Dressing of stonesArticles > Dressing of stones
The stones after being quarried are to be cut into suitable sizes and with suitable surfaces. This process is known as the dressing of stones and it is carried out for the following purposes.
- to make the transport from quarry easy and economical,
- to gel the desired appearance from stone work,
- to suit to the requirements of stone masonry.
Following are the varieties of finishes obtained by the dressing of stones:
This type of finishing is used in hard stones like granites, where the dressing is done with the help of an axes. Such a finish is termed as an axed finish.
Boasted or droved finish:
In this, the dressing is done with the help of a boaster and hammer, forming a series of 38 to 50 mm wide bands of more or less parallel tool marks, which cover the whole surface.
In order to obtain uniform joints in stone work, the margins are placed which may be cither squared or pitched or chamfered.
Dragged or combed finish:
This finish is used only in soft stones: This type of finish, a drag or a comb, which is a piece of steel with a number of teeth, is rubbed on the surface in all directions and surface.
In this type of finish, the surface of stone is made round or circular as in case of a column.
In this type of finish a margin of about 20 mm width, is sunk on all the edges of stone and the central portion is made to project about 15 mm. A number of vertical or horizontal grooves about 10 mm wide arc formed in this projected portion. This finish is generally adopted to make the quoins prominent.
The surface of stone can be moulded in any desired shape so as to improve the appearance of the work. The moulding can be made either by hand or machine.
In this type of finish, the stones are made roughly square or rectangular by means of a waller's hammer. The hammer-dressed stones have no sharp or irregular corners and have comparatively even surface so as to fit well in masonry.
In this type of finish, the surface ol the stone is made approximately smooth with a saw or with a chisel.
This type of finish is used in marbles, granite etc. These are polished either manually or with the help of machines. A glossy surface is obtained.
This type of finish presents a net-like appearance. A margin, about 20 mm wide is marked on the edges of stone and irregular sinkings are made on the enclosed space. A margin, about 10 mm wide, is provided around the irregularly shaped sinking, having a depth of about 5 mm. A pointed tool is used to put the mark on the sunk surface so as to present a pock-marked appearances.
This is another form of rough dressing, usually used for lower portions of the buildings. The exposed face of the stone is dressed with the help of a punch, thus making depressions or punch holes at some regular distance apart. A 25 mm wide strip is made around the perimeter of the stone.
This type of finish is obtained by rubbing a piece of stone with the surface or by rubbing the surface with the help of a suitable machine. The water and sand are freely used to accelerate the process of rubbing.
The stone surface is finished by means of a chisel and parallel continuous marks, either horizontal or inclined or vertical are left on the surface.
This type of rough dressing in which the irregular projections are removed by a scabbling hammer.
Self-faced or rock-faced or quarry-faced finish:
Some stones, as obtained from the quarry posses smooth surface and they can be directly placed on the work. Such a stone surface is termed as the self-faced or lock-faced or quarry-faced finish.
This finish is obtained by sinking the surface below the original level in the form of wide grooves, chamfers, inclined surfaces, etc.
The conditions which govern the selection of stone for structural purposes are cost, fashion, ornamental value and durability, although the latter property is frequently overlooked or disregarded. Cost is largely influenced by transportation charges, difficulties in quarrying and cutting, the ornamental features, and the durability of stone. The type of dressing of stone may make a difference to the cost, particularly with the stones derived from igneous rocks. When the cost of quarried stone to cost of finished stone is considered, it will be found that the labour cost is far greater than the price of the stone. Thus, a difference in the price between two alternative stones is unimportant and it would be unwise to reject a more durable stone on the grounds that it was costly.
Another factor which should he considered is the suitability of the stone for the type of design. For example, for a highly carved design if, by mistake, a harder stone such as granite is selected the cost will be affected. Colour, arrangement and shape of mineral constituents greatly influence fashion and ornamental value. One of the first factors influencing the selection of stone for a particular work will be colour.
It is important that the designer is aware about how the colour is likely to change after long exposure and is particular how il may vary in polluted atmospheres. As an example limestone, being slightly soluble in water, will remain clean in portions facing rain but retain a film of root in sheltered areas. This results in strong colour contrast Resistance to fire and weathering-factors which are largely influenced by the mineral constitution of the rock-are the most important determinate of durability. It is very important to select a stone according to its exposure conditions. Limestones when used in areas not exposed to rain but acted upon by sulphur gases of polluted atmosphere, form a hard and impermeable surface skin which subsequently blisters and flaskes off. It must be noted that flaking of this kind occurs mainly an external work only, although the air inside the building is almost equally polluted, probably due to the damper conditions inside.
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