Know-how - both geological and technical - is of great importance in the natural stone sector. The Brachot Stone Expert Centre, headed by a doctor in geology, has compiled a detailed technical specification sheet for each of the natural stones in the Brachot-Hermant range. These sheets can be obtained from your professional stone supplier. The Brachot Stone Expert Centre will also be pleased to answer any technical questions you may have, relating to placing, use of adhesives, discoloration, etc.
In Europe a distinction is made between three different types of norm:
- norms relating to specific tests, such as porosity, compressive strength, hardness, etc.;
- norms relating to classification and terminology, including the commercial names for European natural stones;
- product norms, which set down the requirements which the natural stones must meet for use in particular applications (such as technical characteristics, tolerance levels for size and flatness, etc.).
Until recently, many of the norms for the testing of natural stone were based on the norms for concrete. Fortunately, more and more norms are now being developed for the specific testing of natural stone. The following is an overview of the most important of these stone-specific norms. Should you require more detailed explanation, please feel free to contact the Brachot Stone Expert Centre.
European standard NBN EN 1936.
This standard expresses the mass of the natural stone in kilograms per cubic metre (kg/m3). This cubic metre is made up of stone and pores (air).
The result gives an indication of the level of compactness of the natural stone. The higher the result, the more compact (and therefore less porous) the stone.
| Limestone |
1500-2800 kg/m©ø |
| Marble |
2600-2800 kg/m©ø |
| Granite |
2500-3000 kg/m©ø |
| Slate |
2650-3000 kg/m©ø |
As might be expected, the granites and slates are far more compact than the limestones.
In addition to the structure of the stone (open, closed, etc.), the composition of the constituent elements (minerals, etc.) also plays an important role in determining the volume mass.
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European standard NBN EN 1936
This standard indicates what percentage of the total volume (apparent volume) is made up by pores.
A distinction is made between two different types of porosity
- closed porosity: the pores in the stone are separated from each other;
- open porosity: the pore cavities are more or less linked by a series of chambers, allowing almost free passage to water.
As a technical characteristic, professional stone dealers will sometime quote an absorption level instead of a porosity level. This absorption level is expressed as a percentage of the mass (i.e., the mass of absorbed water in relation to the mass of the dry piece of tested stone).
Indicative porosity values: (in volume%):
| Soft (white) limestone |
5 to 50 % |
| Marble and compact limestone |
0,2 to 5 % |
| Granite |
0 to 2 % |
| Slate |
< 3 % |
There are large differences between the porosity levels for the various types of limestone. The percentage of pores gives a clear indication of the openness or density of the structure. These differences can be related to a number of factors, such as the depth of the strata from which the stone was extracted and the degree of metamorphosis which the stone underwent during its formation. Porosity levels will also often determine whether or not a stone can be polished - a finishing which is only suitable for stones of a sufficient density. Account must also be taken of the fact that stone types which absorb water with difficulty sometimes have difficulty in shedding this water at a later stage. This water retentiveness can be a disadvantage in terms of resistance to frost damage.
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European standard NBN EN 1926.
This standard expresses the level of pressure/force which needs to be exerted to cause the stone to almost break. This level of pressure is expressed in Newtons per square millimetre (N/mm©÷) and is adjusted to take account of the surface area of the stone being tested.
This standard can be used to choose stone for particular building applications, where the materials will be exposed to specific load levels (taking account of the safety co-efficient). In this respect, the heterogeneity of the stone plays an important role: some metamorphic stones have a grouping or alignment of their mineral elements in parallel lines or bands. This is known as foliation and the different layers have a clearly different mineralogical composition. This can result in widely differing compressive strengths, depending upon whether the pressure is applied horizontally or vertically.
Indicative compressive strength levels (N/mm©÷) of different types of natural stone are given in the table below:
| Limestone (white stone) |
2 to 240 N/mm©÷ |
| Marble |
40 to 230 N/mm©÷ |
| Granite |
80 to 400 N/mm©÷ |
| Slate |
40 to 260 N/mm©÷ |
It should be noted that the levels vary widely, even within the same stone type. Yet again, the highest scores (in average terms) are recorded for granite.
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European standard NBN EN 12372
The flexural strength - sometimes also referred to as the bending tensile strength or tensile strength - is expressed in Newtons per square millimetre and indicates the maximum pressure that a stone can bear for this specific type of load.
The flexural strength is always much lower than the compressive strength (approximately 1/10 for granite/marble and 1/15 for sandstone). The bending capacity can be very important for certain natural stone applications, such as freestanding staircases or overhangs. It is also important to take account of bending capacity when there is a high risk of cracks forming, as a result of shrinkage and/or thermal stresses.
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The hardness of a stone is usually indicated as a number which refers to the so-called MOHS table. This is a list of minerals, so ordered that a mineral can scratch the mineral before it in the table, but not the mineral after it.
Natural stones are compositions of different types of minerals and the concentration of these minerals can vary widely from tile to tile, block to block, etc. The hardness of a particular natural stone is therefore determined by taking the average hardness of the minerals which it contains (but without taking account of the relative quantities of these minerals). This means that the hardness figure is not always fully accurate for every piece of stone of that type, but it does give a good indication of its susceptibility to scratching. The following are examples of stone hardness.
- granite, gneiss: 6 to 7 (based on the presence of quartz and feldspars)
- basalt, gabbro: 5 to 6.5 (based on the presence of feldspar and hornblende)
- marble, limestone: 3 (based on the presence of calcite)
This means, for example, that marble and limestone can be scratched by an ordinary kitchen knife, but that granite cannot.
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This is the ability of the stone to withstand wear and tear caused by abrasive movement. This measurement is especially useful for assessing whether or not a particular natural stone is suitable for use as a floor in a particular location. The wear resistance test examines the abrasive effect caused by the passage of people over a natural stone surface. This gives a more accurate indication than hardness of the suitability of the stone for use as a floor covering.
A distinction is made between two different types of wear:
- Visible wear;
- non-visible (or 'deep') wear.
An example of visible wear is the fading of the sheen on polished stone (there are standardised tests for this type of wear). It must be remembered that - irrespective of the basic hardness of the stone - there is always a risk that a polished surface will be worn away with use. The wear of polished surfaces is not only influenced by stone hardness, but also by a series of external factors, such as the intensity of use, maintenance, the presence of abrasive particles (grit, sand, dirt, etc.).
As far as deep wear is concerned, there now exists a new European norm based on the so-called Capon test (NBN EN 14157). Prior to this, the Belgian Amsler test was widely used (NBN B27-003). In practice, the relative newness of the Capon test (and consequent lack of data) means that the Amsler criteria are still frequently applied.
| Wear resistance |
Intensive collective use |
Normal collective use |
Individual use |
| Amsler (mm/1000m) |
¡Â 4 |
¡Â 8 |
¡Â 12 |
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European standard NBN EN 12371
For this test pieces of natural stone are subjected to a repeated cycle of freezing and thawing. A measurement of the elasticity modulus determines whether or not the stone can be classified as frost-resistant.
Unfortunately, this European norm makes no comparison between the standard freeze-thaw test cycles and the actual conditions to which the natural stone will be subjected. In reality, every application of natural stone out of doors will experience differing levels of frost stress. For example, terrace tiles are normally in direct contact with a surface which can become frozen, whereas a ventilated cladding will undergo a much less severe frost regime. Various organisations are currently investigating whether or not alternative criteria, based on the European norms but more representative of actual conditions, can be devised.
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European standard NBN EN 14231
This standard indicates the extent to which a natural stone limits the risk of slipping. The slip resistance of a tile has an effect on the comfort and safety of potential users, as well as on the cleanliness and general appearance of the floor. The level of resistance can diminish with time, as a result of wear, dirt, maintenance products, etc.
The slip resistance value is to a large extent determined by the surface finishing of the tile.
For applications where slip resistance is an important element, it is preferable to opt for a rougher finish. European criteria for this norm still need to be definitively established.
Brachot-Hermant has invested heavily in know-how. The Brachot Stone Expert Centre is unique in Europe. Our technical data sheets are evidence of our thorough knowledge of the properties of natural stone.
Each sheet contains detailed information relating to appearance, petrographic profile (if available), finishing options, technical specifications in respect of the above mentioned tests (if available), possible uses and recommended placement and maintenance techniques.
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