The method involves forming granular composition (5) by extrusion of a mixture of a heat-treated wood (3) under fragmented form, with a binder (4) e.g. polyethylene, polypropylene and paraffin, where the granular composition contains approximately 90 percentage of the heat-treated wood. Granules in the granular composition are heated and mixed by adding a thermoplastic material (6) e.g. polyolefin. The heating and mixing of the granules are carried out by forming a section through a purlin or by forming another granular composition. An independent claim is also included for a coffin made of synthetic material, comprising a cover.
