This month Barry Hunt turns his attention to a stone that holds an elevated position among the world’s dimensional stones – marble. When people want to make a particular statement, it is marble they turn to.
Marble is a word we learn not just as the name of a class of stone but also as a description of a kind of figuring (marbling). Such is the powerful impression this stone has made on our visual perspective.
We all think we know marble. We all want marble. It is the quintessence of elegance and, dare I say it… class.
Marble was recognised by early civilisations as a stone ideal for carving inscriptions and sculpting statues. It is harder to work than the more ubiquitous limestones and sandstones but its dense structure allowed early sculptors to represent life more convincingly, making it the choice of Kings and Emperors.
Perhaps it is coincidence that the centres of both the Greek and Roman Empires had good local resources of marble and were in reasonably warm climes where it was recognised that marble created an atmosphere of coolness and tranquillity.
Perhaps Socrates, Aristotle and Pythagoras might not have thought so deeply if their surroundings had been made from other stones.
It is easy to imagine the power and majesty of the Parthenon when replete with its (Elgin) Marbles or the Forum in Rome before it was sacked and the marbles burned to make lime.
Today, two of the best known marbles continue to be Carrara from Italy and Pentelikon from Greece.
Marble can be given a high polish with less effort than alternative polishable stones (such as granite and porphyry) but it is not just this property that makes it so favoured. Its typically large calcite crystals allow light to penetrate without great absorption, bringing a depth to the stone that is unmatched. The crystals sometimes sparkle like sugar where there is reflection from cleavage planes and petrologists apply the term ‘saccharoidal’ in recognition of this effect.
The traditional mason regards marble as any stone that may be given a polish with relative ease. This view has become clouded as technology allowed granites and other harder stones to be worked more easily. So the definition of what constitutes a marble in building terms has reverted to the geological view – the product of the recrystallisation by heat and pressure of limestone and related calcium-based rock types.
Because of the continental scale mode of formation of marble, when deposits are found there is rarely a problem with bed height or block size. The principal factors when selecting a marble for use are aesthetics and recovery rates.
The highest quality marbles are usually considered to be those with very little variation from veining, spotting and other features, the pure whites with noticeable translucence. Perversely, such quality lacks the features that make marble so recognisable.
Where veining, spotting and other features occur, the colour and intensity provide the character that can be completely changed in the finished product by altering the orientation of sawing the blocks into slabs.
Most commercial marbles tend to be of the recrystallised limestone variety rather than derived from other stones such as the forsteritic marble that may form from the alteration of less common dolomite.
The recovery rate is critical in many operations and in most instances this should be high – as much as 80% – as most resources allow the cutting of neat, square or prismatic blocks using diamond wire sawing in the quarry.
Unfortunately, the cutting of large blocks in some resources can result in problems if the blocks are not quickly sawn up into smaller units. Residual compression may lead to expansion of extracted blocks once freed from their confinement and the blocks can fracture and split.
This was a problem identified at Ruschita quarry in Romania, where a change from a Russian system of small block production to modern large block production actually reduced recovery rates to less than 10% rather than improving it as had been expected.
To find substantial examples of the use of marble in the UK at first seems difficult because marble is not recommended for external use in these islands. An acidic environment can rapidly remove the polished surface of marble. A notable exception is Marble Arch in London, completed in 1828. It was built using Carrara marble to architect John Nash’s design for the main entrance to Buckingham Palace. It was moved to its current site by Hyde Park when the palace was extended in 1851. The arch has withstood well the ravages of one of the most polluted locations in the UK. However, other examples of the external use of marble have been confined mostly to statues, figurines and gravestones.
The use of marble in the UK has also been restricted by the geology – there is not much marble in the ground. A few years ago Ledmore marble from Scotland was produced commercially. It is a beautiful, forsteritic marble with a wildness of greys, olives, blues and other hues. Now it is only extracted to be crushed for aggregate.
Purbeck and Ashburton Marbles are not metamorphosed stones and are not, therefore, true marbles but hard limestones.
Expanding the search world-wide and away from antiquity, one of the most spectacular uses of marble is the Taj Mahal, described by the Nobel Laureate Rabindranath Tagore (1861-1941) as “a tear on the face of eternity”.
Built to the memory of Arjumand Banu, the wife of Shah Jehan in the middle of the 17th Century, the white marble used was quarried at Makrana near Jodphur in Rajasthan. There is a great tradition of marble working in India that remains strong today.
The most common place to see marble in the UK is in the home. It was used in many Victorian and Edwardian residences for fireplace mantles. Marble has seen its prominence replaced by granite in domestic interiors in the past 15 years but it is making a come-back now and has never been out of fashion for corporate buildings and hotels, where it is used for internal floors, walls and bathrooms.
Fitness for purpose
Marble may be considered a premier stone, although its use has to be considered carefully as it can conflict visually with its surroundings. The coldness associated with marble may not be appropriate where a lively atmosphere is wanted, although underfloor heating has changed the nature of marble as a flooring product.
For larger projects, the architect also has to consider whether sufficient stone is available that is within an acceptable range of variation. It is not uncommon for large floors to be laid out at the processing factory for approval prior to shipping to site. Limits may be placed on background colour variation, frequency and size of veining, spotting and other features, and the presence of voids that can be filled if sufficiently small.
The only other aspect the architect should really be concerned with is maintaining the high quality finish, which is possibly the most difficult aspect, as the purity of the marble might mean that even the slightest stains that would not be noticed with other stones become highly visible.
Marble is a good material for an engineer to use. It is typically strong, has little directional variation and is rarely affected significantly by water saturation. Also, when marble weathers it does not lose strength. Instead it tends to lose its surface gradually, resulting in the thinner stone breaking at a lower stress, but not significantly so.
Where problems begin is usually with the veining if this is incomplete or formed from minerals that reduce rock integrity. As marble is most commonly used in tile or slab form, the flexural strength should be regarded by the engineer as the most important property.
It is also important that this strength is determined using a four point bending apparatus in preference to a three point system (often referred to as the modulus of rupture) to identify more accurately the potential strength variation.
Impact resistance should be considered when employing marble for walling or flooring. When used as an internal wall lining and the occasional external cladding of façades, usually at thicknesses of either 20mm or 30mm, there might be impact at low level from people wheeling trolleys or other equipment, while externally at higher levels cleaning cradles could buffet the stone.
Fixing locations will be stressed with impact, so the type of fixing, how close it is to the stone face and the preparation of the holes to accept the fixings all become critical. Length to width ratios must also be considered.
Floors might be subjected to impact by falling objects and the point impact of metal shoe heels, both of which can cause ‘star’ marking effects. Star marking results when there is local crushing of the crystals, upsetting the light path and causing the marble to lose some of its visual depth. There is little that can be done to prevent star marking during normal use.
Marble faces a safety conundrum because people like to see it polished, but a polished marble floor tends to be slippery, especially when wet. Under CDM regulations, a designer cannot knowingly install a potentially dangerous floor with impugnity.
So are polished marble floors disappearing into history? No. Polished marble floors are fine in dry conditions and so maintenance should be carried out at times when the floors can be closed to traffic. If wetting cannot be avoided, such as in food serving areas or close to swimming pools, treatments specially developed for use with polished stone can be applied or the stone can be given a rougher finish, although if it is, the reasons for using marble in the first place might need to be reassessed.
Problems with marble
The greatest problems are those that affect the aesthetic properties and the first simple advice is to ensure that light coloured marbles are bedded using light coloured cements, preferably white cement. There are now a variety of proprietary brands available with enhanced properties such as rapid drying, which reduces the chances of ‘picture frame’ staining round the edges of the tiles where moisture has caused the migration of minerals that stain the stone.
When using thin marble, dark coloured cements can actually be seen through the marble and darken the appearance of the finished floor.
Another problem with dark cements, most notably Portland type cements, is that alkalis and other salts may pass from the cement into the stone and can sometimes attack the marble. The results of such migration include surface efflorescence that upsets the polished finish and, in the worst instances, provides a nutrient source for bacteria that can bloom below the surface and result in unsightly brown staining.
One problem with all flooring, but especially marble, is installing it at an appropriate point in the construction programme. Floors are often installed at an early stage and then have to be protected from later construction and service installation. A lack of supervision and the unwitting behaviour of other trades can result in damage that may be irreparable.
Sometimes impermeable sheeting laid over marble floors leads to ‘sweating’ and surface reactions that can result in an apparent loss of polish.
Once in service, daily wear and tear can rapidly change the surface appearance of marble unless maintenance procedures are undertaken to a strict regimen.
Unfortunately, in the real world, cleaning contractors, facilities managers and building owners change and the correct maintenance regime can be forgotten. Generally, water and / or neutral pH cleaning agents only should be used… and then sparingly.
When stains occur, some people cannot resist the temptation to resort to bleach or other strong cleaning agents. These will not solve the problem, they will make it worse.
By far the best option is to have marble sealed professionally when it is installed. This will reduce the potential for dirt ingress and make it easier to clean.
It used to be thought that if a stone needs treatment it must be the wrong stone for the job. This attitude has now changed and there are many proprietary sealers, cleaners and enhancers for marble available to the trade and consumers.
The alternative is simply to accept the general dulling of the surface and increase in dirt. The surface can be re-finished periodically to bring the stone back to its original look or it can simply be left to age.
An alternative and often preferred solution to cleaning is the process known as ‘crystallisation’. This involves a fluoride-based compound being placed on to a marble floor and rubbed at high speed with a ‘burnishing cloth’ or fine wire, inducing a chemical reaction that creates calcium fluoride and / or calcium fluorosilicates at the surface. These are more chemically inert than the original calcite of the marble and also harder and thus more resistant to scratching. This process helps to seal the surface, making it less porous, but it is not a permanent solution and crystallisation typically needs repeating every three to four months on a publicly trafficked floor.
Marble surfaces in kitchens are likely to disappoint no matter what maintenance regimes are carried out. The array of products used in kitchens, the different food stuffs, the persistence of water, the use of sharp or heavy metallic implements and frequent turning by people in confined spaces provide an unforgiving recipe for disaster.
The special case of marble warping
One of the most intriguing problems affecting marble is warping. The problem is nothing new and was recognised as long ago as the 19th century in New Orleans in the USA. New Orleans has a high water table because of the Mississippi delta and dead bodies should not be buried in the ground because of the risk of contaminating the water supply, so they are interred above ground. The stench of rotting corpses coming from marble mausoleums first alerted people to the fact that marble was warping and breaking the seals on the tombs.
Move forward 100 years to 1972 and the opening of the Amoco (now renamed Aon) building in Chicago, then the world’s fourth highest structure and certainly the world’s tallest marble clad building. Carrara marble was reputedly chosen because the owner’s wife liked the fact that it had been used by Michelangelo in the Sistine Chapel.
Within 10 years, pieces of marble had begun to fall off the skyscraper as a result of the marble warping, so between 1990 and 1992 the 44,000 panels were replaced with Mount Airy granite from North Carolina.
Since then, reports of similar problems on other buildings have steadily increased. They include La Grande Arch de la Défense and Bastille Opera House in Paris, Finlandia Hall in Helsinki, and other buildings in Germany, Sweden, Italy and Jordan, and at least two buildings in the UK. Research is on-going and has already determined that several different types of marble and even some dense recrystallised limestones can suffer from warping, although not all marbles do.
While there remains debate about the exact cause of the warping, the degree of it is clearly influenced by panel thickness and is greatest when the panels are 20-40mm thick, which is just the thickness of most marble exterior cladding panels.
Efforts to limit the problem so far have proved futile and repair strategies have been implemented, ranging from piecemeal replacement to complete renewal. In one instance in the UK an ‘invisible’ wire mesh has been constructed to try to prevent warping and to stop any panels that become detached from the building from falling on passers-by. The aim of the wire mesh was to buy time to agree a replacement strategy.
Telling good marble from bad
There is little to choose between marbles in general because the geological processes that have created the stone have resulted in what is generally a simple material with little physical and chemical variation.
Marble is generally a fine material for use in building, so choices about which marble to use come down to aesthetics. And what is true in general holds good for marble: Beauty is in the eye of the beholder.
Marble is the king among building stones, although using it comes with a cost that is not measured by the initial price of the stone initially – modern production methods have made it affordable – but by the cost of maintenance, for it does need regular maintenance to keep it pristine.
Although granite worktops have dominated interiors for some years, marble is making a strong come-back as people turn their attention to bathrooms, bedrooms and living spaces where they want elegance and to make a statement as only marble can.