To determine the ash content of paper by heating a sample to a high temperature, burning off all organic material, leaving behind only the inorganic ash residue, which provides crucial information about the paper's quality and composition. this process requires the controlled, high-temperature environment provided by a muffle furnace to ensure accurate results. 

The primary reason for using a muffle furnace in paper testing is to accurately measure the ash content, which is a key indicator of the paper's quality and the amount of inorganic fillers present.