Wood Pulp; Microscopic Features.
Fig. 36.—Mechanical Wood Pulp.
Mechanical and chemical pulps are readily distinguished under the microscope. The former consists of fibres of irregular shape and size, mixed with a large proportion of structureless particles, all bearing evidence of having been torn apart and separated by mechanical methods. The chemical pulp, on the other hand, consists of fibres isolated by a process which preserves them in perfect condition and form. The pulp from the various woods can be differentiated by minute details in fibre structure, some of the woods being determined from the presence of characteristic cells.
The use of aniline sulphate can also be resorted to, and for microscopic work the most useful reagent is a mixture of zinc chloride and iodine. This produces an intense yellow colour with mechanical pulp and a bluish colour with sulphite and other chemical wood pulps.