Although many literatures have been presented about the identification of dirts in paper, we can't see any reports or reviews refering to their major types in DSP.
In this report the methods of physical and chemical classification of dirts; respectively resin, bark, bast fiber, shives, wood particles, gum, iron and iron rust, copper, slime, sand, scale, glass, asbestos, coal, oil and grease, felt fair ; were studied and several photomicrographs were shown.
The greater parts of dirts in commercial DSP were shives and bark, besides them gum, slime, etc. were observed.