Mortar is used to cement the parts of a wall together, and also to prevent the fracture of the bricks or stones by insuring an even distribution of pressure, notwithstanding any irregularities in their beds.
The quantity and coarseness of the mortar that should be used will therefore decrease in proportion as the beds are more perfect; e.g. ashlar masonry has thinner joints than rubble, and good bricks can be set with closer joints than bad ones.
Excessively thick joints should be avoided when possible. They not only injure the appearance of the work, but, when the weight of the superincumbent walling comes upon them, the mortar is squeezed out, projects beyond the face of the wall, catches the rain, and leads it into the wall, rendering the work liable to injury by frost.
In good brickwork (not gauged) the joints should be about ¼ to 3/8 inch thick. For ashlar masonry or gauged brickwork about 1/8 to 1/10 inch thick, while for rubble they vary in thickness according to the nature of the work.
The bricks or stones should be wetted so as to remove the dust, which Would prevent the mortar from adhering to them, and also to prevent them from sucking the water out of the mortar. The mortar should be used stiff, and every joint well flushed, all interstices being filled with bits of brick or stone set in mortar.
Larrying is the method usually adopted for filling in the interior of very thick walls. After the bricks forming the exterior faces of a course are laid, a thick bed of soft mortar is spread between them, and the bricks for the inside of the wall are one by one pushed along in this bed until the mortar rises in the joints between them.
Grouting consists in pouring very liquid mortar over the course last laid, in order that it may run into all vacuities left by careless workmanship in not properly filling up all the internal joints with mortar. Grout is, however, a weak and objectionable form of mortar.
Such joints are not very ornamental, but are suitable for internal surfaces to be whitewashed.
Flat Joints jointed, b P1. I., are the same as those last described, except that an iron jointer is used to mark a narrow line along the centre of the joints, which improves their appearance. Sometimes both the upper and the lower edges of the joint are jointed as in c P1. I.
1 Joints in Stonework are described in Part I.
Flat joint (jointed).
Flat joint jointed.
Struck joint (properform).
Pointing. (flat joint).
l Tuck pointing.
Gauged work (see Part I.) has very thin joints (see d PL I.) formed by dipping the bricks in white lime-putty before laying them.
Struck Joints should be formed by pressing or "striking" back the upper portion of the joint while the mortar is moist, so as to form a sloping surface which throws off the wet (see e P1. I.); the lower side of the joint is cut off with the trowel to a straight edge.1 These joints are usually struck along the lower edge as at f P1. I.; a ledge is thus formed above which catches the rain.
Keyed Joints, g P1. L, are formed by drawing a curved iron key or jointer along the centre of the flush joint, pressing it hard, so that the mortar is driven in beyond the face of the wall; a groove of curved section is thus formed, having its surface hardened by the pressure.
In some cases the moist key is dipped into ashes, which are thus rubbed into the surface of the joints.
Mason's or V Joints, h P1. I., project from the face of the wall with an angular V section. With good mortar they throw off the wet, but when inferior lime is used they soon become saturated and destroyed by frost.
Raking and Pointing consists in removing the original mortar joints to a depth of about ¾ inch in from the face, i P1. I., filling in with mortar, k P1. I., and finishing the joints in one of the methods about to be described.
Pointing is not advisable for new work, when it can be avoided, as the joints thus formed are not so enduring as those which are finished at the time the masonry is built.
During severe frost, however, it would be useless to strike the joints at the time the work is built, for the mortar would be destroyed by the frost.
Pointing is, moreover, often resorted to when it is intended to give the work a superior appearance, and also to conceal the defects of inferior work.
In repairing old masonry or brickwork, the mortar of which has become decayed, raking out and pointing become necessary.
Both in old and new work, before pointing, the original mortar should be raked out with an iron hooked point, and the surface well wetted before the fresh mortar is applied.
The raked joints are filled in with fine mortar, and struck flat with the trowel or jointer, as at k P1. I. They may be jointed as at b P1. I.
Tuck Pointing, I P1. I., is used chiefly for brickwork; the joints having been raked are " stopped," that is, filled up flush with mortar. This is coloured or rubbed over with a soft brick until the joints and bricks are of the same colour. A narrow groove is then cut along the centre of each joint, and the mortar is allowed to set. After this the groove is filled with pure white lime-putty, which is caused to project so as to form a narrow white ridge, the edges of which are cut off parallel so as to leave a raised white line about 1/8- inch wide. This process causes inferior work to look as if it had been executed with large bricks and very fine joints; in carrying it out any defects in the work, such as irregularity of joints, are corrected by smearing over the face and striking false joints, so that badly executed work is disguised and made to present a good appearance.
1 Joints so struck are sometimes called weather joints.
Bastard Tuck Pointing, m P. I., consists in forming a ridge from ¼ to 3/8 wide on the stopping itself, the edges being cut parallel and clean. There is no white line, the projecting part of the joint being of the same colour as the remainder.
Vertical joints are similar to horizontal joints, but in many cases are much thinner.