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Hi Dave, A comment from a few on-line posts from canoe makers and artists who use the bilayered bark. When the bark is peeled usually in spring, it is pliable and curls in the natural direction, with the whiter outside convex as it is on the tree. If left this way for months it will harden up and is impossible to unroll these coils by force without damaging them. One method used to unroll these coils is to heat them or to steam them (also heat, obviously). So your observations using more extreme heat (fire) simply continue to move the bilayer in the same direction as the bark unrolls describe, in your case with the white side apparently contracting even further so that the natural curvature eventually reverses. Of course it could be the dark inside layer that expands, or that both layers are involved differentially a bit like a bimetallic strip used in old thermostats. None of this speaks usefully to the mechanism. Two prominent birch bark components are polymers cellulose and lignin, which cross link, and neither looks likely to form a condensed coiled structure that might mediate extreme contraction. Steve On Jan 17, 2019, at 8:41 PM, David <dwebster@glinx.com> wrote: > Hi All & Bev, > Further to my comments of 7:42 PM Jan. 16, I tested five more strips by heating the pale side of one end and then the darker side of the other end. In all 12 cases the pale side was on the inside of the curl. Whatever the mechanism is it is consistent. > YT, DW, Kentville > > > ------ Original Message ------ > From: "David" <dwebster@glinx.com> > To: naturens@chebucto.ns.ca > Sent: 1/16/2019 7:42:47 PM > Subject: Re[2]: [NatureNS] White Birch bark outside becomes inside of strip > >> Hi Bev & All, >> Drawing on memory, a thin layer of outer bark is generated each year just outside of that brown layer and I refer to a fairly thin layer representing 3-4 years. So the bark cells are under tangential tension. It is not differential loss of water because when a thin layer is peeled off and left exposed to room air the darker side sometimes forms the inside of the coil. >> Perhaps the outer (more pale) sides are under greater tangential tension and contract more when heat decreases adhesion between layers. >> These bark cells stop growing in the year they are generated (I think) and the necessary increase in cell length as the tree grows is due to their being pulled longer. And when trunk growth eventually splits the outer layers I think the free ends always curl pale side in. >> And I checked only a dozen or so pieces of bark from the box so it may have been coincidence of wood pretreatment. >> Yt, DW, Kentville >> >> ------ Original Message ------ >> From: "Bev Wigney" <bkwigney@gmail.com> >> To: naturens@chebucto.ns.ca >> Sent: 1/16/2019 6:25:22 PM >> Subject: Re: [NatureNS] White Birch bark outside becomes inside of strip >> >>> David and all, >>> >>> I hope I am understanding this question as meant. Would the curling >>> action be because the white outer bark is damper and so dries out more >>> (more shrinkage) and also that it is more fragile? I'm thinking of >>> the canoes and various other birchbark objects (round containers, >>> moose calling horns, etc..) I have seen made by Todd Labrador, and the >>> white outer side of the birch is always on the inside. I guess there >>> is also a time when collecting bark - in early spring - when the bark >>> comes away with an inner layer -- winter bark -- that oxidizes, >>> turning dark brown -- and can be etched for designs. I don't know >>> all that much about such things, but I follow Todd's work on Facebook. >>> It is quite beautiful and fascinating. >>> >>> Bev, >>> Round Hill >>> >>> On 1/16/19, David <dwebster@glinx.com> wrote: >>>> Hi All, >>>> I have recently been burning some White Birch and sometimes set >>>> aside some bark to pep up a low fire. >>>> When layers of bark are stripped off of a stick, whether thick or >>>> thin, the outer surface is more pale (white) than the inner. When held >>>> near a flame the bark curls and the paler outer side always forms the >>>> inside of the curl whether the pale outer side or the darker inner side >>>> faces the flame. >>>> Why does the outer surface always form the inner side of the coil ? >>>> Yt, DW, Kentville >>>> >
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