The bond across the thickness of the wall is of still greater importance, either "thorough bonds," extending from one face to the other, should be inserted at regular intervals, or "headers" should cross each other alternately from opposite sides, extending inwards about 2/3 the thickness of the wall.1
Some authorities prefer headers to thorough bonds in walls more than 3 feet thick, because the interior of the wall settles down rather more than the sides, leaving a hollow, so that a thorough bond stone would be unsupported in the middle, and might be broken. Another reason against long bond stones is, that there is danger of the beds not being even throughout, in which case the pressure comes upon a few points, and the stone is liable to break in two.
Masons are very apt to build up the sides of a wall separately, filling in with small stuff, or even dry packing. The wall thus consists of two thin slabs, united only by the thorough bonds.
This should never be allowed. The stones should be made to cross from opposite sides of the wall, and overlap as much as possible, so as to assist the bond stones in giving transverse strength to the wall. The interior of walls of every description should be solidly filled in, every stone being bedded in mortar, and all interstices flushed up.
The width of bond stones may be about 1 1/2 times their height, and the aggregate surface shown by their ends, on each face of the wall, should be from 1/8 to 1/4 of the area of the face. Care should be taken that each bond stone is of sufficient sectional area throughout its length.
Thorough bonds present an advantage over three-quarter headers, inasmuch as they can be traced in the work, and therefore cannot easily be omitted by the mason without detection, but they are more expensive, as each must be cut to a length exactly equal to the thickness of the wall, and, moreover, they are apt to conduct damp through the wall.
The practice of leaving the ends of thorough bonds sticking out beyond the wall, and knocking them off afterwards, should not be allowed, as it shakes and injures the masonry.
Thorough bonds should not be placed directly over one another, but chequer-wise, so that each bond stone in any course is over the centre of the interval between two in the course below.
In work built up to courses the bond stones are generally specified to be from 4 to 5 feet apart in each course, and they should be placed in position before the course is built.
1 Sometimes called "dog's-tooth bond."
Iron work should not be built into stone in positions where, by rusting, it might disfigure the face with stains, or in such a way that it may burst the stone, by its increase in bulk during oxidation or by its expansion and contraction from heat and cold.
Ashlar Masonry is built with blocks of stone very carefully worked, so that the joints generally do not exceed 1/8 or 1/10 inch in thickness.
" In order that the stones may not be liable to be broken across, no stone of a soft material, such as the weaker kinds of sandstone, and granular limestone, should have a length greater than 3 times its depth. In harder materials, the length may be 4 or 5 times the depth. The breadth in soft material may range from 1 1/2 times to double the depth: in hard materials it may be 3 times the depth." - Rankine.
Ashlar is the most expensive class of masonry built, and depends for its strength upon the size of the stones, the accuracy of the dressing, and the perfection of the bond; but hardly at all upon the quality of the mortar.
Mortal: - The mortar used for the superior descriptions of ashlar must be very fine and free from grit. The outer portion of the joint, about 3/4 inch in from the face, is generally filled with putty, as described at p. 42 - either that formed from lime and water, and known as "plasterer's putty," or in some districts white lead is added to it.
The joints, though very carefully dressed, should not be too smooth, otherwise their surfaces will afford no key for the mortar, nor offer sufficient resistance to the sliding of the stones.
Great care must be taken that bed joints are not worked hollow. This is sometimes done in order to show a very fine joint on the face without the trouble of carefully dressing the whole bed. It leads to the entire weight being thrown on to the point in front (C in Fig. 120), and a "spall" or piece, S, is splintered off; the stone is then said to be "flushed " at this point.
1 "Drafted margins," or "drafts," are narrow strips or borders chiselled round the edges of the faces of a stone to enable it to be set with accuracy, and in some cases to improve its appearance.
Fig. 120. Hollow bed Joint.
Fig. 121. Slack led Joint underpinned with a Spall.
With the same object of saving labour, the back of the joint is sometimes worked slack and underpinned, as in Fig. 121. The stone is then supported only at the front and back, and liable to break in the middle, as shown.
"Where bed joints are worked convex, the pressure that comes upon them is concentrated upon a single point, and leads to crushing or splitting of the stone.
Where the beds and joints are not carefully worked throughout, they should be so for at least 6 inches in from the face.
Flushed joints are particularly likely to occur with stones that are not laid on their natural beds.
They may be guarded against by raking out the mortar to a depth of an inch or two from the edges of the stones, pointing up again when the work has settled, or by chamfering 1 them off as in the quoins in Figs. 122, 123.
In any important work with fine joints, especially in columns, sheet lead is often laid between the stones, which is intended to yield to the irregularities on the bed and to distribute the pressure.2 The lead should extend only to within about an inch of the outer edges of the stones, so as to leave a clear space between them, and prevent them from bearing upon one another and flushing.