WONASA GUIDE Frequently asked questions by architects and designers on Natural Stone - and the answers

An initiative of WONASA (World Natural Stone Association) www.wonasa.com


1) Why should I use natural stone?
Stone is a natural product. It is respectful with the environment, it does not destroy it. It has proved its validity as a building product through its use over centuries, proving its resistance and durability. It is easy to use, clean and easy to obtain. Natural stone encapsulates beauty, history, culture, and in the mind of the people in all the cultures of the world, it is intrinsically associated with luxury, quality and permanence.
2) Can my client afford to buy natural stone?
Most probably yes, because there are natural stones which are very economical in price, and even the customer with a limited budget is often surprised to discover how affordable natural stone really is. For each project a natural stone can be found which meets your preferences and fits your budget. When comparing the life cycle versus price with other alternatives, natural stone is definitely affordable. However, there are also stones in the market which are really exclusive and premium and, like all luxury products, command a premium price.
3) How can I find the natural stone suitable for my project?
You should first ask yourself the following questions- in which climate will the natural stone be used, and for what application ? Is the climate of the place extreme, very cold, or very hot, is it moderate, or is it humid, or is it a combination of several climates ?Is the application for which you are looking for natural stone, an exterior or is it for interior ? Is it a wall cladding or is it a flooring ? Based on the answer to the above questions, you should now look for natural stone on the basis of physical and chemical properties and the local standards.
4) What kind of natural stones are suitable for my climate?
 The key is to know the natural stone application- will it be used outside or inside? Will it be used for an interior wall cladding, or will the application be for a high traffic flooring? Or will its application be a kitchen countertop or in the bathroom, or for use on steps, or window sills? The analysis of the physical and chemical properties of the natural stone will guide you to the best choice.
5) What is the range of natural stones that exist in colours?
 Endless! From pure white to the darkest black, and with a dazzling spectrum in between. The widest range of colors and their different combinations that exist in nature.
6) What is the geographical origin of the natural stones?
In just about all countries of the world natural stone is extracted in some way, after this unique, wonderful natural product is a result of geological formation, the process of which began in the initial stage of the formation of our planet, hundreds of millions of years ago.
7) What is the difference between granite, marble, limestone, sandstone, slate?
The difference between different types of natural stone starts right from the origin of its creation.
Granite is an igneous solid rock formed by magma with at least 20% quartz in volume.
Marble is a non-foliated metamorphic rock composed of recrystallized carbonate minerals, mostly common calcite or dolomite. Marble, in other words, is a metamorphosed limestone. Limestone is a non-foliated metamorphic rock composed of recrystallized carbonate minerals, commonly being calcite or dolomite.
Sandstone is a sedimentary rock composed mostly of sand sized minerals or rock grains.
Slate is a fine grained foliated homogeneous metamorphic rock, derived from the original shale-type sedimentary rock, composed of clay or volcanic ash, by means of low grade regional metamorphism.
Each of the different varieties have their own properties and appearance, and determine their suitability for the different applications one may require.
8) Is there a catalogue that shows all the existing natural stones?
 There are thousands of varieties of different types of stones, and new varieties are constantly appearing in the market as a result of extraction in new areas in different countries. The catalogues and the web pages of the different countries, or associations, individual companies, etc. show the stones that are extracted, or processed and marketed by them, and therefore, by definition, are only a partial database. It is also a good idea to check on whether the stone of your choice is still being extracted. Being a unique and irreplaceable product of nature, perhaps the deposits of the stone in question no longer exist !
9) Why can we not have all the different types of natural stones that we are shown in the catalogues? How can I ensure that I finally get the natural stone that I have chosen?
The catalogue is only the first step in the process of choosing a natural stone of your requirement. After analyzing the catalogues you should then ask for a real sample of the natural stone. In the case of selecting natural stone for a big size project, you could follow a further series of procedures. After having obtained a small sample, you could then ask your supplier to show you a small range of that particular natural stone variety initially so that you are, this way, better informed of what variations to expect. Obtaining the technical characteristics of the natural stone is then important. Bigger size samples could be asked for to get a better idea of how the final project will look like. If possible, in some cases, a visit to the quarries and factories may be useful and informative. Often a mockup at the source or the job site is done. And finally, quality control and inspection at job site may be needed. These procedures are often adjusted according to size of the project and country.
10) Is my stone sample an accurate/depiction of the stone?
 Samples only serve as an indication of a stone´s physical characteristics. Being a natural product, absolute resemblance, naturally, cannot be guaranteed. There may be variation in colour and in patterns. Try to see a big size slab or many slabs of the stone, if possible- it will give you a much better idea of what to expect.The modern technologies make your job much simpler- the high quality images that can be seen on the tablets and smartphones can give you a very good idea of what to expect.
11) What are the different finishes that exist for natural stone?
There are a wide variety of finishes, all of them help to highlight the beauty of the natural stone and to serve a certain purpose of usage. Some of the finishes are- polished, honed, sawed, sawed finish with diamond disk ,bush hammered, flamed, aged, brushed, silk effect ( with or without shine), leather finish, antique, split finish, etc. Constant innovation by the industry has now allowed us to enjoy new finishes for different kinds of stones. The properties of each stone defines which finishes can be applied and what changes take place to the aesthetic appearance of the material.
12) What are the different sizes recommended in natural stone?
 Depends on the particular application of the stone. Mosaics are available in the market in sizes as small as 1cm x 1cm. And there are also stone tiles of 1.20 m x 1 m, and sometimes, even bigger. Widely used sizes, however, are 60 x 30 cm, 60 x 60 cm, 120 x 60 cm. The architect can define the size of the project. Recommended sizes by the manufacturers are based not only on aesthetic criteria but also take into account the size of the blocks extracted in the quarries and the optimization in the production systems in the factories to achieve minimum wastage.
13) What are the general physical properties of natural stone that should be taken into consideration?
One should ask for the technical data sheet of the stone in consideration. The data will be of-1-a) water absorption b) Compressive strength   c) Frost resistance d) Resistance to Bending e) Density f) Abrasive resistance g) Resistance of surface polish against weathering. For external applications testing is becoming increasingly important especially in humid climates. In hot countries certain stones alter themselves, the physical appearance may change
14) What has the porosity to do with the water absorption of stone?
 Porosity is related to water absorption and density of the stone. The higher the porosity, higher is the absorption of water phenomenon. Depending on the application, one should refer to the porosity scale.
15) What is the lifetime of natural stone?
Eternal. What is the lifetime of the Pyramids (and they exist for 4000 years now)? Or those cathedrals, temples, mosques, palaces and imposing official buildings the existence of which we are taught of when we are very young and still exist when we are at the end of our lives ? In some places thousand year old buildings have been destroyed and recycled stone used in new buildings. In our modern buildings, however, the natural stone life-cycle is usually considered around 80 years.                       
16) How does natural stone compare to other alternatives in terms of sustainability?
Sustainability in natural stone is best understood in the context of thinking of lifecycle. This would mean analyzing factors like preparation of raw materials, manufacture of pre-products, the production phase, end of life disposal, etc. Impact assessment includes energy consumption, raw material consumption, greenhouse effect, acidification, over fertilization, environmental toxins, waste, etc. Natural stone has already been made over hundreds of millions of years and it is mined and the processing is done by physical, and not chemical means. The sub-products can be recycled. The burden on the environment is low thus making natural stone the ideal hard floor covering, when green thinking and sustainability is the criteria. (For more information, refer to study done in Germany on hard floor coverings at www.wonasa.com).
17) What should be the structure of the building so that natural stone is used in external wall cladding?
The main condition is the structure of the building must be solid enough to carry the weight of the stone, and thus must be properly designed. The stone can be fixed to any structure using suitable assembling technology which is determined by the characteristics of the construction.
18) What is the appropriate thickness when using natural stone ?
 The thickness is often subject to local standards which may vary from country to country. A key aspect to consider is the resistance of the stone to the force it is subjected to, in other words, it depends on the application. Generally speaking, in pavements, using granite,for example, if trucks are going to pass over it, use stone of more than 6 cm thickness, depending on the elasticity of the stone. 1 cm can be sufficient for the wall claddings. For kitchen countertops 2 cm stone can be used, though 3 cm and, in some countries, even 4 cm of thickness stone is also used for aesthetical reasons. Therefore, depending on the application and the properties, thickness may vary from 1 cm to 20 cm.
19) Is there any information available for fixing of natural stone in wall claddings?
There are many official and non-official organizations who have elaborated manuals which define the criteria and specifications in the case of wall cladding and flooring. The different manufacturers of anchoring systems should also be able to give you full and complete information on the existing technologies.
20) Can natural stone be installed on an existing floor/on a dry screed with adhesives?
The important thing is that no stress is caused which leads to deformation of the fixed stone and the whole structure remains durable. In other words, the surface on which the natural stone will be installed should be flat, solid, stable, and durable. There are many adhesives suitable for such floor assembling.
21) Can I install natural stone on my barbeque terrace?
Yes. Just make sure that the stone you use is a high density, low absorption one, this will mean the floor will stay clean for a long, long time.
22) Is the installation of natural stone tiles complicated?
Stone installation is best done and is easy when done by stone professionals. A great stone poorly installed may end up looking bad. If possible, ask your stone installer to show you his previous jobs on installation. The key thing to take into account is to choose the right stone for the right application.
23)  Is there any good method for polishing natural stone after installing the tiles to make the joints look even?
The joints can be filled with a polyester/epoxy based filler or adhesive. After hardening the entire surface is then grinded and polished. Expansion joints are then filled with a silicon sealant suitable for use in natural stone.
24) What kind of maintenance is needed for stones already installed?
In general, natural stone is a construction material that needs very little maintenance if a little care is taken in treating it (depending on the stone). After installation a basic cleaning process is necessary to remove construction and installation related dirt. For daily cleaning water can be used in general; it is also recommended that from time to time special daily cleaning agents from reliable stone chemical manufacturers be used. 
25) How can the natural properties of natural stone be improved by the use of chemicals?
 If the problem is that the stone is breaking up, there are resins which help to consolidate stone. If the problem is that of oxidation, then the oxide needs to be removed first and then the surface protected with specific resins to make them impermeable. Where is the stone installed ? In case it is a wall perhaps a water-resistant liquid may be needed every 5 years. Or perhaps an anti-graffiti. In restoration work, stone can be improved, it can be protected against external agents. The chemicals help to enhance the natural beauty of the stone.
26) How can I protect/seal/impregnate my limestone to prevent staining?
 Depending on the stone, it is advisable to use either non-colour changing or colour enhancing impregnators. Impregnators penetrate inside the stone surface, however, they do not form a coating layer. Impregnators allow the stone to keep its breathability but at the same time protect against the easy penetration of soiling substances ( oil, water, dirt, etc.).
27) Is it true that polished natural stone is very difficult to keepclean compared to ceramic tiles ?
 Not so,especially in the case of granite. In the case of certain marbles and limestone, it is important to follow the right guidelines for maintenance.
28) What are the technical standards for stone used in exteriors?
 There are different standards in different places. In the USA, for example, the ASTM standards are followed, in most of Europe the EN standards are the norm. In the UK, the BSI standard prevails.
29) Is natural stone radioactive?
 Radioactivity is all around us in the environment, it is part of nature, emitted by the sun, it is in the soil, air, etc. It is also emitted by the modern appliances like TVs, etc. Granite does emit very small amounts of radioactive gas radon, and also by direct radioactive decay. Therefore, the appropriate question to ask is -is the level dangerous? The answer is very clearly- NO. Several scientific studies have been conducted, mostly in USA, on this matter and they all ratify this conclusion. (For more information on the matter, consult marble-institute.com/consumers/granite_radoninfo.cfm)
30) What steps should be taken to avoid problems when fixing stone or when doing so in old buildings?
 By properly following existing established standards and guidelines, you should have no problem. How will the stone be installed? There is an appropriate stone for each place and application.                        
31) The natural stone installed is losing polish. What should be done?
If the stone is bland, you can fill it with a liquid resin that strengthens it. If the stone has scratches on the floor but you do not want to polish the whole floor, then use products which even though they do not remove the scratch, end up giving the impression that the scratches are no longer there. Alternatively, in the case of a big floor, you may need to grind and polish it again.
32) There are holes in the stone installed. How do I solve this problem?
Holes can be filled with transparent and coloured fillers. Depending on the size of the holes and their number, either a spot repair with a UV-Filler or a treatment of the whole surface may sometimes be necessary.
1-In very cold and rainy weather climates, for external use, avoid using polished or fine honed finishes in natural stone floorings in exteriors, since they will create problems of slippage. Better to use flamed, or bush hammered or any rugged finish. It is advisable to apply an anti-slippage finish to the stone.
2.-To avoid slippage on stone installed inside a building, anti-slip stripes made of 2-component –epoxy containing corundum should be placed on the stairs. In the entrance areas, anti-slip agents enable roughening up of the polished surface.
3- To avoid efflorescence /salt/moisture stains on stone installed, the stone should be protected on the back side and the flanks with an alkaline-resistant special coating in conjunction with silica sand.
4- When doing exterior cladding made of a soft natural stone (like soft limestone, for example) in a cold climate, care should be taken to follow the right criteria.
5-When deciding on the sizes of natural stone pieces for cladding or staircases, also consider whether there may be difficulty in transport, in handling and assembling of the pieces.
6-When choosing natural stone for countertops, it is not enough to consider only the visual, aesthetic aspects, one should also consider the physical properties. Limestone and marbles should, in general, not be used as kitchen-tops, since they are not acid resistant. To protect against stains from oil and grease, a special impregnator should be used.
7-For countertops use smooth finishes avoiding the rough finishes which are more difficult to clean.
8- Fountains are often made of concrete and then finished with stone elements. Often the joints look ugly after a short period of time due to water penetration effect. This problem can be avoided by using solid and well sized elements eliminating the joints to the minimum number. Alternatively, it is advisable to bond the stone cladding on a fountain with 2-component- epoxy- adhesives and also fill the joint with 2- component-epoxy- adhesives. In this case, no efflorescence from the cement will appear.
9 When using under the floor heating, some stones (like travertine, for example), which are filled, the filler can pop up due to the heat. This is not a defect. Particularly if the heating is done with water, run it before installing the stone tile so that the concrete settles down, this way the risk of the stone cracking is highly reduced.
10. Some stones need to be resined on the face, or reinforced on the back side. This way these stones become more stable, and we can now enjoy a wider variety of what is available in nature of natural stones.