By Peter Hartog

is green marble marble ? It is in the nature of the work of Building Diagnostics Asia Pacific that we are more concerned with specific failures of products, materials and techniques than with their general successes. Sometimes the increasing recurrence of failures indicates a trend. In the past three or four years, the popularity of green marble as a wall and floor finish appears to have revived after a decline in the 1960s and 1970s. We are now seeing a recurrence of many of the defects that caused some green marble to lose favour in the not-so-distant past. Geologists define marble as “...a metamorphic (recrystallized) limestone composed predominantly of crystalline grains of calcite or dolomite or both, having an interlocking or mosaic texture.” Dolomite in turn is defined as a limestone containing at least 40% magnesium carbonate. Calcite is a form of calcium carbonate; it commonly occurs as clear or white crystals in limestone and marble. Standard C119 of the American Society of Testing and Materials (ASTM) offers a so-called commercial rather than scientific definition. It describes marble generally as “...a crystalline rock composed predominantly of one or more of the following minerals: calcite, dolomite, or serpentine and capable of taking a polish.” One obvious difference between these definitions is the occurrence of serpentine in commercial and trade practice. Commercial marbles now also include greenstone, onyx and some limestones capable of taking a polish. The Marble Institute of America *1 has classified marble into four groups on the basis of solidity and working characteristics. Its 1997 publication Stone Selection emphasises that: This classification has no reference to comparative merit or value. It is meant to indicate what method of fabrication is considered necessary and acceptable for each type of marble, based on standard trade practices ... These classifications ONLY indicate what method and amount of repair is necessary prior to or during installation, based on standard trade practices. Such emphasis suggests that the classification is intended to imply what the MIA and its members are reluctant to make explicit; some of the most readily available marbles have very limited architectural applications and very poor durability. The MIA classification is as follows: Group A: Sturdy marbles with uniform and favourable working qualities; containing few geological faults or voids. Group B: Marble, similar in character to the preceding group but with less favourable working qualities, may have natural faults; a limited amount of bonding, filling and waxing may be required. Group C: Marbles with some variations in working qualities; geological faults, voids, veins, and lines of separation are common. It is standard trade practice to repair these by one or more of several methods - waxing, bonding, filling, and cementing. Liners and other forms of reinforcement are used when necessary. Group D: Marbles similar to the preceding group, but containing a larger proportion of natural faults, maximum variations in working qualities, and requiring more of the same methods of finishing. This

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group comprises many of the highly coloured marbles prized for their decorative values. Varieties of marble are often given fanciful commercial names, as much to conceal their origin as for marketing. Well established names are occasionally borrowed by new sources. The MIA’s 1991 Design Manual lists no fewer than 47 varieties of green marble then readily available from quarries in France, Portugal, Ireland, Italy, Greece, Sweden, Guatemala, Taiwan and the USA. The Manual notes that some stones may have multiple listings “... because both generic and proprietary names have been furnished.” For example, Guatemala is identified as the source of Belice Verde, Guatemala Antique Green, Guatemala Emerald Green, Quetzal Green and Tikal Green; it is likely that some of these terms apply to stone from a single source or from a group of neighbouring quarries. Green marble has recently been supplied to a project in Southeast Asia under the proprietary names Verde Guatemala and Verde Venezuela; a German web-site identifies their actual sources as India, lately a major exporter of green marble slabs and tiles. Phoenix Green does not come from Phoenix, Arizona, but from at least two different quarries in Taiwan. It has also been called Empress Green and Verde Antique. The same playful terminology is used for granites which may, to a geologist, be gneiss, syenite, gabbro, granodiorite or even basalt rather than true granite. The granite sold as Sahara Gold comes from Brazil, Balmoral Red comes from Finland. London Grey comes from Russia. In several problematic projects examined by BDAP, laboratory analysis sheets submitted to architects have clearly categorised selected green (and some other) marbles as being in Group C in the MIA’s system. It is unlikely that these stones would have been approved if the implications of their MIA classification had been recognised by designers and building owners. The MIA Manual of 1991 classifies 39 of the 47 listed varieties of green marbles in Group C. Another two occur in Group D. In other words, all but six of the listed green marbles are likely to require, as standard trade practice, some form of repair as part of finishing before installation. These include Taiwanese serpentine in floors at Sydney’s Star City Casino, South American (or so it is claimed) green marble used in the lobby floor of a five-star hotel in Malaysia, Indian green marble tiles laid in lift lobbies of a 27-storey commercial office building in Bangkok, tiles used in bathrooms of recent residential developments in Sydney and Melbourne, and wall finishes in a shopping centre in Kowloon and an office building in Wanchai, Hong Kong. In 1989 the consultants AMDEL, Ove Arup & Partners and Stone Tech Inc of New York presented seminars, in Sydney and Melbourne, on the then state-of-the-art in use of Stone in Modern Buildings. The published proceedings of the seminar contain the following advice on harmful minerals: A number of minerals have been found to be detrimental to the performance of building stones due to their dimensional instability (for example swelling clays such as smectites, or zeolites e.g. laumontite), solubility (gypsum and solubility salts), loss of strength when wet (clays in general), reactivity (carbonates), instability (sulphides and as pyrite), or colour change (chlorite/serpentine, siderite). This very general advice is remarkably similar to a corresponding clause in an architects’ 1996 specification for a Sydney building now the subject of

protracted dispute over poor performance of its green marble floors: Stone shall be free from minerals which are detrimental to the performance of the stone due to their dimensional instability (for example swelling clays such as smectites, or zeolites e.g. laumonite), solubility (gypsum and solubility salts), loss of strength when wet (clays in general), reactivity (carbonates), instability (sulphides e.g. pyrite), or colour change (chlorite/serpentine, siderite). Laumontite in the 1989 seminar proceedings became laumonite in the 1996 specification; otherwise, the two documents are almost identical. The specification explicitly prohibits stone containing serpentine. It is therefore noteworthy that analyses of the two varieties of green marble ultimately used in the building have shown the predominant constituents - from 75% to 90% by volume - to be the prohibited mineral serpentine. Even more remarkable is that product data sheets, submitted to the architects before these stones were approved, describe one as Taiwan Green Serpentine with a MIA Soundness Classification of C and both as Marble (Serpentine). How can such seemingly erudite specification clauses be reconciled with the architects’ evident failure to recognise the implications of information submitted in data demanded for the architect’s review and approval? A reasonable suspicion is that the compilers of the specification did not understand many of the technical terms conveniently borrowed from seminar proceedings. A further suspicion, one confirmed in many disputes arising from selection of inappropriate materials and specification of inappropriate fixing methods, is that architects have come to rely on supply-and-install contractors to detect and alert them to such lapses in knowledge. In practice, this reliance is often further contorted into the notion that skill-based trade contractors have a duty to guard head contractors and proprietors against errors of judgement made by architects and interior designers. The theme recurs in arguments over selection of moisture-sensitive and excessively thin natural stone tiles in wet areas; that by executing the work as specified, tilers and stone-fixers in some way warrant the fitness for purpose of materials and techniques selected and specified by others, and that they are ultimately liable for the consequences of errors of judgement made by others. Who’s confused now? Published information on the suitability of green marble in architecture is alarmingly inconsistent. Depending on where one looks for information, serpentines are found to be either extraordinarily hard or relatively soft; they may possess exceptional chemical integrity while tending to expand and warp when wet to decay when exposed to polluted air and moisture. Erhard Winkler’s Stone in Architecture - Properties & Durability is the 1994 version of the respected reference work Stone: Properties, Durability in Man’s Environment first published in 1973. The original edition noted that the “... great durability of the mineral serpentine permits both interior and exterior application.” The current edition advises that serpentine is “... a very stable interior decorative stone, but is readily attacked outdoors by a polluted urban atmosphere with a high SO4 content.” Modern Stone Cladding: Design and Installation of Exterior Dimension Stone Systems, a design and installation manual written by Michael Lewis and published in 1995 by the American Society for Testing & Materials, also

extols the durability of serpentine marbles: Serpentine is a siliceous metamorphic stone of igneous origin, composed originally and essentially of hydrated magnesium silicate. Veined with calcite or dolomite, it is usually green to greenish-black. Not a “true” geologic marble, its strength, chemical integrity and durability are among the highest of any natural stone. By contrast, an architectural stone handbook published in instalments in the British Architects’ Journal in 1975 and 1976 was more cautious. It noted that serpentine: ... is a soft stone unsuitable for outside work, soon assuming a dingy appearance. Installation of Modular Stone Floor Tile: Thin-Set Method, published in 1998 by the American Marble Institute, cautions that: Special attention must be made when setting certain green coloured marbles as they tend to warp in most solvent-emulsion adhesives (water-based). Water from the setting bed is drawn into the stone from the bottom and is held to the stone by surface energy. The force tends to weaken the intercrystalline space and expand the wet side of the stone. Avoid using water-based adhesives not specially designed for green marble when installing certain green-coloured stones. Obtain additional information from supplier. That advice is repeated in the trade literature of several tiling adhesive manufacturers. Laticrete, Mapei and ABA caution that not all of their products are suitable for fixing green marble. ABA’s Guide Tiling Specification

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warns: Green marble: Confirm fixing methods with marble supplier. Do not use water-based fixing methods. Green marble tiles from some of the sources noted above are now supplied with moisture-resistant epoxy resin barrier coats on their undersides. These coatings cannot prevent moisture infiltrating through joints and polished surfaces. Natural Stone: A Guide to Selection by Frederick Bradley, published in 1998, distinguishes three aesthetic types of green marbles. The largest, Type 2, includes serpentines. Bradley places Verde Guatemala, Rajastan Green and Taiwan Green in this group of rocks: ... with a mostly silicatic composition known as ophiolites. The groundmass of the different varieties occurs in various shades of green and is crossed by whitish-coloured veins which vary in distribution and conformation. They may create a real veining pattern or appear as irregular speckles which blend into a green groundmass. The frequency and distribution of the veins may vary significantly, even in the same variety. In some varieties there is also a considerable variation in the colour of the green groundmass. The groundmass sometimes contains ferrous minerals a few millimetres across which are subject to oxidation and may therefore cause rust spots. These materials require a good seasoning period and often have structural defects, i.e. areas with low mechanical resistance. Bradley notes that many varieties are readily available, but that the quantity of “good quality materials” is limited. He recommends that Type 2 green marbles are best used: ... in interiors for facings and special pieces in quality projects. In the case of floorings, these materials should only be used for low traffic areas, since the polish could quickly disappear. In exteriors, their use should be limited to areas protected from atmospheric agents and direct sunlight. Slabs, in particular, of materials with many white veins and materials with a breccia pattern might need reinforcement before installation. Direct Adhered Ceramic Tile, Stone and Thin Brick Facades, a design manual by written Richard Goldberg and published by Laticrete International in 1998, notes that: Some stones, especially dark and highly colored marbles and certain slates, contain minerals such as serpentine which are reactive with water; that means that crystal growth occurs when exposed to water, and the volume of stone literally expands. This results in two problems that may occur if thin, moisture sensitive stones are installed on facades using the direct adhered method: Progress of installation - if water based cement or latex cement adhesive mortars are used, the side in contact with the adhesive will expand, and the outer surface will remain dry, resulting in differential movement with enough pressure to cause a thin stone to warp or distort from a flat plane. Post installation - even if moisture sensitive stone is installed successfully on an exterior wall, the stone may still be subject to cracking and spalling or adhesive bond failure from excessive volume expansion after exposure to constant humidity or repeated cycles of rain. The same phenomena occur, of course, in moisture-sensitive stones installed

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on interior floors and walls in wet areas, near water features, in kitchen and servery benches, on vanity tops and wherever service conditions require frequent damp mopping, such as hotel lobby floors near portes cochere. White blisters or eruptions, followed by powdering, pitting and erosion along veins, are the characteristics of moisture-sensitive serpentine marbles. White spots may occur in bands or clusters within areas of light-green stone. They may also occur in isolation within dark green fields or hard dense stone as cycles of wetting and drying cause the hydrous magnesium silicate Mg3 Si2 O5(OH)4, i.e. the mineral serpentine, to decay to the hydrated form Mg3 Si4 O10(OH)2, more commonly known as talc, the softest material in the Mohs scale of h0rdness. The proceedings of the AMDEL-Arup-Stonetech seminars held in Sydney and Melbourne in 1989 also record that “... some stones rich in serpentine or chlorite bleach strongly with exposure to strong sunlight.” Examples of faded grey-green marble can be seen in shopfronts and street-level veneers on office buildings in the CBDs of Sydney and Melbourne. One of the most spectacular examples recorded by this writer occurs on a bank’s modern headquarters in the heart of Edinburgh, Scotland. The vivid natural colouring remains wherever stone is permanently shaded by a cast bronze bas-relief of Henry Moorish proportions and openings. Elsewhere, white veins contrast feintly with grey patches once dark green. Although much has been written about problems of green marble, the relevant publications (some free for the asking) are rarely found in architects’ office libraries. That says much about the design professions’ priorities. Hours seems to be spent perusing and comparing a narrow range of near-identical samples in order to satisfy an architect’s aesthetic intent, whereas related specifications commonly make suppliers wholly responsible

for demonstrating and warranting the prudence of the architect’s selection. I regard this approach as unprofessional. It is commonly defended by the rhetorical question “Who should know more about the performance of a product than its supplier and/or fixing contractor?” An honest answer is the architect. Architecture, its recent agoraphobic sibling interior architecture and interior design (which seems to be interior architecture practiced with a diploma rather than an undergraduate degree) are supposed to be knowledgebased professions whose practitioners have ready access to information through technical libraries, specialist journals and the internet. Their fees are ultimately based on percentages of the cost of executing their designs. Upstream stone supply tends to be a rugged business practised by persons whose familiarity with explosives, heavy equipment and cross-border cash transactions may come courtesy of secretive fraternal organisations in Sardinia or remote self-funding outposts of the People’s Liberation Army. Downstream stone supply is often merchandising pure and simple; display and deliver samples, take orders, arrange letters of credit and carry the risk of importing whatever takes the customer’s fancy. Stone fixing is a physically demanding skill-based trade with small margins. It is often undertaken by recent immigrants not fluent in English and unlikely to be familiar with ASTM standards, British codes of practice, geological terminology, performance criteria and other paraphernalia or boiler-plated performance specifications. In this chain of responsibilities between quarrying and fixing, who is best placed and adequately rewarded to investigate the track record and fitness for purpose of a specified material? I suggest the reasonable answer is none of the above. It is ultimately the duty of the specifier to determine whether

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a preferred stone is fit for the purpose anticipated, even if to do so requires the architect to recommend engagement of a more specialised consultant such as an engineering geologist.*2 Architects tend to extol innovation, so locally unprecedented materials sometimes have more appeal than familiar alternatives. Is it fair to specify a material without a local track record and then to make the importer and stone-fixer responsible for locating reliable test data, for explaining its meaning to the designer who wrote the specification in the first place and for carrying the risk that the designer does not really understand the information submitted? I believe it is imprudent to expect tile-fixers and stone-fixers to be familiar with the geological, chemical and engineering characteristics and occasional peculiarities of hundreds of varieties of commercially available stone. It is also unfair to expect them to interpret and validate test certificates and chemical analyses and to anticipate all the service conditions of the installation. Those are properly the responsibilities of the designer and specifier. A paragraph in the last issue of Tile Today foreshadowed the

topic of this article. That brief note has already provoked three separate enquiries from tiling contractors involved in disputes over poor performance of floors and wall finishes of green marble selected by others. In each case it is being claimed that the contractor should have known about the weaknesses of the stone. Such confusion of responsibilities needs to be addressed by the tiling and stone-fixing industries. Until neophyte designers learn from their mistakes and enthusiasm for green marble again subsides, be wary of endorsing the fitness for purpose of anything described as serpentine and all marbles, green or otherwise, in Groups C and D in the Marble Institute of America’s system of classification. Recommend to whoever chooses such stone slabs and tiles that they identify and inspect successful precedents or else commission tests to demonstrate satisfactory moisture stability and durability. ❋

*1 A misnomer; the Marble Institute of America and its publications cover all the main geological varieties of stone used in architecture. *2 The architect’s duty to recommend specialised consultants when operating outside his or her field of experience is likely to become an issue in forthcoming litigation over early failure of 30 mm thick limestone cladding on six apartment buildings on The Peak in Hong Kong. The architect selected a stone with no relevant local precedent. The specification required the stone importer and fixer to demonstrate that the material was suitable for Hong Kong’s climatic and atmospheric conditions. The local supplier did not do so, allegedly in the belief that the architect would not have travelled with his entourage to France and selected the limestone unless satisfied of its likely performance. The supplier’s design calculations relied on a safety factor of 3.0, suitable for homogenous stone such as granite but far below the recognised minimum of limestone. Shop drawings and calculations were submitted to the architect for approval, as required by the specification. Engineering analysis has subsequently determined the stone to be dangerously underdesigned for typhoon wind loading. The installation methods uncovered during current demolition also deviate far from the details shown in shop drawings. The owner of the development alleges that he relied on the architect’s explicit recommendation. The architect in turn asserts that he claimed no particular knowledge of the weathering characteristics and engineering properties of the stone; his preference for the imported limestone was supposedly based exclusively on its appearance.

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