Insoluble impurities include cellulosic material, grit, and polymers that may impact the operation of the burner but may not be a constituent of ash (because they may be combustible materials). Aside from causing operational problems, the insoluble material may represent a significant portion of the TBF thus lowering the value of the commodity. The specification limit for insolubles in all grades of TBF is a maximum of 2% mass.