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Cork for building
From Building for a Future Autumn 2002

When was the last time you opened a bottle of plonk? How did you select the bottle? By colour, taste, label, brand, price, randomly or by whether it had a natural cork stopper in it or not? Natural corks are getting rarer amongst the average priced bottles of wine. Similarly when was the last time you specified cork as the material of choice for your building project? Cork it seems is also becoming rarer as a building, industrial and product material.

Is its decline in use a problem? Well if you’re into sustainable solutions it is. Cork is one of the sustainable wonder materials and its disappearance is a major blow to all looking to renewable resources as the future basic materials for our industrial economies. Cork has a myriad of uses and before I investigate those that are relevant to the building industry, it is first worth examining its pedigree.

Quercus suber
Cork is the bark of a species of oak tree [Quercus suber] which grows in the thin soils and scarce groundwater regions of the rolling dehesas landscape of western Spain and Portugal. These woodland regions have been used to produce cork and graze livestock for hundreds of years and between them account for over 80% of the world’s cork production.

The centuries old, beautiful oak forests with their glades and clearings provide a prime natural habitat for a huge range of local fauna and flora. The diversity of this is now being recognised as that of international importance, with the RSPB amongst others championing their continued sustainable management. The cork forests are home to 42 recorded species of birds including the endangered Imperial Eagle with a global population down to only 130 pairs, and other rare species such as black stork and black vulture. Smaller birds, such as robins, chaffinches and song thrushes, migrate to the Dehesas from northern Europe, along with blackcaps from the UK. In spring and summer they play host to a rich variety of butterflies and plants, with over 60 plant species recorded per square metre. The Dehesas are also home to an impressive range of mammals, the star of the show being the rare Iberian Lynx, found only in the remotest parts - this has been declared the world’s most endangered big cat.

Harvesting cork
Harvesting cork is more akin to picking apples or olives than any agriculture or manufacturing practice, albeit on a much longer time scale. For ten months of the year a walk in the cork forest is as tranquil as entering a cathedral. For two months, June and July, there is the thwack of axe on wood. Not the crack a lumberjack might make, the sound is of a skilled cork harvester first making an incision and then splitting off the bark. This is a highly skilled job and is undertaken by local craftsmen. Those who are well apprenticed in the craft can take off the whole of the bark, below the first branches of the tree, in one piece. Each tree has its bark harvested every nine years and can live for between 150 and 250 years, being harvested at least 15 times during its life. Young trees take 30 years to reach maturity, after that the bark can be harvested, but they need about 40 years before they are suitable for wine corks. Cork harvesting utilises the unique ability of cork oaks to regenerate their bark, strip or ring the bark from any other tree and you are more than likely to kill it. I can’t think of another material that is harvested in a manner that leaves the plant intact and growing, it seems definitely more like a fruit.

Processing
Cork was first used in wine bottles by the French Abbott, Dom Pierre Perignon (born in 1639), who is credited as being the man who invented Champagne. After harvesting, the bark sections are dried in the sun and then put through some cleaning, fumigating and straightening processes. From this the natural cork stoppers for wine bottles are bored out. This forms the basis of the cork industry, with wine corks accounting for approximately 90% of all the cork harvested. The waste left over is granulated, then with the addition of a resin [approx 0.02% by volume] is made into blocks or logs. From these sheets, reels or any other shapes are cut. It is from this material that all the other uses of cork rely. Post consumer wine corks or most other cork materials can easily be put through this granulating process. I, along with most people I know, keep my natural cork wine corks without much of a clue as to what to do with them - maybe we should have cork collection banks along side bottle banks encouraging a secondary cork manufacturing industry. There would be a neat symmetry to placing the bottle in one bank and the stopper in an adjacent one.

Problems
However, three and a half centuries after the first black eye from a champagne stopper occurred, the cork industry is in something of a crisis. A rapid ten per cent annual increase in the use of plastic stoppers over the last decade has already given plastic a one per cent share of the world market (three per cent in the UK) and this is severely impacting on the farmers of the dehesas and all the supporting industries. Over 80,000 rural people rely on the cork industry and its decline will severely effect their lives. The increased use of plastic is partly due to the presence in some natural cork of TCA [trichloranisole], a chemical which can cause a mouldy taste known as ëtaintí. This was partly due to a quality problem due to poor management of the cork forests, which was a hangover from the Portuguese revolution in the seventies, during that time the necessary husbandry was somewhat neglected. The replacement with plastic is also due to pressure from the wine retail industry which wants to reduce the number of bottles returned with allegedly ‘corked’ wines. Although this seems a contentious issue, there are many factors that might make the wine go off, and the complexities of this debate will be avoided here. However, with the reduction in the quantity of cork required, the forests from which cork is being harvested have begun a slow decline as they become uneconomic for farmers to maintain trees that have little market value. Instead intensive agriculture is replacing the forests with cash crops such as sunflowers, or fast-growing [nutrient greedy] eucalyptus trees. This is likely to destabilize the local economies and radically change the social support patterns, resulting in impoverished soils and water shortages, It will quickly turn the Dehesas into desert without trees - virtually uninhabitable for humans and a disaster for the indigenous wildlife.

So what’s being done about it? The Portuguese are running a massive public relations campaign to promote humble wine cork and the industry is looking at new ways of marketing itself. One positive action is that a new standard and logo is being promoted by some distributors and supermarket chains which would tell the user that the wine bottle they have chosen has a natural cork plugging its narrow throat.

Cork has many other uses beyond the wine stopper, and the UK has a long established cork industry, manufacturing a huge range of products. In fact many of the products available around the globe today were developed in the UK. For instance cork rubber [which uses a synthetic rubber binder in varying quantities], and insulation cork board both originated in the UK and are now global.

Use in buildings
In buildings, cork has a range of uses from the foundations up. It can be used for its anti-vibrational, thermal, and acoustic absorbing properties in specific locations such as around plant rooms or sound studios, or with all these properties for more general applications such as floor, wall and ceiling tiles or underlay. It can also be used for seals and gaskets, in expansion joints and for intumescent strips. I’ve also seen it being used as an external cladding, it protects the trees so why not a building. However, along with natural wine stoppers, cork’s use in buildings has declined over the past 20 years. This has been attributed to a number of somewhat linked factors. Firstly, fashion. In the 1960-70’s cork was used quite extensively in the home but as fashions changed so it fell out of use. This has had the knock on effect, as with many other materials, of becoming ‘out of sight, out of mind’. As its use faded so did our knowledge of its properties (and conceivable uses), until we no longer register it as a possible material solution. There was also a quality problem with some cheap, imported tiles available in DIY stores during the 80’s and 90’s. These, if anything, had a detrimental effect on the industry, as the tiles were not very robust and the glues that were sold with them often failed after a short time. In terms of glues, solvent based ones have traditionally been used, which seems to contradict the eco-friendly nature of the cork produ. Fortunately these are now being superseded by a range of water based glues, which, while not being edible are much less toxic than the solvent glues. When using cork tiles they are often sealed with a water based acrylic but a wax coating is also available.

By far the biggest sticking point to the widespread use of cork within the UK public or volume building industry is with Building Regulations. For public buildings the Building Regulations limit the amount of cork tiling that can be used as it is considered a fire hazard. However this appears to be something of an anomaly to the cork industry, and to the situation on the European mainland where it is widely used, as cork itself is a fire retardant, something that you can easily test yourself by putting a match to a natural cork. The thing that might catch fire is the binder or the coating, and careful design and manufacture and material selection for coating could avoid this risk. In the UK, the cork suppliers to the building industry are generally small independent businesses, and without major players to champion its use, question the current building regulations and conduct new testing, it is likely to languish in the doldrums. A quick survey of the green building material suppliers didn’t bode too well for the industry. I found that none of the obvious ones currently sell cork products. If you check out GreenPro, however, you will find that it is not impossible to obtain as there are around fifteen UK suppliers of tiles, boards, insulation and acoustics listed. Also the Green-Building Store in West Yorkshire are considering bringing a range of cork tiles onto the market in the autumn. However, as the new European Standards come in and replace the British Standards, potentially cork could have a reprieve and float to the top of the material specifier’s wish list.

Future
There are a number of challenges ahead for Quercus suber. The industry must promote its use and environmental advantages whilst at the same time make the cork based products competitive against its substitutes and look into the issue of Building Regulations. Specifiers should start to incorporate it into their buildings or products and a fledgling cork recycling industry must develop.

Author: Scot Fletcher