Building with stone : Travertine
Travertine has become the material of choice for domestic floors, but this tough, durable contender for Marble’s crown as king of building stones is ideal in so many building applications. Barry Hunt explores its strengths and weaknesses.
Travertine is an unusual stone formed by the precipitation of calcium carbonate salts from the cooling of mineral-laden hot spring waters typically derived from volcanoes. Some try to class it as a sedimentary rock type, but this implies sediment being laid down, which is not the case. Precipitation rates vary daily and seasonally and on the vagaries of the volcanic structures driving the springs. This results in a layered structure to the deposits that can take on some fantastical forms, such as those around Yellowstone in the US and Pamukkale in Turkey, where there is active travertine formation today.
The ancient city of Hierapolis was built on the travertine deposits at Pamukkale, and Roman tombs, now half buried, allow the rate of deposition to be determined, with several metres formed over a couple of thousand years.
Until quite recently, travertine was listed by the American Society for Testing Materials (ASTM) under both limestone and marble classifications, adding to the confusion. It now has its own standard (ASTM C1527). Geologically, travertine is classed as a type of ‘tufa’, which should not be confused with ‘tuff’, a rock formed from volcanic ash fall deposits.
The reason travertine historically has been referred to as marble is because it can take a high polish. But travertine is closer to limestone because it has not undergone the metamorphosis that turns limestone into marble (explained in NSS August 2013). In fact, it deserves a separate classification.
Travertine forms as a surface deposit and all manner of detritus may fall upon it and eventually be included within the structure as it builds up.
Most of the detritus is in the form of leaves and twigs, but occasionally animals can be included, perhaps as a result of having been overcome by the hot spring water from which the travertine is formed.
This organic matter rots away, leaving abundant voids that may remain empty but can also develop linings of crystals as the mineral waters continue to flow through the formed deposits. This gives travertine a highly voided structure within an otherwise strongly cemented and dense matrix of mostly calcite.
Really, travertine is little different to the deposits found inside an old kettle used in a hard water area – it is an elegant form of limescale.
The mixture of layers and voids means that the appearance of the stone can alter dramatically, depending on the direction in which it is cut, which can be either parallel to the layers (cross cut) or across them (vein cut).
The layers themselves typically exhibit shade variations that depend on the size and structure of the crystals within the layers and the presence (or lack of) trace minerals such as magnesium and iron. Colours typically range from white to brown, sometimes with yellow, pink and red hues.
When preparing travertine for sale there is a decision to be made whether or not to fill the voids. Some choose to fill the voids with dyed fillers to create dramatic effects. There is more about filling below.
Travertine has been used to create some stunning statues, the fabric providing an organic essence with the twisting and turning of layers of the stone interrupted by macro-chasms. Emily Young is a fine UK sculptor in stone and she has a wonderful array of travertine artworks, some of which are on display around Paternoster Square near St Paul’s Cathedral in London and should not be under-looked (as they sit on top of columns).
Travertine producing regions
The name ‘travertine’ is believed to derive from the ancient Roman town of Tibur, where lapis tibertinus, or the Tibur stone, was first quarried.
Tibur is now known as Tivoli, which is approximately 20 miles to the East of Rome. The quarries here fed the building of Rome and continue to produce high quality stone for use as flooring, paving, walling and cladding. The Colosseum in Rome is reputed to be the largest building constructed entirely of travertine.
Many European countries have travertine deposits somewhere, although they are often not commercially viable as the deposits are typically small, due to the restricted mode of formation.
Bulgaria is one country the Author has visited where travertine is present but does not appear for sale on the open market.
The UK has no sources of travertine of its own, not even a close relative.
EN 12440 lists 30 sources of travertine: four from Germany, three from Greece, nine from Italy, three from Romania and 11 from Spain.
Turkey produces the largest volume of travertine sold in Europe, much of it coming from the Denizli region, which includes Pamukkale. The bulk of the material produced is modular tiling for internal floors and walls.
It is often cross cut, as is fashionable – although it is a fashion that has resulted in the problems discussed below.
EN 12440 does not include a listing for Chateau-Landon travertine, a French variety that bleaches white. It was used to spectacular effect for the construction of the Sacré Cœur (Sacred Heart Basilica) of Montmartre in Paris.
Looking further afield, Iran has a wealth of unusual travertines, mostly from the north-west of the country, that exhibit a good range of colour variations. Some unusual red and pink travertines are available for highly decorative work.
The current trade embargoes and sanctions on Iran do not include natural stone, although the UK government does not encourage trade with or investment in Iran and has withdrawn all commercial support for trade.
This has not stopped ships from other countries with on-board cutting and polishing facilities anchoring in the Persian Gulf to process blocks of Iranian stone. Some of the stone is used in Iran, some is taken away to be sold on the open market.
China also has travertine that it is now exploiting, although little is making its way to the UK as yet.
Travertines are also found in the Americas, with good deposits in Mexico and South-Western USA. These resources have been identified relatively recently and apparently were not exploited by the various earlier civilisations of these areas.
Aside from the voids, travertine behaves similarly to dense limestone and marble, with relatively high strength and low absorption and porosity characteristics.
The strength can vary significantly, depending on the orientation of the bands and the degree of connected voids.
Both slip and abrasion resistance are similar to marble, with slip values dropping significantly in the wet when the stone has been honed or polished. However, slip resistance can be significantly better than for marble and limestone if the travertine is vein cut and the voids are left unfilled.
Travertine can be used in a wide variety of applications and will typically provide good performance. Table 1 (on page 33) gives some generic test results for travertine, although there can be considerably greater variation than those shown.
Travertine is resistant to most forms of weathering and can stand up to the rigours of both cold and hot climates. Like marble, it tends not to lose strength but does slowly lose its surface over time. Unlike some marble, travertine does not appear to suffer from warping. (For a discussion of marble warping see the August 2013 issue of NSS.)
The greatest threat to travertine is attack from acids, which tends to comprise initial granular disintegration followed by slower surface etching.
Potentially, the layered structure makes travertine more susceptible than marble to significant breakdown under acidic conditions. This is not something to lose sleep over as travertine has the ability to last potentially thousands of years, as many surviving Roman structures now testify.
Evidence of the longevity of travertine when used for internal flooring and walling can be seen within many of the great Art Deco buildings in London. Major metropolitan complexes such as Bush House on the Aldwych or Senate House on Malet Street followed a similar template: They have austere Portland stone facades contrasted with Italian travertine finishes in the lobby areas, with the travertine partially extending to external areas.
The floors of these and other similarly styled buildings of the time have stood the daily grind of sometimes thousands of people and the movement of trolleys and other items, as well as daily maintenance.
Even after 50million or so pedestrian journeys across some areas, travertine can typically be regarded as aged but not tired.
For many years, Senate House was the second tallest building in London. It was rumoured to have been favoured by the German Nazi Party during World War II for their headquarters had Britain been invaded. It also inspired George Orwell’s description of the Ministry of Truth in his novel 1984. The building deserves recognition for the way in which natural stone was used in it in such innovative ways.
By its nature, travertine should not be considered to be a poor performer but it does exhibit different qualities based on the colour, layering and void geometry. A travertine with a higher number of voids might provide better slip resistance but people often prefer to see the voids filled and a smooth, polished surface presented.
To fill or not to fill
Traditionally the voids in travertine were left empty. It was only in the latter part of the 20th Century that fillers started to be used, including experimentation with coloured fillers.
Inspection of various external claddings in the UK carried out by the author typically reveals that fillers in vein cut travertine panels have remained intact after as long as 50 years with minimal loss of surface, although the older fillers are cementitious materials while more recently resins have tended to be used.
Lime is probably the most appropriate filler as it works in close harmony with the calcite of the travertine as well as offering the best colour match. Portland-type cements set harder but can be susceptible to shrinkage and consequent debonding.
Resins can be made in any colour and they penetrate the stone fabric to provide deeper filling with a secure bond. Sometimes clear resins are used to great effect when there are larger voids. Whether travertine is filled or not does not seem to affect its overall weathering and wear performance significantly, although it can reduce slip resistance.
For domestic environments, filling the voids and lowering slip resistance – which really only has the potential to be a problem when the surface is wet or greasy – is not as much of a problem as leaving the stone unfilled, when it can collect food stuffs and other materials and prove difficult to clean. The same might be said for any environment where food is served.
In one major London development, the open voids of the travertine were actually used as part of the drainage system for a large area of externally laid travertine. Normally, significant falls are required to drain rain but the stone was laid flat and drained from below via the voids.
There were two issues with this: The voids could block up with dirt and prevent drainage; different densities of voids resulted in a patchy appearance that highlighted a lack of careful stone selection.
A note of caution must be given if using resins for fillers: It can mean the travertine has to pass various fire rating tests. Normally, of course, stone is regarded as non-combustible and is not required to be subjected to fire testing. However, resins are organic and should make up no more than 1% by weight of the stone in order to avoid such requirements.
Problems with travertine
The greater bulk of recent travertine production has been as floor tiles, with cross cut stone proving particularly popular. Cross cut travertine provides what is considered one of the most pleasing or aesthetically interesting natural stone appearances, as random variations are the essence of a natural material.
The typically light and warm hues of travertine also suit both traditional and modern styles of construction.
But with a surge in demand of travertine for domestic applications (in particular) in recent years, concern has grown about holes appearing in the surface after the floor has been laid. The principal problem with cross cut travertine is the fact that there can be concentrated bands of voids lying just below the finished surface.
The skin of stone between the upper surface and the void below can be just a fraction of a millimetre and will easily collapse under normal trafficking, especially under the point pressure of thin heels on women’s shoes. The size of the voids will dictate the resistance of the surface to collapse.
Not everyone realises that polished travertine tiles might have been extensively filled on the wearing surface. When selecting travertine, just look at the unfilled rear of a tile to gauge how voided the stone is and, therefore, the potential for the surface to collapse into hidden voids.
Often going hand-in-hand with surface collapse is loss of the filler, which might be more susceptible to popping out in cross cut, rather than vein cut, travertine due to the void geometries.
During the original stone formation the detrital organic matter that eventually rotted away to produce the voids typically laid flat on the surface as the stone was being built up. This means that voids tend to appear elongated or flattened parallel to the layering. When travertine is cross-cut the surface voids are normally shallow and the shapes often do not help the filler to interlock.
It is always worth picking at the surface of fillers in samples of travertine to see how well bonded it is. Sometimes sawing and finishing dusts have not been properly cleaned away from the voids and the resins are basically stuck to films of loose material and will readily fail.
Many stone problems can be resolved, but the collapse of a travertine surface is not easy to repair and ultimately almost always requires complete replacement. Certainly collapsed surfaces can be filled but the refinishing required as a result can open up more hidden voids.
Often it is the smaller voids that are impossible to prevent and then fill in situ. Surfaces containing them will retain dirt, so when selecting cross cut travertine for internal domestic flooring when voids are not desirable, select low void materials, even though they usually cost more.
The stone with many faces
If marble is the king of stones then travertine is the young pretender.
Marble is melancholy and, frankly, staid. Travertine is complicated and it is difficult to see what is really below the surface. There is disorder and emotion in this stone and we need to approach it with care to bring out its true potential. Travertine is variable but as a class of stone lacks variety. More than any other, however, it can be cosmetically altered using fillers, perhaps making it the stone for our time.
You could say it is the ideal stone for vanity units – and what other stone could live up to such a claim? We are only now really beginning to realise the true potential of travertine and marble, the king of stones, might be in danger of being usurped by this young pretender.