Production of Natural Stone
Natural stone must be carefully quarried and preferably removed as large blocks. The first step is to take down the "primary block", which is a large dimensioned chunk of rock loosened from the quarry wall. This is most often achieved by drilling, blasting or/or sawing. A primary block can vary in size depending on the stone type, and up to 1000 cubic metres, 2700 tons!
When drilling and blasting, it is important not to crush the stone. To avoid this, the stone is cut as straight possible by first drilling a series of holes to define the cut line. Some rocks are easier to blast than others, so the distance between the drill holes varies accordingly. It is important to limit the use of explosives as much as possible, choosing only weaker types. In Norway, dynamite is forbidden.
Long ago, black powder was used for blasting, but today highly specialized explosives and detonating fuses are more common. Usually, the blasting technique is tailored to the properties of the specific rock type.
Today, the most common method to quarry the dimension stone is by cutting out the primary blocks with a wire gang saw. Diamond wire cables are threaded through the drillholes, and tightening the wire during the sawing results soon in large cuts. Sawing in this way is often the only means of extracting blocks of marble and limestone. For these types of rocks, chain saws and sword saws might also be used. Even very hard stone types can be extracted with the wire saw, but most often the quarry manager will find it more efficient to remove one or two blocks in this way, and then blast the remaining blocks.
After the primary block is loosened from the quarry, it can divided into smaller blocks by using the drilling/blasting, sawing or by inserting wedges into the drill holes and striking with a hammer until the stone cracks. In the slate production industry, the primary blocks are often small, and slabs of slate are split up into gradually thinner slabs.
When the blocks have properly shaped, they are ready to be transported to the nearest factory. If they are to sold on the international market, there are strict quality standards to follow. Blocks must have rectangular shape and preferably with dimensions no less than 2.4m x 1.2m x 1m. Moreover, they must be flawless, that is not show any small cracks that could develop into larger cracks later during processing.
Blocks to be sold must be uniform in color and structure and without stained areas. The natural stone deposit usually contains numerous cracks and other flaws, so it is not surprising that there is only a small percent that achieves market standard. As a rule, only five to twenty percent of what can be extracted from a natural stone outcrop is marketable, although there are exceptional cases in parts of Europe where higher utilization rates are achieved.
Most of the natural stone extracted will produces of different forms of slabs. Slate is split along its natural cleavage plane repeatedly, where the distance between these layers determines the thickness of the slab. Roof slate is usually less than 1.5 cm in thickness, while roofing slates used on traditional alpine cottages and paving stones can be up to 10 cm thick. After been split along the cleavage planes, the stone surface can left natural and untouched.
Dimension stone is usually shaped by sawing. There are a number of types of custom-made natural stone-saws, including diamond covered blade, gang sawing (or frame saw) which holds many parallel, straight saw blades, as well as circular saws of various sizes. Today, there are many high-tech methods of sawing of natural stone which include advanced control systems and integrated solutions for assembly line production.
After sawing the stones, its surface can be finished. Polishing heads are applied. These start very coarse, and then move to finer and finer grit abrasives until high gloss polished surface is achieved. The stone's natural color and texture is enhanced. Alternatively, different tools can be used to create a rough surface; including, chiselling, sand-blasting or heat and flame treatment. The flame treatment process allows small flakes of stone to fall off, due to the expansion of certain minerals (especially the quartz). Flame treatment is often used in the processing of granite slabs that will be used outdoors (surfaces or façade).
Today, stone processing may follow traditional principles, especially when creating stone sculptures, ornamental works or stone restoration. Therefore, the stone mason's skill and knowledge will always be important. A number of stone products, such as dry brick and paving stones, are made by wedging and cracking the stone. Wedges can be placed in drill holes or deep depressions, and used to split large stones, while stone guillotine can be used to cut smaller stones. Both methods are labour intensive and require knowledge and experience.
Cost of production is one of the reasons why Indian, Portuguese and Chinese cobblestone is chosen over local stone for use on the streets in Norway.