Birch bark is a lovely material that can be woven, folded, and lashed into a great variety of projects. If done mindfully, birch bark can be sustainably harvested without permanently damaging trees.
The images in this Instructable were taken by Alex Kamerling at Week in the Woods: http://weekinthewoods.org/
Step 1: Tools
Bark may be harvested without any tools, but it is nice to have a short sharp knife, ladder, string, ruler, straight edge, soft wax, and scissors.
Step 2: Time of Year
Birch bark should be harvested in the spring when the sap is running in the trees. In Alaska and Minnesota (and presumably many other places) the best time to harvest birch bark is when the wild roses bloom. When done at the right time of year the bark will literally pop off the tree. However, if done too late in the season the cambium (inner bark) will come off with the outer bark. If the cambium is damaged the tree may die. If done right, a tree will re-grow its bark over the course of 10-20 years. I have heard that the best bark comes from trees that have already been previously harvested, and traditionally people would return to harvest in the same grove over the course of several generations.
Step 3: Selecting a Tree
Bark can be harvested from any birch tree (at the right time of year). However, there is a danger that the tree will be injured in the harvesting process and many people do not like the look of a tree that has been harvested. For several years after a tree has been harvested the tree's bark will be black.
The majority of the bark used at Week in the Woods is harvested from logger's trees. Philosophically, the staff likes working with trees that have been, or are about to be, cut down. Loggers generally do not use the bark from their trees, and it is easier to remove large continuous sections of bark once the tree has been felled. Generally, the State of Alaska regards harvesting birch bark as a subsistence activity and allows people to harvest bark from State Land (please check with your local government before harvesting on public land).
Each tree is unique. There is a great variety of color in birch bark. The larger diameter the tree is - the wider the piece of bark will be. The longer it has been since the tree has been harvested the thicker the bark will be (the tree adds a thin layer of bark each year). The more gnarled the tree is - the more heterogenous the bark will be.
Step 4: Cutting
Make a cut down the tree trunk. To get the whole piece of bark the knife must cut down to the cambium. It is important that the cut be as vertical as possible. If the cambium is cut all the way around the tree (horizontally) the tree will die (this is called girdling a tree). Charlie Mayo, one of the teachers at Week in the Woods, contends that if you start high in the tree and make one short vertical cut, the tree can be unwrapped in one great downward spiral.
Step 5: Popping
If it is the perfect tree at the perfect time of year the bark will pop off as soon as the vertical cut is made. Usually, to remove the bark the harvester needs to push their hands between the outer bark and the cambium. By gently working their hands around the tree the bark will pop off.
Step 6: Project Decisions
Birch bark can be used as is directly off the tree. Large fresh pieces are ideal for origami and paper engineering style projects. However, for many projects bark will need to be stored, cut into strips, and/or thinned.
Step 7: Storing Bark
If bark is not going to be used immediately it should be rolled into a bundle. The bark should be rolled longways (down the trunk of the tree) to counteract the way it would naturally curl (around the tree). If it is not wrapped in this direction it may be impossible to use later. As the bark dries it will become less flexible and more brittle. Bark can be somewhat reinvigorated by exposing it to moisture and heat. John Manthei, camp director, recommends letting it relax in a hot sauna (steam bath).
Step 8: Making Strips
A straight edge, ruler, and utility knife or scissors can be used to cut bark into strips. There also are commercial bark stripping machines that use a guide and pizza-cutter-like wheel to produce uniform strips.
Step 9: Thinning
Each piece of bark is composed of many thin layers (one produced by the tree each year). Often bark is too thick and inflexible as it comes off the tree. Like starting a roll of tape, a fingernail can be pushed between the layers of bark at one of the corners. Working slowly and carefully, one may divide the entire piece of bark into two layers. I have also seen Charlie Mayo use a small wood plane to thin gnarly sections.
Step 10: Start a Project
Here are a few birch bark projects from Week in the Woods: http://weekinthewoods.org/