to the accuracy in dressing the joints. The variations in the coursing of the stones may be the same as for ashlar. The term rubble is usually applied to stone masonry on which but little work has been clone in dressing the stones, al though the cleavage planes may be such that very regular stones may be produced with very little work. Rubble masonry usually has joints which are very irregular in thickness. In order to reduce the amount of clear mortar which otherwise might be necessary in places between the stones, small pieces of stone called spalls are placed between the larger stones. Such masonry is evidently largely dependent upon the shearing and tensile strength of the mortar and is therefore comparatively weak. Random rubble (Fig. 31), which has joints that are not in general horizontal or vertical, or even approximately so, must be considered as a weak type of masonry. In fact the real strength of sti..12 walls, which are frequently built for architectural effect, depends on the backing to which the facing stones are sometimes secured by cramps.
145. Stone Cutting and Dressing. Many of the requirements and methods of stone dressing have already been stated in the defi nitions given above. Frequently a rock is so stratified that it can be split up into blocks whose faces are so nearly parallel and perpendicu lar that they may be used with little or no dressing in building a sub stantial wall with comparatively close joints. On the other hand, an igneous rock such as granite must be dressed to a regular form.
The first step in making rectangular blocks from any stone is to decide from its stratification, if any, or its cleavage planes, how the stone may be dressed with the least labor in cutting. The stone is then marked in straight lines with some form of marking chalk, and drafts are cut with a drafting chisel so as to give a rectangle whose four lines lie all in one plane. The other faces are then dressed of with as great accuracy as is desired, so that they are perpendicular (or parallel) to this plane. For squared-stone masonry, and especially for ashlar masonry, the drafts should be cut for the bed-joints, and the surface between the drafts on any face should be worked down to a true plane, or nearly so. The bed-joints should be made slightly concave rather than convex, but the concavity should be very slight. If the surface is very convex, there is danger that the stones will come in tact with each other and produce a tration of pressure, unless the joints are made undesirably thick. If they are very
concave, there is a danger of developing transverse stresses in the stones, which might cause a rupture. The engineer or contractor must be careful to see that the joints are made truly perpendicular to the face. A frequent trick of masons is to make the stones like truncated wedges, as trated in Fig. 32. Such masonry, when finished, may look almost like ashlar; but such a wall is evidently very weak, even danger ously so.
To produce a cylindrical surface on a stone, a draft must be cut along the stone, which shall be parallel with the axis of the cylinder. See Fig. 33. A template made with a curve of the desired radius, and with a guide which runs along the draft, may be used in cutting down the stone to the required cylindrical form. A circular template swung around a point which may be considered as a pole, may be used for making spherical surfaces, although such work is now usually done in a lathe instead of by hand.
To make a warped surface or helicoidal surface, a template must be made, as in Fig. 34, by first cutting two drafts which shall fit a template made as shown in the figure. After these two drafts are cut, the surface between them is dressed down to fit a straight edge, which is moved along the two drafts and perpendicular to them. Such stonework is very unusual, and almost its only application is in the making of oblique or helicoidal arches.
The size of the blocks has a very great influerce on the cost of dressing the stones per cubic yard of masonry. For example, to quote a very simple case, a stone 3 feet long, 2 feet wide, and 18 inches high has 12 square feet of bed-joints, 6 square feet of end joints, and 4.5 square feet of facing, and contains 9 cubic feet of masonry. If the stones are 18 inches long, 1 foot wide, and 9 inches high (just one-half of each dimension), the area of each kind of dressed joint is one-fourth that in the case of the larger stones, but the volume of the masonry is only one-eighth. In other words, for stones of similar shape, increasing the size increases the area of dressing in proportion to the square of the dimensions, but it also increases the volume in proportion to the cube of the dimensions. Therefore large stones are far more economical than small stones, so far as the cost of dressing is a factor.
The size of stones, the thickness of courses, and the type of masonry should depend largely on the product of the quarry to be utilized. An unstratified stone like granite must have all faces of the stone plug-and-feathered; and therefore the larger the stone, the less will be the area to be dressed per cubic foot or yard of masonry. On the other hand, the size of blocks which can be broken out from a quarry of stratified rock, such as sandstone or limestone, is usually fixed somewhat definitely by the character of the quarry itself. The stratification reduces very greatly the work required, especially on the bed-joints. But since the stratification varies, even in any one quarry, it is generally most economical to use a stratified stone for random masonry, while granite can be cut for coursed masonry at practically the same expense as for stones of variable thickness.