What is kraft paper made of?

Long ago, "kraft paper" was made of calf leather. This kind of "kraft paper" is now only used when making drum skins. Kraft paper for book packaging is made from softwood fibers after people learn papermaking technology, and then it is pulped by chemical methods, then put into a beater, and then rubber compounds and dyes are added. Finally, it is made into paper in a paper machine. Because this kind of paper is yellowish brown, tough and looks like cowhide, people call it kraft paper.