Tapping rubber in Sri Lanka
One of the most versatile natural commodities used by man for thousands of years is wood. He increasingly found the best ways to use it in construction, coming to realise that some woods suited certain purposes better than others. While some woods have been grown and harvested commercially for centuries, the value of others is only just now being fully appreciated. One of these is rubberwood.
by Michal Brink, Forestry Solutions
Left: Elephant used for logging. Centre: Rubber trees being harvested for lumber. Right: Rubber tree being tapped.
Brazil provided the world with the rubber tree, Hevea Brasiliensis, but that country no longer plays any significant part in the world rubber trade. Seeds were exported from the lower Amazon area of Brazil to London UK, a local planter acting for the British Government in 1876. The seeds were germinated at the Tropical Herbarium in Kew Gardens, London later that year. From there seedlings were exported to Ceylon (now Sri Lanka). In 1877, 22 seedlings were sent from Ceylon to Singapore, where they grew strongly, and the technique of tapping was developed. Prior to this, the trees had to be felled before the latex could be extracted.
By 1900, most of the techniques and agricultural practices required to establish large plantations had been developed. One key technique was bud grafting. This is essentially a cloning technique which ensures that genetically identical trees can be produced in unlimited numbers. The trees should be grown between 600 and 1 000 metres above sea level in well-drained soil, at a temperature of 20 to 28 degrees C and with a well-spread-out annual rainfall of 2 000 millimetres.
Latex is often described as the sap of the Hevea tree. This is not an accurate description. The sap runs deeper inside the tree, beneath the cambium. Latex runs in the latex ducts which are in a layer immediately outside the cambium. This highlights the skill of the tapper. If the cambium is cut, then the tree is damaged, because the cambium is where all the growth takes place. Too much damage to the cambium, and the tree stops growing and stops making latex.
All natural rubber originates in the rubberwood tree, and it starts its journey when the tree is tapped. Trees are rarely tapped more often than once every two days. A tapper starts the trek around the plantation before dawn. At each tree a sharp knife is used to shave off the thinnest possible layer from the intact section of bark. The cut must be neither too deep, nor too thick. Either will reduce the productive life of the tree. This starts the latex flowing, and the tapper leaves a little cup underneath the cut. In ordinary circumstances, this latex will normally coagulate into a lump in the bottom of the cup, called 'cup lump.' If the plantation manager wants to make latex, then the tapper must add a stabilising agent to the cup. Usually this is ammonia, which prevents the latex from coagulating. The tapper returns a few hours later and collects the contents in the cup – either cup lump or latex.
If solid rubber is required, the cup lump is collected and processed. That processing involves heat, which destroys many (but not necessarily all) of the proteins. It ends up as solid rubber. Depending on the method of processing and the final purity of the material, the industry refers to it either as TSR (technically specified rubber), or sometimes sheet rubber.
When latex is required, the material is gathered on the tapper's return journey, poured into containers and delivered to a processing station where it is strained and concentrated. At no stage in the process is the latex heated. This means most of the proteins remain in the latex.
The older practice was to just burn the tree when, at the age of about 25 to 30 years, it ceased to produce latex. During the 1980s, rather than just being cut into logs and used as fuel for fires, the commercial potential and suitability of rubberwood as a material for the construction of furniture and other objects was realised in Sri Lanka. Its qualities are that it is easily worked, light and attractive in colour and close grained. It has a notable tendency to warp, which can be kept under control (mostly) by applying pressure during drying. Products made of rubberwood are a significant export commodity for Sri Lanka and include toys, cutting boards, furniture and wood panels.
Rubber trees are today grown commercially in thirty countries across Asia, Latin America and Africa.
Published in October 2010