The bamboo forests in China have largely been cultivated there for many hundreds, and in some cases thousands, of years. Every year, in spring and summer, new poles (called culms), grow from a shoot underground. One shoot of the larger timber varieties of bamboo weighs between 2-4 kg when it is less than 30cm high. At this stage it is quite soft and can easily be cut with a knife. It is also eminently edible, when prepared properly. If left to grow, this shoot reaches its full height of say 20 metres in an incredible 3 months, (height depending on variety). When mature, the timber is extraordinarily resilient and strong. It has a tensile strength quite similar to mild steel.
Bamboo textile fibre is made from bamboo timber which has matured in the forest for at least 4 years. Even in remote areas of China bamboo forests are highly valued and carefully tended and managed. In summer, when new shoots reach their full height, they are marked with a year code which makes sure they are harvested at the right maturity. When harvested they are taken to mills where they are crushed and submersed in a strong solution of sodium hydroxide which dissolves the bamboo cellulose. With the addition of carbon disulfide it renders the mix ready to regenerate fibres which are then drawn off, washed and bleached to a bright white colour and dried. The resultant fluff is very long in staple and visibly finer than other fibres. Then they are spun into yarn, like any other textile fibre. The longer staple and higher tensile strength is what makes a tough, soft yarn – which is not as susceptible to wearing and fraying as many other yarns. This is what gives bamboo fabrics excellent durability. The hollowness of the fibre contributes to its very high level of absorbency. But it also takes longer to dry on a clothesline. The hollowness of the fibre also enables it to hold dyes and pigments more readily and permanently, thus making it much more colourfast.
The two main chemicals used in the process are sodium hydroxide and carbon disufide.
It was only discovered that carbon disulfide was a nerve poison after many years of exposure at high concentrations by factory workers in Italy in the 1930s and 40s. With adequate ventilation it is not a problem these days and it breaks down when in contact with the natural elements. Neither carbon nor sulfur are poisonous elements.
Sodium hydroxide is also known as caustic soda, and it is true that it is strongly alkaline and will react with many substances. However, it is not toxic at all and is used extensively in cooking. Used in quite high concentrations it is what gives traditional pretzels their distinctive flavour.
We have produced a video to give you a fair idea of how bamboo fibre, or bamboo rayon is made.
The demonstration shows not exactly how it is made commercially, because that takes much more time, but the principles are much the same.
We have named the video “Kitchen Chemistry” because the substances involved are almost as tame as those used in everyday kitchens and households.