Natural Stone Identification
Know your stone
Competent stone craftsmen should be familiar with the basic geology of stone. A stone craftsman should also be familiar with the formations and changes of the stone he is working on. The study of these changes is known as geology.
Stone flooring comes in such a variance dependant on whether it is an interior or exterior installation, the stone type determines the appearance and performance of the floor. Typical interior stone types are; marble, granite, limestone, travertine, slate, terrazzo, sandstone, terracotta, quartz, onyx, basalt and minton tiles. Each stone type has a rating on the Mohs scale of hardness, this will dictate how the surface will wear with regular abrasion from footfall or traffic. Exterior stone types will be on the higher end of the Mohs scale and will be more resistant to the elements and abrasion, these will typically be slate, granite, sandstone and concrete based materials. Stone floor restoration techniques will differ between stone and installation types, for instance a high quality marble within a hotel entrance will require higher grade of diamonds to polish the stone whereas a terrazzo floor will require a slightly lower grade of diamond to achieve a highly polished finish. Due to different densities of stone, the hardness and types of diamond abrasives varies to cope with the hardness or softness of the stone, the identification of stone types is a vital part of stone refurbishment. Natural stone floors which are badly pitted, scratched, dull or damaged from poor maintenance the use of diamond grinding machinery will be required. The top surface layer of the stone will need to be removed using metal bonded diamonds, these can remove stock of 5mm if required, the following stages of grinding involves using numerous grades of resin diamonds to achieve the desired finish for the client. The grits of diamonds range from 16 grit which is a very coarse abrasive all the way through to 3500 grit which will be a very fine grade to obtain a ultra-gloss finish. When wet or dry grinding, it is important to mask the surrounding areas to protect against dust or slurry affecting any adjacent materials, the dry grinding process will require the use of industrial fine dust vacuums to contain the dust. If a floor has become dull and dirty but is in need of a slight appearance lift, the use of abrasive cutting pastes that lightly remove the top surface, deep clean the stone and grout are beneficial towards cost savings. This can then be polished using the correct polishing techniques and chemicals for the particular stone type. The results are not always as impacting as a full resin diamond cut but the transformations in appearance are always dramatically improved. Beaver Floorcare will provide you professional and effective stone floor restoration services, contact us today for a survey and no obligation quotation. Why is geology so important to the stone craftsman? There are many reasons. For example marble and granite differ enormously. Granite is almost totally acid resistant but marble is not. Granite is denser than marble therefore different abrasives should be used when refinishing or refurbishing them. Granite and marble for example differ greatly in their hardness. A harder material such as granite may require more dense abrasives for refinishing. Certain limestone contains very sharp minerals that can quickly damage a polishing pad or set of diamond abrasives. The craftsman must identify which stones these are before any work can be started. A stone craftsman is exposed to many different varieties of stone. It is believed that there are somewhere near 9000 different varieties. Of course it would be impossible for anyone to know every one, but all stone can be classified into three distinct groups. These groups are how geologists classify the many types of stone. Learning these three types and their characteristics should be the basis for any training that a stone craftsman receives. Knowing the differences between these groups can solve most of the problems a craftsman will face. For example: granite, which is an igneous rock, contains quartz that is very hard. For this reason, the proper abrasive must be chosen to refinish it properly. Marble, which is a metamorphic rock, contains calcium carbonate, which reacts to acids. This knowledge would tell you that acidic cleaners will damage most polished marble surfaces. Travertine, which falls into the sedimentary group, consists of small grains of minerals, which are bonded to each other with softer minerals. The minerals are very coarse and can damage a diamond abrasive very rapidly.
The following are the three classifications of stone and how to identify them.
Igneous Rocks (Granite)
Igneous rocks are formed from the solidification of magma deep in the earth they are 45 to 66% silica (Quartz). The remaining minerals are mostly feldspar, mica and iron ores. Granite is the most abundant igneous rock found on earth.
Identification of Igneous Rocks
Exhibits a Crystalline form with grain size ranging from very small to several inches. The large crystal granites are fonned when the magma cools slowly. The smaller crystals are formed when cooling is very rapid. All this takes place deep in the earth before the magma reaches the surface. If it does reach the surface then we have what we call lava.
- Hardness ranges from 6 and higher on the Mohs scale of hardness.
- Performing a scratch test with an ordinary knife blade can identify igneous rocks very easily. If it is difficult to scratch, it is most likely igneous.
- Igneous rocks will generally not react with acids. However Hydrofluoric Acid (HF) will affect a polished surface. Many stone cleaning agents designed for sandstone and brick contain hydrofluoric acid. Make sure to read product labels and MSDS when using any stone cleaner or chemical. Avoid using HF on all stones if possible.
- The minerals contained in igneous rocks are usually dense and packed tightly.
- They will lack bedding or foliation. However, be aware of a granite look-a-like known as Gneiss. Gneiss looks like an igneous rock but is a transitional material between metamorphic and igneous. It is a brittle material and does not have a bedding plane.
- White granites are rich in potassium
- Combinations of mica and quartz result in grey.
- Pink contains sodium and calcium rich feldspar.
Sedimentary Rocks (Limestone & Sandstone)
Sedimentary rocks are simply produced from erosion of other rocks as well as compression and underground water erosion (Lithofication). The sedimentary group of rocks can be classified into two types: Limestone and Sandstone.
Limestone is formed in the shallow waters of the sea shelf. It consists of calcite but may be mixed with other minerals depending on how clear the water was when it was formed. Much limestone contains a mineral known as magnesium carbonate (Dolomite). Dolomite does not react with acid unless it is first crushed. Any rock with more than 50% carbonate minerals is classified as limestone. Some common limestone includes travertine, Bottacinno, coral, shell stone or coquina.
Identification of limestone
- Because limestone is formed in the sea it will often contain fossils and shell fragments.
- They will have a bedding plane
Sandstones are also sedimentary rocks that are primarily quartz minerals loosely cemented together with calcite, iron oxides and/or mud. Sandstones are and have been used frequently as a dimensional building stone. Major cities throughout the world contain buildings made with sandstone. Some common sandstones are brown stone, shale, etc ..
Identification of sandstone
- Sandstone can be recognised by its distinct sand-like appearance.
- Fractures very easily around the individual grains.
- Exhibits a distinct bedding plane.
Metamorphic Rocks (Marble and Slate)
Metamorphic rock is limestone that has been exposed to high temperatures and high pressures over a long period of time. This change is known as metamorphoses and hence the name metamorphic. The change causes the minerals to go through a molten phase. For this reason marbles often have distinct swirls or bands.
To understand how this change takes place a simple experiment can be performed as follows: Take a handfol of snow and examine the snowflakes carefully. Next compress the snow into a loose snowball. Now compress the snowball as hard as you can without breaking it. Break the snowball and examine the crystals again. They have undergone a metamorphosis and have changed their shape.
Identification of Metamorphic Rocks
Most marbles have distinct veins, swirls or bands. There are exceptions to this rule, so do not rely on this parameter alone. Light veins can contain quartz and feldspar. Dark veins contain hornblende and/or biotite.
- All marbles will have calcite and/or dolomite.
- Will react with acid.
Is it Marble? Granite? Limestone?
Due to the vast amounts of stone types available, it can be difficult to get the exact name of the type of stone, however it is possible to narrow the search by classifying the stone type into one of the three types we discussed. The following is the steps you will need to take to classify them into one of these types:
a) Visual Identification -Does the stone have veins running through it? If so it may be marble. Absents of veins may indicate it is granite. There are also several stone posters that can be supplied or purchased from stone wholesalers that will help you with identification.
b) Hardness Test -Perform the hardness test as previously outlined. If the stone is hard and it is difficult to scratch, you probably have granite. If it is soft and scratches easily it may be marble or limestone.
c) Acid Test -Place a drop of hydrochloric acid on the stone(vinegar or cola if easier) If the acid bubbles and/or the polished surface dulls, it's marble. If no bubbling occurs it may be granite.
d) Finish -Take a close look at the surface finish. If it has a flame textured finish, then it will not be a marble. Be a little cautious though as some limestone can look to be flame textured but be 'bush hammered' (more of that later), so be careful in coming to a conclusion.
THE NAMING OF STONE
As we have already mentioned there are nearly 9000 types of stone and probably three times as many names. Do not let names confuse you. Many importers will give stone a different name. For this reason it would not be unusual for one stone to have several different names.
Over 50% of the worlds fabricated stones are sourced from Italy. Italy has its own deposits, however the majority of stone is purchased in block form from quarries in other countries. The Italians then cut and fabricate he stone to give it the added value of say a polished slab or cut tiles. As a result of the Italians dominance in this area, many stones are given Italian names. The names usually reflect the colour, region or pattern of the material. For example Bianco Carrara is a white (Bianco) stone from the Carrara area of Italy. You will also find that the names describe a feature of the stone. Breccia is a common Italian term used for the stone that has a broken appearance. Breccia stone is a marble that is fonned in areas that have experienced earthquakes. The violent movement shatters the stone and voids are created between the particles that have broken up. These voids then fill with various minerals cure and cement the shattered piece together. Breccia can be recognised by its broken and fragmented appearance.
There are many materials that can be mistaken for the real thing. These are often tiles that look like real stone but are man made or a combination of man made and real. The craftsman needs to be able to recognise these artificial materials. Many of these imitations mimic stone on their surface only and therefore if you try to polish or re-finish them, you could very easily remove this finish and end up replacing all the tile.
Following are 'Look a likes'.
This is a plastic material. It is common on bathroom vanity tops and shower walls. Cultured marble is made by pouring a plastic resin into a mould. During the pouring process and as the mixture cures, vein like patterns are introduced resulting in a marble like appearance. In order to give the cultured marble a surface gloss, a thin gel coat is applied. This gel coat is the are that will create all the problems for you. If you try to put an abrasive on it you will rapidly remove it and be left with a dust. This will then be non-repairable.
Cultured marble can be identified by its warm touch. If you suspect it is not real marble, touch the surface with the palm of your hand. Cultured marble will always feel warm, where as stone will have a cool touch.
NEVER refinish a cultured marble. The only works that can be done to this surface is to apply some solid wax and burnish it with a lambs wool bonnet.
This another man made material called Corian. Like Cultured marble, Corian is a plastic resin. It differs from Cultured marble as this has no false surface coating and can be reface and polished like stone. Corian is very popular and can be found as kitchen tops, sinks and most surfaces in bathrooms. Corian looks very much like granite but feels warm to the touch. Corian also lacks the depth of shine associated with granite.
Ceramic tiles are clay a fired man made material. Many of the new generation tiles are manufactured to look like stone. Some of the stone imitations are excellent copies and can easily fool the experienced eye. Although they may look like real stone they do not react like the real thing. Ceramic has a baked on finish that is very thin (the cross section would look like a chocolate digestive biscuit). This finish or bisque can wear and break away under sudden stress and if sanded will disappear rapidly. As the tile is a clay material it will feel very cool to the touch so be careful it does not catch you out. The best way to tell if it is a ceramic is to carry out the knife test in a discreet corner. If this is not possible go to an edge and try to view the side of the tile where it has been cut. If the side is deep red or brown then this is the clay and you have ceramic.
Porcelain is similar to ceramic but differs in its manufacturing process. It is fired for longer and at higher temperatures than ceramic and therefore is a very tough and durable material. Porcelain tiles are an homogenous material and therefore do not have the baked on surface that a ceramic would have (the cross section would like a plain digestive biscuit). It is possible to finely hone and polish porcelain with diamonds and very fine powder abrasives, however it is very difficult and very time consuming.
Terrazzo is a mixture of Portland cement (matrix) and marble/limestone chippings (aggregate). Traditionally this was poured, laid and fabricated on site in large sections; more recently it has become the norm to fabricate the tiles at the factory in 305mm x 305mm fonn and ship to site. The terrazzo is built up with two layers comprising an overall depth of 40mm. The first 20mm will be cement only and on to this bed will go a further 20mm of the blended cement ( coloured to choice) and chippings. This is roiled to remove air, cured then ground to a finish with grade 60 to G 120 blocks. Sent to site they will be laid with a liquid grout mixture flooded over the surface. The grout is worked between the tiles to fill the void. The floor is then ground flat to give a uniform and even surface.
Unlike natural stone, Terrazzo has a very uniform appearance and can be supplied in almost any colour. On close inspection the small marble chips can easily be identified though when you first encounter Terrazzo it is possible to confuse it with granite. The best way to tell is to perform the knife or acid test. If it scratches or etches than it is terrazzo. Terrazzo can be ground, honed and polished just like marble.
Agglomerates are produced from broken pieces of marble or limestone bound together with a polyester or epoxy resin. The refinishing and polishing characteristics of the binder is similar to the stone except for one feature. The process can often capture air, which creates minute cavities in the cured material. These cavities can become exposed during the refinishing process and are extremely difficult to fill. Always note this and inform you customer of the possibilities of this occurring during your work.
To identify agglomerate, look for distinct shapes and different types of stone in one tile. Different colours are a give away that it is agglomerate. Agglomerate like terrazzo can be worked just the same as marble.
Terracotta is similar to ceramic and made from clay. Terracotta is used both on building exteriors and floors. It can be fired at a lower temperature than ceramic and for a shorter time; consequently it can be very porous and it is very abrasive. Terracotta cannot be refinished.
OTHER STONE TYPES
Marble granite and limestone are the most common floor types that you will enconnter. There are however other stone types you may come across. The following is a brief description of these.
Slate can be found both indoors and outdoors. It is a stone that has been metamorphosed from shale and consists of clay like materials. It can be recognised by its sheet like appearance and is found in blacks, greys, browns and blue/greys. Slate will not accept a high polish, however if it has a gloss then the slate has had a glossy coating applied. Slate is a challenge for the craftsman since it contains a high clay content and will often flake, spall and develop efflorescence. Slate is best treated with a good quality silicone impregnator to increase durability and a satin finish topical coating to enhance the colour and form.
Sandstone is a sedimentary material that consists of sand crystals cemented together with clays or calcium. It is most often used as a flooring material externally. Sandstone is very abrasive and porous and will get dirty very quickly. Often the craftsman is asked to clean or restore the floor as a consequence of its poor maintenance qualities. Never put diamond abrasives on sandstone as the resin holding the diamonds will wear away in seconds. If abrasives are required then use large silicon carbide abrasives to carry out a light reface. Use a dust extraction system on the floor machine as sandstone dust is regarded as dangerous and carcinogenic. An effective wet method that can often give good results is the use of Bi-carbonate of soda. Apply a kilo of the powder to the floor, place a black or green pad to the floor machine and add water. Work the mixture but it is crucial you keep it wet. TI1e surface will rapidly clean up though be aware that you are abrading the surface and a thick slurry will be created. Work only 2 sq.m. at a time. Vacuum up and rinse thoroughly. Leave the floor for 24 to 48 hours. Check with a moisture meter that the floor is dry, if so then apply a silicone impregnator, if not then wait until the floor has dried.
This is a rare material but is being used more today than before. It is a metamorphosed stone and is classified as sandstone. For this reason it exhibits all the properties discussed under the sandstone category.
Shell stone / Coquina
Shell stone or Coquina is limestone composed of broken fragments of shells and corals. It is a sedimentary stone and is very porous. It can be easily identified by its shell and coral content. It can be a very abrasive stone and like sandstone can wear resin diamond abrasives away in seconds. Always be aware of this if you are asked to work or reface this material.
Flagstone is a term given to all stone material cut into thin irregular shapes. Found extensively as paving, flagstone can be almost any stone but is most commonly sandstone. Never mechanically work a flagstone only ever clean and seal.
Onyx is a type of marble that has been formed by deposits of cold solutions. It is translucent material with veins running concentrically to one another. It is often expensive and can be found on table tops and small pieces of furniture. It can be treated just like marble.
Soapstone is one of the softest materials composed of the mineral talc. For this reason it makes an excellent carving material and can be found on :fireplace surround and hearths. It is treated like marble and should be sealed.
Travertine is limestone that has been formed in Hot Springs. Water movement caused the erosion of the stone creating numerous holes of varying sizes. A polished travertine will have its holes filled with colour matched Portland cement. This will not take a high polish and this will give the surface a blotchy appearance.