Mechanical durability simply refers to how dense the pellet is, and how well it is formed. The denser, pellets are of course stronger. The advantage is that pellets withstand transportation better, and work more efficiently in the pellet burner.

When a qualified pellet comes out of the small pellet mills, it should have a smooth surface, with little or no cracks. If the pellet is cracking and expanding, it is because there is too much moisture within the pellet, or poor compression within the small pellet mills. Once a qualified pellet has cooled, it should be like a coloring crayon. The surface of the pellet should be smooth, and glossy.

Wood pellets tend to be shinier than others; the most important factor lies in the pellets smooth compact state. Try tapping the pellet against a hard surface, to see if the pellet stays intact, or if they crumble or easily crush and separate. The length of the pellet is not really that important. However if pellets are too long (above 1 inch) they can cause damage to the auger in the pellet burner.