Bast and Basswood

This is the third in a series of blogs all about tree bark. In the first we looked at some of the chemical components in bark, namely tannins. In the second we looked at the outer bark.  In this blog we are focusing on the inner bark or  the phloem and in particular the phloem of one family of trees.

The phloem represents the inner most layer of the bark sometimes referred to as the bast layer and is a vascular layer which transports sugars produced by photosynthesis in the leaves to other parts of the tree and also  other chemicals such as proteins and RNA.

The main cells in the phloem layer which allow the flow of these chemicals differs slightly between deciduous and coniferous trees (angiosperms and gymnosperms) in they way that they function and are normally called  sieve tube members in the former and sieve cell in the latter,  but essentially these cells align to form vertical columns through which the sugars can flow.  Each cell has either a sieve plate (angisperms) or sieve pores (gymnosperms) which allows the sugar to pass from one sieve cell to another.

Each of these sieve cells has one or more companion cells associated with it which  provide the sieve cell with essentially it’s life support system providing all the metabolic processes required for the sieve cells to survive because as they  mature they loose their own cell organelles and therefore cannot metabolise without a companion cell.

Surrounding these paired sieve cells with their associated companion cells are parachyma cells which transfer materials across the phloem to the sieve cell/companion cell pairs and they also act as packing between the sieve columns.

To provide structural support with flexibility are the  sclerenchyma cells which  form  strengthening tissue which made up  of short sclerids and long lignified fibres with thick cellulose walls.

It is these bast fibres which provide us with strong, flexible materials from which we can manufacture cordage or string.  There are many species of tree, world wide, which have strong enough fibres in the phloem layer that can be removed and processed to make string.  With each species the way the bast is removed and then prepared to yield usable material is different, but one family of trees stand out, particularly in Europe and North America, as having a long history of providing bast fibres for cordage and that is the Lime or Tilia family.

Species of Lime (of which there are over 30) range from North America across Europe and through Asia and throughout their range the bast fibres are used. . In Europe the use of lime bast can be traced back to the Mesolithic right through to the modern day; we know Otzi the Iceman was using it and had several examples found with his body, the Vikings used it for making rope and it was used in Japan to yield fibres for weaving.  Associations with the bast fibres being utilised is evident from some of the common and latin names for some of the speices; The species growing in Eastern parts of North America is referred to as basswood….derived from bastwood and the small-leaved lime that grows in the UK the latin name is Tilia cordata. It is also frequently called Linden thought to derive from the Old English word lithe.

The reason that lime is used above other tree species which also yield bast fibres such as willow, sweet chestnut and elm is partly because of it’s superior strength but also because of the volume of material obtained.  The bast fibres in lime are arranged in 10 or so successive layers each separated by the vascular sieve columns, through a process known as retting it is possible to separate out these individual layers.. The bast fibres occur in bundles that are glued together by pectin and calcium ions, retting breaks down the pectin and removes the calcium allowing  the fibre bundles to separate into elementary fibres.

In the UK as well as the previously mentioned Small-leaved Lime we also have the Large-leaved Lime T. platyphyllos, both of which are now relatively uncommon trees in British woodland, fortunately the hybrid of the two of them T. x europea is frequently found in parks and towns….most towns have a Lime or Linden Avenue which you can be certain will be lined with Lime trees.

The time to start processing lime bast is in late spring and early summer.  When the sap is flowing the phloem and outer bark can easily be removed from the vascular cambium and xylem layers beneath. Ideally find a tree that has previously been coppiced as in the picture below or in towns the hybrid limes often produce suckers from around their base.  Either way always seek permission before removing any material from any tree.  Remove as long and as straight a shoot as possible with as few side branches as possible, cutting as close to the base as possible.

 Bast from growth less than 10 years old is more durable than  that from older trees and branches, and bast from trees more than 15-20 years old is often coarse and stiff although growing conditions have an influence.

Using a knife or axe score through both the inner and outer bark all the way along the length of the pole.  It is possible with patience to then peel the bark away completely from the wood underneath but this can take some time, so a quicker method is to remove the bark in thin strips.

Score another line parallel to the first and then in the middle of the pole use the tip of your knife to lever up the center the strip.  When it has come away enough to get your hand in your should then be able to pull the strip away along the whole length of the pole.

Keep repeating this process or you could try making and using the bark stripping tool that Adam described in a previous blog, until you have removed all the bark.  Coil the strips of bark up loosely with the bast layer outermost to prevent it cracking.

Place the coiled bark into a pond or other water source so that is is submerged and securely anchored to the bank.  Leave for 4-8 weeks depending on temperature and bacterial content of the water.

Once sufficiently retted, you should be able to peel away thin multiple layers away from the inside of the bark. The bark will be slimy and smell of rotting vegetation.

 Rinse, in fresh water and the peel away all the separate layers of bast fibres from the coarse outer bark. The inner most layers are fine and strong, whilst the outer most bast fibres are coarser often with a netted appearance and much weaker.  Once all the layers have been removed and separated from each other, give them a good rinse and then hang to dry.  They can then be stored indefinitely in a dry place until needed for cordage making.

 In woodland with larger amounts of lime it is sometimes possible to find natural dead wood which has been down long enough for nature to have undertaken the retting process.  It tends to be more haphazard, with some areas of bark not retted enough and others have rotted away to nothing.

But it is often possible to obtain sufficient fibres to make a reasonable length of cordage.

 

 Kev Palmer