Understanding Colour in Stone
By Scott Worthington
The simple answer is that some natural stone does fade especially varieties like the well documented water sensitive verde (or green marble – serpentine). The reasons for fading are varied however it is important to note that many of these processes can be managed by proper use of man made technologies such as sealers, good adhesives and waterproof membranes. So how and why do certain stones fade? To understand this phenomenon you must firstly understand how colour is created in stone. We see colour in stone in the main body and also in the veins that exist in marble and limestone. Colour is created by several different processes all working and modifying the minerals and chemistry of the stone.
Some minerals have their colour as an intrinsic part of their chemistry and this is called inherent coloration. The majority of the minerals that form rock, calcites (calcium and magnesium carbonate), quartz and feldspars are inherently colourless being either transparent or white in their pure form. However when these minerals exhibit a specific colour (exotic coloration) it is due to trace amounts of a particular pigmenting element. These are mineral solutions and gases or organic matter that is absorbed during the formation of the mineral or the overall process of rock formation. For example when limestone is transformed into marble (metamorphic process) the limestone is heated and becomes viscous. During this phase super saturated mineral solutions (water and gases) can pass through the viscous mass under great pressure. The result is a variety of colour in the form of streaks, bands, swirls etc.
Veins are the other main part of the colour palette we see in stone. These are restricted mainly to the limestone and marble family as granite is a silica rich crystalline stone whose chemistry and formation is more immune to the processes causing veining. The veins in limestone and marble are created after the formation of the original host stone and hence are a secondary process. They result from geological movement which stresses or shatters the stone causing cracks or fissures. Over a period of time super saturated mineral solutions infiltrate these fissures depositing minerals that eventually totally fill them. The type of mineral not only determines the colour but also the hardness of the vein creating both hard and soft veins. The softer the vein the more likely the minerals are calcites whereas the harder the vein the minerals will more likely be quartz based.
The last factor in creating the colour we see is the interaction between the minerals and light. Each mineral responds differently to different wavelengths absorbing and reflecting different parts of the visible spectrum, finally reflecting that part of the spectrum that we see as a particular colour. Add to this that each stone is made up of different minerals each reflecting different parts of the spectrum resulting in the huge variation of colour we see between different stone varieties as well as between stone of the same type.
So how and why do some stones and their mineralogy fade? The first and perhaps most common theory is that stone fades due to the suns UV light. UV rays or radiation delivers bundles of energy to the host which in turn creates chemical reactions. For example leave a newspaper in the sun and it will slowly turn yellow. These are called photochemical reactions. The importance of these chemical reactions is that they can set off a chain reaction of other chemicals that no longer necessarily require any light. This can explain why a stone may continue to fade even after installation in an area void of natural sunlight. UV rays will fade colours in stone that have an organic (from plants and animals) component in their coloration. Black marble, travertine and limestone can all be altered this way. It is also important to point out that there are other sources of UV rays other than just sunlight. Incandescent light bulbs emit a low but significant (over long periods of time) amount of UV radiation. Fluorescent light has a slightly higher level with Tungsten Halide emitting the most.
Although UV radiation is a significant source of fading in stone it is not the sole process at work. There are in fact several other processes that create the stone to fade to our eye. The first is the weathering process. Many stones from the limestone and marble family have soluble minerals in them such as pyrite. When combined with water and oxygen (sourced from the water) pyrite (an iron sulphide) forms sulphuric acid which dissolves the calcium greatly changing the stones coloration. Water from rain or even humidity dissolves the soluble compounds in the veins especially if the veins contain carbonaceous (organic) materials and in doing so creates different coloration. Any force that erodes or changes the stones surface texture (even microscopically) such as wind or water erosion or even foot or vehicular traffic can change the way the stone will reflect and absorb light and hence change the colour we see. Maintenance chemicals can also have an affect on the colour of stone. The chemistry of stone is basically alkaline (high hydroxide count) and hence when exposed to acids (hydrogen ions) the minerals can be damaged especially the calcium based stones such as limestone and marble. This damage is in many cases manifested as fading. Highly alkaline chemicals can in some cases also appear to fade stone. However in most cases this is due to a build up of the alkaline salts in the stones pores due to insufficient rinsing which causes a hazy dulling effect rather than true fading. However there are some cases where soluble organic compounds in the stone are affected by alkalines especially bleaching compounds such as chlorine and sodium hypochlorite resulting in fading. This is especially true of iron rich coloured stone where some of the iron is soluble.
In conclusion the fading of stone is a real issue – it does happen. It must be remembered that it is mainly a problem in the limestone and marble (sedimentary and metamorphic) families as far as commercial dimensional stone is concerned. In contrast to popular opinion fading is not due just to UV light and radiation but to a number of processes that in many cases work simultaneously. In many instances factors such as UV light cannot be controlled but many of the other processes can be managed using man made technology. Fading due to improper cleaning chemicals can be addressed using ph neutral cleaners. The proper design and specification plays its part by ensuring the right stone is used in the right environment. For example a soft stone in a high traffic area will wear quickly dramatically changing the colour of the stone. Finally and perhaps most importantly fading due to moisture related issues can be addressed by reducing and regulating the moisture in the stone and the installation system by installing good penetrating sealers as well as waterproof membranes and latex modified adhesives, mortars and grouts. All of these work together: sealers: reduce surface absorption, waterproof membranes: reduce moisture content from the substrate as well as speed up drying cycles by reducing the amount of overall water under the stone: latex modified adhesives, mortars and grouts: create denser materials that hold less overall moisture. Specifying and installing quality high performance versions of these technologies will go a long way to managing the issue of the fading of these problematic stones.