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Lignin

7 Oct

Ash

Ash is one of my favourite hardwoods for making furniture. Light and strong, tight grained and easy to work, it accepts precise joints – like the dovetail in this little cradle made from olive ripple ash (Professional photographer Alan Howden has taken all the photographs in this blog). Our beautiful ash trees are under threat however, from a fungus called Chalara fraxinea. First spotted in Europe it has devastated 60 to 90% of Denmark’s ash trees. Whenever I use it now I am conscious of the changing status of our woodlands.

The term ‘olive ash’ refers to the distinctive stripy figuring found in older ash trees resembling the spectacular grain found in Zebra wood or Zebrano:

Table 06

This piece forms the top of a lovely long coffee table I made for a friend

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I am endlessly fascinated by the possibilities of revealing the best in timber, decades of growth, peculiarities of climate and the biology of lignification. The latter process – laying down the thick protective walls in the tree’s plant cells throughout its roots, stem and branches – is thought to be the primary way in which these long lived plants protect themselves against the main pathogens which attack them – fungi. Lignin is a truly spectacularly beautiful polymer made from phenolic (aromatic hydrocarbon) residues with the fearsome chemical formula – C9H10O2, C10H12O3, C11H14O4 n

Lignin plays a vital role in conveying water up from the tree roots via the sap wood to the crown of the tree. If you cut a tree in the spring it will literally bleed with water (sap) flowing up from the roots under the tremendous capillary pressure of the lignin impregnated water tubes – the Xylem – in the sap wood.

In most timbers the sap wood is paler and can be found directly under the bark surrounding a core of darker heart wood.

The cradle shows heart and sap wood:

Cradle 2 copy

In the ash tree the olive coloured heart wood is completely blocked and will not convey water, but the paler sap wood is the principle water highway of the plant. Chalara fraxinea grows within the water tubes of the ash tree to block water flowing to the leaves, which is why the tree ‘dies back’ as the common name of the disease – ash die back – implies. The leaves blacken from the extremities and drop off prematurely. In a way, this fungus exploits the very defence mechanism of the tree (the tubes lined with fungus blocking and strengthening lignin) by blocking the irrigation system with its tiny invasive threads, or hyphae.

The Forestry Commission are doing their best to report and mitigate the spread of Chalara nationally and we can be on the lookout for symptoms in our native ash trees on our walks through native woodland. A useful source of advice can be found on the Forestry Commission’s web site.

In the mean time I will continue to seek sustainably sourced timber and work with it in such a way as to respect and reveal it’s deep underlying structure.

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