This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Of the different kinds of stonework, rubble masonry requires less preparation of the material than any other use of the stone, and covers a wide range of construction, from ordinary foundation walls, such as we have already considered, to the handsome well-pointed masonry of churches and other buildings.
Two definite classes of rubble work are recognized: (1) uncoursed rubble, in which stones of irregular shape are laid as they come to hand with no attempt at level courses, as in Fig. 133; and (2) coursed rubble, in which the blocks are levelled off at regular heights to a horizontal bed, as in Fig. 134. A wall of rubble is finished by pointing up the joints with cement mortar colored to taste, usually to the same color as the stone, when this is at all even colored. Sometimes a false joint of red or white mortar is run upon this pointing to imitate ashlar work.
Uncoursed rubble is sometimes laid with irregular pieces having hammered joints which are fitted together with no "spalls," or small stones, between. (Fig. 135.) This is an expensive and tedious process but is very effective when well done. The coursing of rubble is not necessarily uniform, or at the same level throughout, but may rise and fall by level stages to accommodate the size of the materials.
Fig. 133. Uncoursed Rubble.
The superintendence of this class of work, beyond a general inspection of the quality and soundness of the stone, will be mainly to see that the stones are well laid after being suitably prepared by roughly squaring with a hammer, and knocking off all weak angles and projections. The stones should be clean and free from dust, and should be moistened before laying. Mortar should be used in sufficient quantities to permit of each stone being firmly imbedded, and all hollows between the large stones should be filled with small stones carefully bedded in the mortar. All large stones should be laid on their natural bed, and should be so used that the side parallel to the bed shall be the largest, so that the stones shall lie flat and in no case be set on edge or on end. Care must be taken to break joints, and no side joint should form an angle with the bed of less than 60 degrees. The bonding of a rubble wall must be carefully watched, and bond stones freely used. If the rubble is backed with brick, as is often the case, iron clamps and ties should be inserted, which may run through the wall and turn up up behind the brick, if the back of the wall is to be concealed, and they should run to the inside course of bricks in any case.
These bond stones may be left rough at the back and sides, but the upper and lower beds should be level, so that they will have no tendency to wedge off the backing. This backing should In-curried up at the same time as the face work with the coursing leveled off at the same place. A good proportion of thick stones running two-thirds or more across the thickness of the wall is better than a few extending through the wall.
Fig. 134. Coursed Rubble.
Fig. 135. Hammered Joints with no &8226;&8226; Spalls.
Ashlar Masonry consists of blocks of stone which have been cut to a regular figure, generally rectangular in shape, and laid in courses of usually a foot or more in height. (Fig. 137.) If the courses are not maintained at the same level continuously, but are laid of stones of unequal height but still level and plumb, the work is called "broken ashlar," shown in Fig. 138.
In all ashlar work of soft material, such as limestone, no stone should have a length greater than three times its height. In harder stone the length may be four or five times the height. The thickness in soft stone may be once-and-one-half or twice the height; in hard stone, three times the height.