Random rubble (fig. 3) is the roughest form of stonework. It is built with irregular pieces of stone usually less than gin. thick, loosely packed without much regard to courses, the interstices between the large stones being occupied by small ones, the remain ing crevices filled up with mortar. Bond stones or headers should be used frequently in every course. This form of walling is much used in stone districts for boundary walls and is often set dry without mortar. For this work the mason uses no tool but the trowel to lay on the mortar, the scabbling hammer to break off the most repulsive irregularities from the stone, and the plumb rule to keep his work perpendicular.
Coursed rubble (fig. 4) is levelled up in courses I2 or i8in. deep, the depth varying in different courses according to the sizes of the stones. The stones are dressed by the workman before he begins building, to obtain a level bed and perpendicular face.
Irregularly coursed squared rubble is a development of un coursed random rubble, the stones in this case being squared with the hammer and roughly faced up with the axe. The courses jump from one level to another as the sizes of the blocks demand ; the interstices are filled in with small pieces of stone called "snecks." For coursed squared rubble the stone is faced in a similar man ner and set in courses, the depth of each course being made up of one or more stones. In regular coursed rubble all the stones in one course are of the same height.
Block-in-course is the name applied to a form of stone walling that has some of the characteristics of ashlar but the execution of which is much rougher. The courses are usually less than 12in. high. It is much used by engineers for waterside and railway work where a good appearance is desired.
The angles or quoins of rubble-work are always carefully and precisely worked and serve as a gauge for the rest of the walling.
Frequently the quoins and jambs are executed in ashlar, which gives a neat and finished appearance and adds strength to the work.
The name ashlar is given, without regard to the finish of the face of the stone, to walling composed of stones carefully dressed, from 12 to i8in. deep, the mortar joints being about an eighth of an inch or less in thickness. No stone except the hardest should exceed in length three times its depth when required to resist a heavy load and its breadth should be from one and a half to three times its depth. The hardest stone may have a length equal to four or perhaps five times its depth and a width three times its depth. The face of ashlar-work may be plain and level, or have rebated, chamfered or moulded joints.
Backing to Stonework.—The great cost of this form of stone work renders the employment of a backing of an inferior nature very general. This backing varies according to the district in which the building operations are being carried on, being rubble stonework in stone districts and brick or concrete elsewhere, the whole being thoroughly tied together both transversely and longi tudinally with bondstones. In England a stone much used for backing ashlar and Kentish rag rubble-work is a soft sandstone called "hassock." In the districts where it is quarried it is much cheaper than brickwork. (For brickbacking see BRICKWORK.) Ashlar facing usually varies from 4 to gin. in thickness. The work must not be all of one thickness, but should vary in order that effective bond with the backing may be obtained. If the work is in courses of uneven depth the narrow courses are made of the greater thickness and the deep courses are narrow. It is sometimes necessary to secure the stone facing back with iron ties, but this should be avoided wherever possible, as they are liable to rust and split the stonework. When it is necessary to use them they should be covered with some protective coating. The use of a backing to a stone wall, besides lessening the cost, gives a more equable temperature inside the building and prevents the trans mission of wet by capillary attraction to the interior.