Birch (Latin Bétula) —is a genus of deciduous trees and shrubs in the family Betulaceae. Birch is widespread in the Northern Hemisphere and is one of the prevailing wood species in the territory of Russia. The total amount of birch species is about a hundred or a little bit more!
Many parts of birch are put to good use: wood, bark, white upper layer of bark, birch juice. Birch buds and leaves are used in medicine. Some species are used for forest shelterbelts and in landscape gardening.
Species of Birch genus
List of Betula species
The species of this genus feature high polymorphism. Different investigators have different vision of status of some taxons, referred to the genus. Commonly, the amount of species is estimated at about one hundred or a little bit more.
According to the data of Royal Botanic Gardens Kew’s web-site, the genus numbers 113 species and hybrids, among which the best known are:
Cherry birch (Betula lenta). A species from North America with glossy reddish-brown or almost black bark.
Dwarf birch (Betula nana). A shrub which is usually up to 1 m high with small roundish leaves. It grows in Europe on wetlands, in waterlogged pine forests.
Kamchatka birch or arctic dwarf birch (Betula humilis). A shrub up to 2 m high with oval leaves; it grows in Europe along water banks and on wetlands.
Silver birch or warty birch (Betula pendula). The species is widespread in Europe and Siberia; it can be also found in Nothern Africa. The bark varies from white snow to light-gray. The usual height is 10-15 m, sometimes up to 30 m. The browses are bare and warted (in contrast to downy birch, which has floccose browses with no warts).
Himalayan birch (Betula utilis). A species native to Himalayas up to 18 m high with light smooth bark.
Downy birch, also known as white, European white or hairy birch (Betula pubescens). In European Russia it often grows beside warty birch. See above about how to distinguish these species. This birch is of nomenclature type.
Black birch or river birch (Betula nigra). A species native to eastern Northern America. Saplings have white and smooth bark, the older trees have dark and wrinkled bark. Suggestively, it is an ancestral type.
Erman’s birch (Betula ermanii). Can be found in Kamchatka, Sakhalin, along the shore of the sea of Okhotsk. It is called “stone birch” in Russian due to uncommonly hard, dense and heavy wood, wich drowns in water.
We use the following species in our manufacture:
-Silver birch or warty birch (Betula pendula)
-Downy birch, also known as white, European white or hairy birch (Betula pubescent).
Physical properties of birch.
The freshly felled birch has the following moisture content (average): wood- 78%; bark - 58%.
Unlike with heartwood trees the moisture here is evenly distributed across the bole. To calculate the processes of drying and humidifying of wood one has to know moisture conductivity
coefficient. Moisture conductivity of birch at a normal temperature is about twice as small as of soft wood (spruce, pine), which is explained by higher density of wood. As temperature rises moisture conductivity increases, while the correlation mentioned above is maintained.
Due to quite low values of moisture conductivity, the birch sawn lumbers tend to cupping, therefore it requires scrupulous attitude to management of a drying process.
Birch belongs to highly desiccating species. Its wood swelling (desiccation) coefficients are the following:
* 0.26- 0.28 radially
* 0.31-0.34 tangentially;
* volumetric 0.54-0.64.
When contacting water directly, the maximum moisture content of birch wood is 135%, bark – 63%.
The main kinds of birch (silver, downy, yellow and black) are referred to medium hard species. Their average density (at 12% moisture content) is 640 kg/m3. Density of Korean birch is 685 kg/m3; Dahurica birch – 725 kg/m3; Schmidt’s birch – 975 kg/m3.
It is an important property when applying coatings. Birch with its characteristics is referred to well-soaking species.
Mechanical properties of birch.
Wood of the main kinds of birch features quite high strength values, ranking just a little below oak, while Erman’s birch and all the more Schmidt’s birch even outperform oak.
Tensile strength (Silver and Downy birch):
* at static bending – 109.5 MPa;
* at stretching along the grain – 136.5 MPa;
* at compression along the grain - 54 MPa;
* at splitting along radial plane – 9.02 MPa:
* at splitting along tangential plane 10.9 MPa.
Modulus of flexibility at static bending – 14.2 hPa.
Processing and performance properties of birch:
* impact strength – 92.9 kJ/m2; hardness:
* at ends – 46.3 N/mm2;
* radial – 35.9 N/mm2;
* tangential – 32.1 N/mm2.
Durability (abrasion resistance) of birch wood can be evaluated as high. With this factor birch doesn’t underperform oak, thereby birch can be used for parquet manufacturing. It is easy bendable and suitable for other kinds of processing, can be easily used for imitation of valuable species, is easy to paint on and polish (minimum height of microroughness - 30-60 µm).
Fields of use.
Birch wood is most widely used for producing rotary cut veneer in manufacture of plywood and densified wood laminates. Beyond that, birch is used for production of parquet, wooden parts of guns, turnery and household goods. Application of birch in constructional elements is limited due to its tendency to cupping. Wood of Karelian birch (with a beautiful texture pattern caused by wood flaws, like curly grain and knots) is used for manufacture of high-end furniture and various handicrafts.
Crushed wood is used for wood chip boards and fiber boards, cellulose, furfural, xylitol and other forest chemical products.
Dry distillation enables to obtain chain chemical products, which serve as a basis for production of varnishes, formalin, perfumes. Birch logs are also used for charring.
Birch logs are a valuable fuel with high calorific capacity. Soot is used for production of inks and paints.
Resistance to plucking of metal mountings (like nails or screws) is slightly higher than the same of oak: for nails it is 160-190 N/mm, for screws – 130-140 N/mm (tangentially and radially). Decay resistance of wood species (resistance to biological factors – fungi) is commonly expressed in conditional units (with respect to resistance of linden sap-wood). According to European standard EH 350-2:1994 regarding this factor, the species fall into five classes. The first class of high-resistant species involves teak (India) and eucalyptus (Australia), Russian oak and larch belong to resistant species (the second class), while birch (the most common species) is referred to the last five class of low-resistant species. The value of relative resistance of birch is 1.8 (oak 5.2, larch – 9.1)