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Some Aspects of Wood Structure
and Function
by Ephraim Segerman
(for publication in the Journal
of the Catgut Acoustical
Society, 2001)
Introduction
A scientific theory is an assumed picture of what is happening that can
reasonably explain all of the relevant evidence. That evidence can be
just simple observations of phenomena. Careful measurements under
controlled conditions are good to make. They create more evidence that
challenges a theory to try to explain, and if it fails, the theory is
disgarded. Only a failure to adequately explain evidence can invalidate
a theory, not (as some assume) a lack of supporting careful
measurements.
A series of theories are presented here that offer explanations of
observed phenomena in terms of the wood structure, either physical or
chemical (including adsorbed water). They include explanations of
permanent and recoverable inelastic bending, sound absorption during
playing-in and reduced sound absorption in stewed and aged wood.
Basic wood chemistry
Wood is a remarkably simple material, both chemically and physically.
Chemically, if we ignore the adsorbed water, 99% of wood is comprised of
three types of chemicals: about half is cellulose, and about a quarter
each is of hemicellulose and lignin. Cellulose molecules are long linear
(unbranched) polysaccharide polymers, hemicellulose molecules are
shorter branched mixed polysaccharide and polyuronide polymers, and
lignin molecules are phenolic polymers. Cellulose is largely
crystalline, organised into microfibrils, and is very stable in normal
environments. Hemicellulose and lignin are not crystalline, with
hemicellulose being rather unstable and lignin very stable.
Hemicellulose is the only component that absorbs water to any extent.
All of the changes in the dimensions of wood with changing weather are
due to how much the hemicellulose is swollen with adsorbed water. Small
ions like lithium and sodium can join with and stabilise more adsorbed
water molecules (sodium ions can raise the equilibrium moisture content
by up to 2% - significant at low moisture contents). When dry,
hemicellulose breaks down into carbon dioxide and water. At 20 degrees
C, this would reduce wood weight by 1% per century. When there is water
present, acid breaks down the hemicellulose somewhat faster by
‘hydrolysis’, mostly into sugar molecules. Added acid speeds this up,
but this happens normally because of the natural acidity of wood. These
processes of hemicellulose breakdown are called ‘degradation’, and they
get very much faster with higher temperatures1.
The basic physical structure of wood
Physically, wood is a collection of long thin pointed cells made up of
cell walls on the outside and air in the inside. Each cell wall has four
layers, with the ‘primary’ layer on the outside and three ‘secondary’
layers inside. The amount of hemicellulose is about the same in each
layer because the layers need to swell and contract together without
stress between them when moisture content changes. The cellulose content
of each layer increases steadily from the outer ‘primary’ layer to the
innermost ‘secondary’ layer. The lignin content consequently decreases
in that sequence. The cellulose microfibrils lie parallel to each other
within each layer, and spiral around the cell’s long direction.
Different layers have different angles of spiralling. The hemicellulose
combined with the lignin acts as a glue that holds together the layers
and the cellulose microfibrils within each layer.
Most cells have their long directions parallel in the direction of tree
growth (some bundles of cells lie perpendicular to this majority,
forming the ‘rays’ seen in a radial section). The wall of each cell has
a cross-sectional shape that is rectangular with slightly rounded
corners. Adjacent cells have their walls glued to one-another by a
mixture of about three-quarters lignin and a quarter hemicellulose. The
glue layer is about as thick as an individual cell-wall layer except at
the rounded corners, where it fills the space. Pairs of glued-together
adjacent cell walls act as structural units. There are evenly-spaced
holes in the cell walls called ‘pits’ which are usually lined up with
similar holes in adjacent cell walls. These holes allow the passage of
water or air between cells, and ultimately between the inside and
outside of a piece of wood.
Bending and taking a set
When wood is bent at normal temperatures, the cells on the convex side
of the bend tend to be stretched, and those cells on the concave side
tend to be compressed. The cell walls can hardly be stretched. They lie
along the grain (long direction), the sides of the cross-sectional
rectangles line up along the radial direction, and lie along but are
staggered in the tangential direction. There is thus little scope for
deforming cell shapes to respond to a stretching force along the grain
and radial directions, a bit more scope along the tangential direction,
and a lot more scope in other directions in-between these. There are no
directional constraints in changing the shapes of the cell walls in
response to a compressing force. If the bending force is released, the
cell walls spring back to their previous shapes, but if the force is
applied for a period of time, the wood takes a 'set', and only some of
the bend would spring back if the force is released. That time can be
shortened by higher temperature and moisture content and by internal
mechanical stresses such as vibration and moisture gradients due to
changes in humidity. It is very short if the temperature is over 90
degrees C, with enough moisture to avoid drying, when the glue between
cell walls becomes plastic and flows readily.
In taking a set, the individual cell walls in the pairs slip past one
another in the direction that tends to relieve the stress caused by the
bending force. The glue between cell walls 'gives', allowing sliding.
This is called 'creep'. In the grain direction, the long thin pointed
cells can slide along their long directions relative to one-another. In
directions perpendicular to the grain direction, the slips between
adjacent cell walls are towards making new rectangular cell shapes that
are shorter in the direction of compression and longer in the direction
of stretching.
After creep, if the bending force is released at normal temperature, the
cell-wall glue holds. When the creep sliding is along the grain
direction, there are no residual internal stresses in the wood structure
caused by the movement, and so the bend is permanent, with no ‘memory’
of the original relationships between cells. This happens when bows are
bent to shape. If the creep sliding changed the shapes of the cells,
there are internal stresses that were not there before, as a 'memory' of
the original shapes, and under the right conditions, the original shapes
can be recovered. These internal stresses could well be in the
originally-grown corners of the cell walls having to open out in
straight regions, and the regions that were originally straight having
to bend at corners. After the original bending force is removed, with
time (which can be shortened dramatically by high temperature and
moisture content), the original shapes will largely be restored. Thus a
'warped' bridge (which has creep due to stresses only in the radial
direction of the tree) will spontaneously straighten with heat and
moisture.
More destructive aspects of wood bending
If the cell walls are distorted, as occurs when wood is dented, it can
often be swelled back by moisture and heat, but only if the dent is
fresh. The denting compresses by bending some doubled cell walls. A
dog’s-leg double-bend zig-zag shape should be common. At each bend,
there is more cell wall on the convex side than on the concave side. In
the dog’s-leg, each cell wall is on the concave side at one bend and on
the convex side at the other. In the denting process, that shape is
taken by shearing the glue between the two cell walls in the region
between the two bends. The internal stresses are in the bends in cell
walls that originally grew straight. These residual stresses can drive a
swelling-out of the dent if the surfaces between cell walls is made
mobile by heat and moisture. If the dent is not swelled out quickly, the
bends can migrate, either to each other or to the cell corners. In
either case, the walls straighten out, but at the expense of the cell
corners no more being at right angles. Possibly because of cleavage of
the primary layer of cell walls at the acute-angle bends, the original
cell shapes cannot be restored by heat and moisture.
Curvature of wood perpendicular to the grain direction can be made
permanent by high heat. The side on which it is applied becomes concave.
The scorching or near-scorching heat breaks down much of the
hemicellulose in the cell walls, thus contracting the wood on that side.
This makes the heated wood lose most of its capacity to absorb water, so
the contraction is permanent. Staves of bent-stave English 17th century
viol bellies were bent this way with scorching irons, with the bends
forming parts of the arching curve.
Sound absorption by creep
When one first tightens up the strings of a new bowed instrument, there
are new bending forces on the structure. The string tensions tend to
compress the length of the top plate, increasing the longitudinal
arching curvature and raising the arching height. This is complicated by
the downward pressure from the bridge, which tends to straighten out
both the longitudinal and sideways arching curvatures in the bridge
region. So the longitudinal arching curvature away from the bridge
region tends to become even greater. These changes cause other
distortions and changes of curvature over the body. Each change of
curvature is subject to creep, and while creep occurs, it most probably
absorbs vibrational energy whenever the instrument is vibrated. This is
suggested because creep in string stretching absorbs vibrational energy,
dulling the sound2 . The material uses that energy to speed up the
creep. Creep is greatest at the beginning (most during the first week)3,
and it slows continuously, eventually settling down to a negligible
rate. Vibrating the instrument (as well as heat, moisture and the
stresses of moisture gradients during humidity cycling4) will shorten
the time it takes to settle down. The speeding-up of creep by vibration
appears to be the mechanism by which playing-in helps5 .
If an instrument is left with reduced string tension for some time, the
changes of curvature along the grain cannot recover, but those
perpendicular to the grain direction can to the extent allowed by the
constraints (recovery would be helped particularly by the moisture
gradients of humidity cycling). Then, when tuned up again, some creep
could occur again, so some playing in may be appropriate.
Changes of moisture content in equilibrium with a particular relative
humidity
We should not confuse the above effect of vibrating the wood during
playing-in with any improvement of sound that can result from warming-up
playing. There is evidence suggesting that vibrating wood has a lower
equilibrium moisture content than wood under the same conditions but not
vibrating6. The more moisture there is in the wood, the more absorption
of vibrational energy there is. So, according to this theory, during
warming up by playing, some of the moisture that was in the wood before
is freed, and so less sound is absorbed. It is unlikely that this
physical effect is large enough to be noticed.
Since vibration affects the equilibrium moisture content of wood at
constant temperature and relative humidity, we would expect static
stresses to do the same. This seems to apply when two pieces of wood
with different grain orientations are glued together at one relative
humidity, and then the humidity changes. The glue joint holds, so one
piece is under compression and the other under extension in the glued
area. The wood region in compression can hold less water than it
normally could at the new humidity, and that in extension more. It seems
likely that wood under static stresses takes the equilibrium water
content appropriate for its constrained dimensions rather than that
appropriate for unstressed wood at the ambient relative humidity. Thus
the weight of plywood varies much less with changes in relative humidity
than normal woods.
Another example of this principle is the observation that the cracks on
centuries-old lute soundboards tend to terminate at the cross-bars glued
underneath. The grain direction is along the long dimension of each
cross bar, and its length would have varied very little with variation
in humidity. This kept the soundboard wood glued to the cross bars from
expanding or contracting, and thus it was kept at a relatively constant
moisture content. The soundboard wood that was not next to the bars
expanded and contracted with humidity changes, and the stresses
resulting from moisture gradients associated with these changes enhanced
the normal ageing effects of degrading the hemicellulose that contracted
the wood. Thus the soundboard cracks between the bars result from
contraction there that did not occur at the bars.
Sound absorption by water
The adsorbed water is a major contributor to the sound absorbed by the
wood of musical instruments. Adsorbed water converts some of the energy
of sound vibration into heat energy7 . There is typically a 3.5%
decrease in damping coefficient for each 1% decrease in moisture
content8. Since hemicellulose is the component of wood that adsorbs
water, and its capacity to adsorb water depend on how much hemicellulose
remains in the wood, the hemicellulose content is directly related to
the amount of sound absorption. Thus instrument response, which depends
on the sound vibration that is not absorbed, would improve as the
hemicellulose degrades. This is probably the main reason why instruments
made of matured wood have more response than those made of freshly dried
wood, and why old instruments seem to have more response than newly-made
instruments. Since hemicellulose itself most probably absorbs sound
energy, its loss increases response more than just that due to the
reduced moisture content.
The closeness of the label date and dendrochronological date of some
Guarnieri instruments suggests that wood maturation was sometimes
considerably shortened, probably by stewing, which greatly accelerates
hemicellulose degradation. It was traditional then to ‘salt’ wood to
stabilise and preserve it9, and impregnated salts have been found in
Guarnieri wood10. The salt helps dimensional stability by raising
moisture content at low humidities, but the main effect on stability and
sound is due to the hemicellulose degradation of stewing. Some makers
today are stewing the wood used in their instruments to give the effect
of aged wood.
There is additional sound absorption by moisture gradients in the
wood11. It appears that the sound energy absorbed is used to speed up
the movement of water from regions of higher to lower moisture content.
If there has been any change in the relative humidity around an
instrument, this is probably a more important reason for warming up an
instrument before performing on it than the small lowering of the
equilibrium moisture content.
Conclusion
These theories explain all of the relevant evidence the author is aware
of. Such theories are not for believing in, but should be respected
unless and until theories that better explain the evidence emerge. In
principle, these are all testable. They could be considered rather
speculative since most have not been challenged by careful experiments.
If such experiments were easy to perform, they would have been performed
long ago, and appropriate theories formulated. Theories can result from
or precede experiments. It is hoped that these theories will stimulate
appropriate experimentation.
--------------------------
1 A. J. Stamm, Forest Products Journal, Vol. 6 (5) (1956), p. 210. Cited
in A. J. Stamm, Preprints of the Contributions to the 1970 New York
Conference on Conservation of Stone and Wooden Objects, Second ed.
(1971), Vol. 2, pp. 1-11. Almost of the statements made here about the
chemistry of wood are derived from this source.
2 Players can often tell when a gut string is going to break soon by it
needing more tuning (some fibres are already broken and the remaining
fibres stretch more because their share of the tension is increased) and
the sound gets dull (the fibres stretching absorb vibration energy).
Also, it is well known in harpsichord circles that newly mounted brass
strings don’t sound fully when first mounted, and only sound fully when
they have stopped their initial stretching.
3 A. Beavitt, The Strad, (Nov. 1996), pp. 916-20. Beavitt claimed that
all of the sound improvement in the life of a violin is associated with
creep, which is facilitated by humidity cycling.
4 R. Hearmon and J. Paton, Forest Products Journal, Vol. 14 (8) (1964),
pp. 357-9. They showed that the rate of creep in stressed wood is
increased by humidity cycling.
5 E. Segerman, FoMRHI Quarterly, No. 84, (July, 1996), Comm. 1471, pp.
53-5. When new instruments are strung up, the wood deforms in response
to the forces. The creep in this deformation absorbs sound vibrations,
reducing response. In ‘playing in’, sound vibration accelerates the
creep, making the instrument settle in faster.
6 D. G. Hunt and E. Balsan, Nature, Vol. 379, (22 Feb,.1996), p. 681.
Sound absorption increases considerably in the non-equilibrium situation
of rising moisture content. I interpret their experiment (Segerman,
1996) as showing that when wood is vibrated, the moisture content in
equilibrium with a given outside relative humidity decreases. The effect
is small and has only been observed at very high humidity.
7 D. Noak and H. Becker, Wood Science & Technology, Vol. 2 (1968), pp.
213-30. They showed that the damping of sound is strongly increased by
increased moisture content.
8 D. G. Hunt and E. Balsan, op. cit.
9 R. Gug, FoMRHI Quarterly, No. 52, (July, 1988), Comm. 881, pp. 44-55.
He reported that in 1580, Palissy wrote ‘Salt improves the voice of all
sorts of musical instruments’. The impregnation of wood by salts was
common practise. It was done by stewing the wood in the salt solution.
The purposes usually stated for salting wood were to avoid rot, to repel
woodworm and to stabilise it dimensionally (so it reacts less to weather
changes). In my interpretation, it is likely that most of the sound
improvement and stabilisation was due to hemicellulose degradation by
the stewing.
10 J. Nagyavary, Chemical & Engineering News, (May 23, 1988), pp. 24-31.
11 D. G. Hunt and E. Balsan, op. cit.
--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html
http://www.hf.uib.no/grieg/personer/cv_RB.html
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