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Hi Steve & All,
We have a communication problem and I should have checked Birch bark
anatomy first and used technical terms. The bark in White Birch consists
of the inner fairly thick dark layer, the phloem, which is not readily
separated into annual layers, and the white bark (cork layer) which can
be separated into many thin or less numerous thicker layers. Each of
these annual layers is pale on the outer side and slightly darker on
the inner side.
I was dealing exclusively with about four to 10 year thicknesses of
this cork layer which in these samples have about 10 annual layers per
mm thickness.
When four samples from the same strip were heated in the microwave
for 50 seconds the two placed dark side up arched upward in the middle
about 5 mm on 5 cm spans and those placed pale side up were arched
upward at the ends slightly less but all four had the pale side forming
the inside of the curl.
About 20 years ago I collected some bark for an American who had
asked me to send him some bark for making canoes about 10" long but was
not satisfied with the quality so instead sent him a Quebec contact of
an outfit which sold large bark samples for craft use. The samples I
collected have been sitting in the wood porch since then, are exposed to
freezing sometimes, and most have gradually split into very thin annual
layers.
When eight 5 cm long samples of annual bark (representing the
increment of one year) were heated for 50 seconds in the microwave, half
pale side down, all were curled somewhat (2 mm to 15 mm) and the darker
side always formed the inside of the curl. Just the opposite of curling
when strips representing multiple years were heated.
This contradiction can be rationalized by assuming that when bark
one year in thickness is heated it curls dark side inward as on the
tree; i.e. assumes the original shape. But when bark representing
multiple years is heated, the outer pale bark being under more tension
than inner layers shrinks in length but remains in contact with inner
layers thus forming the inside of the curl.
This also explains how one can easily waste much time.
Yt, DW
Kentville.
------ Original Message ------
From: "Stephen Shaw" <srshaw@dal.ca>
To: "naturens@chebucto.ns.ca" <naturens@chebucto.ns.ca>
Sent: 1/18/2019 1:42:41 AM
Subject: Re: [NatureNS] White Birch bark outside becomes inside of strip
>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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