This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The mode of "getting" the stone varies in accordance with the nature of the material. Some stone is blasted, other sorts are picked and split up by wedges, other sorts again are capable of being split along the planes of cleavage.
The stone thus procured is then rough hammer-dressed, picked or sawn to the sizes or scantlings ordered.
When it reaches the "banker" on the works the mason proceeds to "dress" the stone, usually commencing with one of the beds. The necessary dimensions being fixed on one surface, drafts are chiselled along one or more faces and tried by the bevel or square. These being found correct, cross drafts are worked, and the intervening rough stuff is knocked off and worked down with the tool until the surface is perfectly true and on a plane. The other faces are treated in the same way until a perfectly square or rectangular solid is formed.
For curved and twisted surfaces the faces are brought to the square as above, and the curved or twisted faces marked off with templates or bevels and the stone brought down to the required shape, as in Fig. 219, which shows the squared surface by the dotted lines, the firm lines giving the twist or bevel required for the finished work.
For mouldings a section of the mould is cut out in zinc or other metal and applied to the ends of the dressed stone, which is indented or marked with a steel pointer to the required profile. A diagonal draft is first cut to an angle coincident with the extreme edges of the mould, and then the mouldings are cut down to the finished shape (see Fig 220).
1 represents the finished moulding; 2 represents the rough angles enclosing the moulding; and 3 shows the square stone before the process of cutting the moulds is commenced.
In the case of curved surfaces, such as a semi-cylindrical coping, the stone is first brought to the form of a rectangular block with the height of the apex of the tabling of the curve. Straight drafts are then run from end to end of the stone at regular distances from the apex, coinciding with the section scribed on the ends; the intervening irregularities are then knocked off and brought to surface. Here the frequent use of the square and bevel is very necessary to ensure good and accurate workmanship (see Fig. 221).
For arches or curved work the voussoirs are usually marked off in the rough stone, reversed alternately, and then sawn down diagonally almost to the sizes required. If care and judgment are exercised in this operation at least one of the sides of each stone can be utilised without further preparation (see Fig. 222), where the voussoirs 1, 3, 5 are reversed, while those numbered 2 and 4 are normal, the sawn joints a, b, c, d, e being finished, or with very little need of further treatment.
-There are several methods of finishing surfaces of stones for the building, and in a measure they depend on the nature of the material to be operated on.
In sandstones a chiselled face is usually considered the finish, although in the softer varieties the comb or drag is sometimes run over the surface to give a smoother effect.
In hard limestones such as Purbeck, Portland, etc., or in the case of grits such as those of Yorkshire, the claw tool is often used. For marble a small chisel with a slightly concave cutting surface is used to give a very smooth effect. Soft stones of a more crystalline nature, such as Bath stone and other oolites, receive a very even surface by means of drags of varying fineness, which remove all marks of the tool, leaving the face perfectly homogeneous. Droving leaves the surface in more or less regular parallel lines diagonal to the arris of the stone (see Fig. 223). Tooled work is generally square with the arrises. Ashlar is similar to tooled work, but in continuous lines across the face and parallel. In section it gives the appearance of small concave curves.
Fig. 223. Drafted Margin
Some work, especially for quoins and angle stones, is tooled on the margins, and picked, punched, or broached as desired in the central portion. Rusticated work has a tooled margin, and the centre cut out into an irregular pattern, of hollows sunk below the plane surface from 1/4 to about 1/2 inch. Vermicul-ated work is similar in many respects to rusticated, and needs no special comment.
There are other labours peculiar to granite, such as hammer blocking, bushing, tooth-axing, etc. Granite is also capable of a high degree of polish.
The ordinary type of buildings composed of enclosing walls with the usual apertures for access and light present no difficulty in setting out. It is in such cases as curved surfaces, niches, groined vaults, domes, or wing walls that great skill and care are requisite, and these will be treated fully later on.
The usual method employed by the masons is to lay out, on a platform of boards secured to prevent movement, an elevation (or part thereof) of the structure, with the beds and joints accurately divided off as shown by the small scale drawings and details provided by the architect, allowance being made for the thickness of mortar joints, etc.