Cork oak is found in warm climates and is principally grown and used in southern Europe, in Portugal, Spain and North Africa. Cork can be harvested as a regular crop every 10-12 years. This special kind of bark may have evolved to withstand the frequent forest fires that occur around the Mediterranean. The trees are ripe for stripping after 25 years and can then be stripped every 8 to 15 years. The material is used as thin boarding or crumbled for loose thermal insulation. It has high compressive strength and is therefore often used on flat roofs. It is also quite resistant to fungus.
Cork is composed of dead cell combinations of cork cambium and resins. It is usual to expand the cork to increase its thermal insulation value. In loose form, cork can be used for fillings in walls and as lightweight aggregate in concretes and earth constructions. Boards can be pressed at a temperature of 250-300 °C, thereby releasing the cork's own glue which eventually binds the board together. This process can be stimulated by adding laccase (Lund, 2003). Today, however, it is usual to bind the boards with bituminous material, gelatine or other glues in a cold process.
Many of the cork forests are now dwindling. Birch bark. The bark of birch trees has traditionally been widely used as a waterproof membrane under turf roofs. It then has to be kept permanently damp to prevent it cracking.
Pieces of bark are taken from large birches, conveniently known as roof birches. A roof has between three and twenty layers of bark, depending upon the required durability. The bark is laid on the roof in overlapping fashion like any other waterproofing material. It is very resistant to rot and can be used as waterproofing also in other potentially damp areas, e.g. foundation walls. During the restoration of the Church of St Katarina in Stockholm in the early 1990s, 300-year-old birch bark was found at the end of inbuilt beams. They were exceptionally well preserved. The same method was therefore used in the restoration. In 1948, the Danish engineer Axel Jorgensen wrote: 'Building traders should set up an import of birch bark from Sweden or Finland, so that we could once again use this excellent protective medium' (Jorgensen, 1948).
Birch bark has also been used as insulation in walls, especially cavity walls, where its considerable resistance to rot and its high elasticity produces a durable solution.
Birch bark should be removed as carefully as possible, so as not to damage the tree's vital layers. The resulting squares of 60 x 60 cm or more, depending on the diameter of the trunks, are piled up and weighted down with stones to reduce their curling. The tree can then continue to grow, though it may not produce more building quality bark. Bark is loosest during spring, and the best time to take the bark is after a thunderstorm (Hoeg, 1974).
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