Mechanical pulp yields over 90% of the wood as fibre is produced by forcing debarked logs, about two metres long, and hot water between enormous rotating steel discs with teeth that literally tear the wood apart. Alternatively, logs can be pressed against grindstones which is why this process is also known as groundwood pulp.
Trees contain up to 30% lignin, a material which is sensitive to light and degrades, and turns brown in sunlight, which explains why papers made from mechanical pulp will discolour. An example of this is newsprint. Newsprint is designed to have a short life span, and if left for a long period of time will lose its whiteness and strength. The special advantages of mechanical pulp are that it makes the paper opaque and bulky.