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Pellets are produced by compressing the wood material which has first passed through a hammer mill to provide a uniform dough-like mass. This mass is fed to a press where it is squeezed through a die having holes of the size required (normally 6 mm diameter, sometimes 8 mm or larger). The high pressure of the press causes the temperature of the wood to increase greatly, and the lignite plasifies slightly forming a natural 'glue' that holds the pellet together as it cools. Pellets conforming to the norms commonly used (DIN 51731 or Ö-Norm M-7135) have less than 10% water content, are uniform in density (density in excess of 1 ton / cubic meter, so the do not float if placed in water), have good structural strength, and low dust and ash content. Because the wood fibres are broken down by the hammer mill, there is virtually no difference in the finished pellets between different wood types. Pellets can be made from nearly any wood variety, provided the pellet press is equipped with good instrumentation, the differences in feed material can be compenstated for in the press regulation. Pellets conforming to the above norms cannot contain any recycled wood or outside contaminants. Recycled materials such particle board, treated or painted wood, melamine-coated panels and the like are particularly unsuitable for use in pellets, since the may produce noxious emissions and / or uncontrolled variations in the burning characteristics of the pellets. See also
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