GROUND-WOOD PULP 



49 



MICROSCOPIC COMPARISON OF GROUND-WOOD FIBERS. 



Plates IV to XII are photomicrographs of the ground-wood fibers 

 obtained from the various species tested. It is not possible, of 

 course, to gauge accurately from such photographs the pulp-making 

 qualities of the fibers. Length of fiber does not necessarily mean 

 strength, for a stronger pulp can be obtained from fibers which, 

 though quite short, will felt well. Steamed white birch pulp, for 

 example, will test as high in strength as longcr-fibered material from 

 the pines, larches, and hemlocks. However, the photographs make 

 it possible to compare the characteristics of the fibers of one kind of 

 wood with those of another kind, especially since the lodgepole pine, 



Fig. 37. — Loblolly pine (Pinus txda). 



western hemlock, western larch, red fir, balsam, white birch, jack 

 pine, hemlock, and tamarack were ground under the same conditions 

 of pressure, speed, surface of stone, etc. For some of these species 

 photographs of both cooked and uncooked fibers are shown. The 

 cooked-wood specimens were not treated in the same manner, nor 

 were they ground under like conditions. 



In the case of birch (PI. IV) it will be seen that the uncooked pulp 

 is very short and contains much wood flour, while the cooked fibers 

 are fairly long and very fine. When run into paper without the 

 addition of sulphite the steamed pulp showed a strength under test 

 of 0.51 point per pound and 5.8 points per thousandth inch of 

 thickness. 



14852°— Boll. 343—16 1 



