Most of the smaller plants must have leaves, seeds and flowers removed before being used as roof covering, thermal insulation, etc. Extraction of fibre from plants such as flax, stinging nettles and hemp is carried out by retting on the ground, left to mould, bacteria, sun and rain for two to three weeks, until the fibres loosen and can be collected for the further final cleaning process.
Thereafter, fibres are woven, needle-punched or glued into mats or sheets, sometimes also used as loose fill. The seeds are often pressed for oil. Pressing may be cold or with heat to extract more. Additives are often used in these processes, either to facilitate the process or to improve the properties of the final product. These include fungicides, adhesives, fire retardants and water repellents. Many of these may be suspect chemicals.
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