Some people regard the darker board colour which results from including bark depreciates its marketability. Despite these disadvantages most manufacturers do not debark their logs before further processing for cost reasons, About half of the bark is removed from the wood furnish, however, at two stages: primary chipping, where large pieces of bark fall oll and are removed; alier drying where bark dust makes up a high proportion of the dust collected at the air-cleaning cyclones. The bark is usually burnt in the factory's boilers as it has a high calorific value ; often providing up to 30% of a factory's thermal energy need.