by Les Dowdell 

This article was taken from the Oct. 2007 Fort Wayne Newsletter. The article apparently was a series of exchanges on a forum. GLBS assumes no responsibility for its accuracy. Sorry for the small font.- ED.

Jason Krozel wrote:  There was a post (I forget who from) about Peter Adams discrediting copper wiring in his Flowering Bonsai book. I looked up this reference, and he states that copper wire is more likely to damage the delicate bark of many flowering species, and that since it conducts heat better, will burn the tree while in the sun and will ice up in winter.

Pseudo science rides again. Mr. Adams’ premise that copper wire will burn bark on a hot sunny day tends to fall into the realm of modern (bonsai) myths. By studying the science (physics in particular) of the situation, there is no sound, logical basis upon which to support this hypothesis.

1. Solar heating during the day will gradually increase the temperatures of both the tree and the copper (or any) wire at a rate that will not cause their temperatures to be noticeably different.  The copper may be a fraction of a degree warmer than the bark next to it and, considering the extreme case of thick, dark copper on white bark, possibly a difference of one full degree (Celsius of course).  This does not lead to stress within the bark.
2. Due to the high thermal conductivity of copper (thermal conductivity coefficient of 4.01 watts/sq. cm./degree K) heat will be lost quickly to the air, parts of the wire in the shade, and the bark of the tree.  Again, these are all very small amounts.  Most of the energy redistribution will be to the air and to the slightly cooler parts of the same wire which are shaded.  Only a small fraction is transferred to the bark due to the facts that 1) the bark is just about the same temperature as the wire and, 2) the bark is a good insulator and cannot conduct heat away from the wire as fast as the air and other sections of the wire. (Note: aluminum is also a good thermal conductor with a thermal conductivity coefficient of 2.37 watts/sq. cm./degree K.  Compare the values of copper and aluminum with iron which has a coefficient of 0.804 watts/sq. cm./degree K.)
3. Since there are normally no sudden inputs of large amounts of energy, solar or otherwise, the copper wire is able to maintain thermal equilibrium with its surroundings by transferring the small amounts of excess heat to the air.
4. At least half the wire is in shade (on the side of the trunk/branch facing away from the sun) and most of it is usually also shaded by the leafs that form the canopy of the tree. Therefore, there is very little actual wire being heated by the sun.  Consequently, any slight heat buildup on this small amount of exposed wire can be quickly and harmlessly dissipated to shaded sections of the same wires as well as the aforementioned air.

In summary:

1. the low temperature differential between the wire and the tree, 
2. the slow rate of temperature increase due to normal daytime warming,
3. the rapid loss of excess heat (small amounts) due to excellent thermal conductivity lead to the conclusion that solar heating of wire on a bonsai will not lead to heat damage of the bark in contact with the wire.  For cold weather, the same properties of small temperature differences and good conductivity will negate the possibility of freezing damage due to the presence of the wire.

If Mr. Adams would like to protect the "delicate bark" of his trees, he might consider wrapping soft paper around the wire before applying it to the bonsai. This traditional technique will prevent mechanical (as opposed to thermal) damage to the bark from the friction of the wire rubbing against the bark as it is applied to the tree.

Best wishes in bonsai (and bonsai science)

Les Dowdell
Somewhere in Alberta (Zone 3)

ADDENDUM:
Les Dowdell wrote:  Pseudo science rides again. Mr. Adams' premise that copper wire will burn bark on a hot sunny day tends to fall into the realm of modern (bonsai) myths. By studying the science (physics in particular) of the situation, there is no sound, logical basis upon which to support Mr. Adams' hypothesis.

I read this post, and, while appreciative of the exact nature of the information, the references to Peter Adams’ beliefs left me feeling a little uneasy.  I read that reference immediately following reading the post that referred to it, but replied a couple of days later, giving the gist of what I thought I had read.  Going back to the book, I have found that Mr. Adams does not, in fact, specifically say that Cu wire heats enough to burn the bark.  Please allow me to quote him directly, now that I have the text in front of me:
The metal used is aluminium or copper.  Anodized aluminium wire made for bonsai is soft and gentle to the tree.  Copper hardens and conducts temperature rather too well, heating up in the summer and icing up in the frost.  Aluminium is the wire to use for flowering plants, most of which have sensitive bark.

I had no intention of misquoting Mr. Adams, and the bark burning was an inference I made over the course of a couple of busy days.  Just wanted to set the record straight.  However, all other info in Les’ excellent post is relevant to the icing and heating up, but I still think I’m not worthy to pinch Peter Adams’ trees.  He has written a lot of informative and accurate educational material.

Jason Krozel   USDA Zone 5
University of Illinois at Chicago, Dept of EECS

Reiner Goebel wrote:  In a somewhat similar vein, Colin Lewis, during his demo here in Toronto, suggested that aluminum wire will cut in more slowly than copper wire because it requires a heavier gauge, which means that more of the wire touches the wired surface.

It sounds OK, but is it?

Reiner,

The short answer is "No". The long answer comes to the same conclusion but with some reasoning behind it.

One reason that copper wire appears to cause more damage than a larger diameter aluminum wire is because of the visual aspect of the wound. With identical depths of penetration of the two wires, the edges of the wounds give different impressions, both physically and psychologically. The angle that the inner edge of the depression makes with the adjacent bark surface is steeper with the narrower wire. The exact relationship is:

angle = arccos(1-(x/R))  

where x is the depth of the wire penetration and R is the radius of the wire.

Therefore, if both wires have become imbedded to a depth that equals the radius of the copper wire, the edge of the copper wire wound will be at 90° to the surface while the aluminum wire wound will be at 60° (assuming the aluminum has twice the thickness of the copper). The smaller edge angle is interpreted by most people as indicating a less severe entry into the bark despite the fact that the depth is identical in both cases.

Thus Alan Walker wrote:  The narrower gauge might be a little more likely to actually lacerate the bark (similar to pressure from a butter knife compared to a razor with the same pressure), but it seems to me that the larger diameter wire would cut in at the same rate as the smaller diameter provided that all other variables are equal, such as the tightness of the wire. The difference is that the wider wire will have a wider indentation which might be more easily disguised later than a narrow indentation.

When wire is initially applied, there is some slack or looseness in the wire. As the tree grows and the branches thicken, the slack is gradually taken up until the wire can no longer avoid being in contact with the bark. The length of time required for this to happen depends on how tightly it was put on initially. After this point the wire, no matter what its thickness will become imbedded in the branch. Thus, if the branch radius increases by 1 mm, the wire will become imbedded 1 mm in the bark.  Thus, all wires, whether thin copper or thicker aluminum, will indent the bark to the same depth. As the last sentence of the above quote from Alan Walker states, the difference is the width of the indentation.  The width is a function of the wire radius and is: 

width = 2 X sqrt(x(2R-x))

with x and R defined the same as above. 'sqrt' means square root.

The next point to consider when compared the damage caused by wires of differing thicknesses arises from this difference in width of the wire indentations.  The amount of damage to the cambium layer is a function of the depth of the indentation AS WELL AS the width of the indentation.  Thus the wider indentation of the aluminum wire will affect (damage) more tissue within and under the bark.  Consequently, the bigger injuries due to aluminum will take longer to heal.

In reference to Reiner’s initial question Walter Pall wrote:  Yes, I think that’s right. Anyway it is my experience. This is a good reason not to wire deciduous trees with copper wire! Sorry, Walter, but it is actually the aluminum wire that will cause greater damage to the tree and, therefore, it is safer to use copper wire.  Although the copper wire may appear visually to cause a more severe wound, in fact the thicker aluminum wire will damage a larger area when it is left on too long.

To close, I will leave the last word to Alan Walker:  The key, I believe, is to practice your wiring skills, so that you wire tight enough to bend and hold your branch in the desired position, but not so tight that bark bruising and lacerations are likely to occur before they are noticed.   Also, routine inspections should prevent these sort of accidents. 

Best wishes in bonsai,
Les Dowdell
Zone 3 somewhere in Alberta