Hard and soft stone

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Typical appearance of hard rock, the Hauzenberg granite
Typical appearance of a soft rock, Naxos marble from Greece

When processing natural stone , stonemasons differentiate between hard and soft stone with regard to the use of tools, equipment and machines. It is a technical distinction. Both terms are not scientifically founded, but a classification that is common and useful in stone processing. Nevertheless, technical parameters, such as the abrasion resistance of rocks, play an important role depending on the intended use.

Rocks are in the earth sciences according to their origin in igneous , metamorphic and sedimentary rocks divided, and according to their degree of consolidation in unconsolidated and consolidated rocks . These distinctions are familiar to stonemasons, but they are of secondary importance in technical processing. They also differentiate between soft and hard rock when working on them. In practical use for consumers in the building industry, this means, for example, that hard stone is not scratched with a kitchen knife or scratched in normal use.

General

The term hardness from the point of view of technical rock science cannot be exactly transferred in the conventional sense to a material hardness or mineral hardness . A classification attempt according to rock types, such as for granite, quartzite or sandstone, does not lead to any useful results because the measurable differences in rock-physical values ​​within the rock groups can be very large. The widespread but misleading term “hardness of rocks” is mainly based on the differentiated consideration of strength values ​​depending on the respective application. In road construction, the compressive strength (cube compressive strength, etc.) is of paramount importance for assessing the suitability of rocks, for example in the case of gravel . In the case of floor slabs or paving stones , abrasion resistance plays a decisive role. For structural parts with a load-bearing or cladding function, the flexural strength , cube compressive strength, shear strength or impact strength are of interest. These and other rock-technical values ​​provide information about the resistance a certain rock in a defined spatial position opposes to the processing tool and the loads occurring in the application. The colloquial term, which is also used by technicians, is mainly derived from the values ​​of the mean compressive strength and is often perceived empirically .

Various factors have an impact on the strength values ​​with measurable consequences. These include the ratio of the mineral components contained in the rock, the grain binding ratios in the rock structure and the grain size . Also of importance are the spatial alignment and distribution of the crystals or clasts , any macro or micro fissures that may be present , the porosity , the water content and the forms of various weathering phenomena .

Distinction

The classification into soft or hard stone follows the logic of the use of tools and is a sensible classification of how work stones are manually processed, machine sawn or ground. It is not always possible to derive definitive statements about the functional properties or later installation and application possibilities of natural stone from this distinction, because this includes profound knowledge of geology. For example, the soft rock dolomite can be more acid-resistant than the hard rock basanite . Ultimately, water absorption or strength cannot be derived either.

Manual stone processing

The following abbreviations are stamped on the chisels that stonemasons use for stone processing and with impact:

If the tools are used incorrectly, they can be damaged or even destroyed. A chisel for writing in sandstone will break off on a gneiss . If, for example, the Ruhr sandstone , a hard sandstone, is worked with the hard rock tool, the hammer , the surface of this stone is bruised in such a way that the surface can weather off like a shell.

Machine stone processing

The state of machine stone processing depends primarily on the use of the machines. Stone saws and grinding machines are still essential. But now there are also robot systems that are electronically controlled.

Stone saws

Ashlars are formatted with stone saws. Stone saw blades are used that are fitted with so-called disks. The trim on the edge of the steel blades that wears out is called a disc. The metallic disks are soldered on and are set with diamonds. The disks consist of different metals depending on the rock. In general one can say about the types of bonding of the diamonds in the saw blades:

A stone saw blade that is suitable for marble use is likely to be destroyed sooner or later when dividing granite.

Grinding and polishing

Nowadays, abrasives are mainly used, which hold the diamonds in metallic bonds (see above).

Soft rock

Soft rocks include, for example

Hard rock

Hard rocks, according to a classification of rocks into stone families, which are nowadays processed with hard stone tools, are for example:

jobs

other topics

Individual evidence

  1. ↑ A. von Moos, F. de Quervain: Technical rock science . Basel 1948, pp. 56-57.
  2. ↑ Otfried Wagenbreth : Basic scientific knowledge for civil engineers. Volume 3: Technical Geology . Berlin 1977, DNB 780237269 , pp. 98-101.
  3. ↑ Erhard M. Winkler: Stone in Architecture. Properties, durability. 3. Edition. Berlin / Heidelberg / New York 1994, ISBN 3-540-57626-6 , p. 38.
  4. ^ Ralf Villwock: Industrial rock science . An introduction to the technical petrography of usable rocks . Offenbach / Main 1966, pp. 118-119.
  5. ↑ Reiner Flassig: Machine, tool and device knowledge . In: Educational center for the stonemasonry and sculpture trade Königslutter (Ed.): Steinmetzpraxis. 2nd Edition. Ebner Verlag, Ulm 1994, ISBN 3-87188-138-4 , p. 361.