This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
Keyed Joint is a modification of a flush joint produced by grooving its centre with a jointer, or key, or pointer having a rounded edge, which is rubbed along the joint so as to press the mortar back and leave a slight concavity.
As already pointed out, this is the fundamental joint of all proper bond. In a slate damp course all joints must lap to retard water rising by capillarity, and when the walls are finished off with a parapet or coping, an upper damp course is advisable to prevent water soaking down.
This occurs between blocks of terra cotta when used for facing or for sewer inverts, etc, and is described under the name of that material.
Loose Joint is easily imagined without explanation. It is characteristic of careless beam filling, indifferent bricknogging, ill-laid paving, etc.
This is more fully explained under the Mason. It is formed in brickwork in panelled walls, chimneys, etc, as shown in Figs. 9 and 10.
Mortar Joint is at once the commonest and most mys-terious of all joints; most mysterious because neither the reasons of its setting nor the time required for the purpose can be even now predicated with certainty, and because there is no satisfactory agreement amongst the leading architectural and engineering experts as to the best and simplest way of making it properly, notwithstanding the thousands of hands daily employed in its fabrication; and, finally, it is most mysterious because in some unaccountable way rubbish heaps rise up and disappear in the vicinity of the spots where the mortar men are at work. A mortar joint, however, is commonly formed by laying bricks with a composition of lime and sand in the proportion of 1 part of unslaked lime to 2 parts of dry sand (or more or less as preferred) mixed with sufficient clean water to slake the lime and work and temper the mass to a stiff or sloppy paste, as thought most desirable. When more than 2 parts of sand are used it is questionable whether under the usual conditions of mixing it becomes properly incorporated or blended with the lime. The mixing should be done upon boards or a brick or stone platform where no dirt or gravel can get worked in with it. Three cubic yards of mortar are allowed to a rod of reduced brickwork, in which about 192 gallons of water are taken up when stone lime is used. The lime ought always to be more or less hydraulic, else it will not set nor be exempt from injury by frost. It should be fresh burnt, free from core or lumps, and cautiously kept from exposure until used. The sand requires to be clean and sharp. Blue lias lime may be well used hot from the kiln; it must be thoroughly slaked and mixed in small quantities as required with just sufficient water to bring it to a proper consistency, and on no account must it be allowed to partially set before using. 24 hours after being made, this sort of mortar, if the lime be technically of poor quality, will probably set in air, and if left undisturbed water will have no effect upon it. The thickness of a mortar joint in ordinary brickwork should not exceed 5/16 in., though excellent work is known to exist with much thicker joints. The less that is used of inferior mortal the better, provided the bricks are pretty square and wetted till soaked, and that the mortar bed is thick enough to prevent the bricks touching, which necessitates a minimum theoretical thickness of 1/8 in. or a practical one of ¼ in. A joint of this thickness has the advantage also of looking well, especially if throughout the work they all maintain the same size.