The Pellet Fuels Institute has two grades for pellets: premium, which is usually made of only wood, and standard, which contains some bark. Premium pellets are marginally more expensive than standard—about $5.50 for a 40-pound bag— but they produce less ash. Another indicator of quality is the amount of fines, or dust, at the bottom of the bag; there should be less than 0.5 percent, which works out to half a cup. Any more dust can create fused ash chunks, called clinkers, that block airflow into the stove. The last variable is sodium content; pellets should have fewer than 300 parts per million (ppm) to ensure an efficient burn.