Many of the participants at Week in the Woods (http://weekinthewoods.org/) made stools. Each person wove a unique seat for their stool out of birch bark strips. These techniques could also be used to make or repair the seats and backs of chairs.
Almost all of the pictures in this Instructable were taken by Alex Kamerling.
Step 1: Materials and Tools
The materials needed for this project include a chair, hammer, tacks, scissors, soft wax, and birch bark strips. Refer to the Instructable Harvesting Birch Bark (https://www.instructables.com/id/Harvesting-Birch-Bark/) for instructions on how to collect and strip birch bark.
The strips need to be about a foot longer than the width of the chair. The combined width of all of the strips needs to be at least twice the width of the chair.
Step 2: Lay and Tack the Warp
Lay and tack strips across the chair one direction (the orientation is a matter of preference). Leave a 1/4-1/2" gap around each chair leg. Leave a gap that is wider than the thickness of the strips between each strip. John Manthei, the camp director, recommended starting in the middle and working out to the edges. It is best to have an odd number of strips. Test fit each strip before tacking. The strip should wrap all the way around the back side as far as possible and should be under enough tension that it can only easily be deflected half an inch. Each strip should be cut to fit. Cut the corners off each strip before they are nailed down to reduce curling.
Step 3: Weave the Weft
Weave a series of strips across the warp (this will be the weft). The pattern, orientation and width are a matter of preference. Most participants chose to weave over one and under the next, but more complex patterns are possible. Most also used one width of strip. Many people created color contrast by alternating which side was up. If the warp is too tight it will be difficult to weave the weft. If it is difficult to pull the bark through the warp, try cutting the ends to a point, rubbing the ends with soft wax, and/or pushing the ends through with a flattened stick (fid). Most participants re-wove a second layer of narrower bark strips to double the strength, tighten things up, and create a more interesting pattern.
Step 4: Tack the Weft
After the weft has been woven across the warp it can be tacked under the chair. If the chair has been double woven it is important to tack the first (wider) strips as low as possible and then to tack the second strips half an inch away so it is less likely the bark will split.
Step 5: Washers
To further reinforce the bark, pieces of bark may be tacked horizontally along each edge on the underside of the seat (my father is calling these washers). Wherever possible tack the washers between the strips so that they will not split the bark that is already tacked down. Trim the corners of the washers to reduce curling and the sides so that they are not visible from the top of the stool.
Step 6: Try It Out
Woven birch bark makes a very satisfying seat. It is just giving enough.
(The tops of the legs of the stool pictured will be cut flush with the seat once the pegs dry.)