The ground and sorted pulp is made into thick sheets by means of the board machine shown in Fig. 31. The pulp mixed with water passes down the shoot D into the vat B in which the cylinder K revolves. This cylinder is covered with wire-cloth, and as it revolves it takes with it a certain quantity of pulp in the form of a continuous sheet. This sheet is taken on to the endless travelling felt F by the small couch roll E. When it reaches the rolls C it is wound round the upper one, from which it is removed when a sufficient thickness is obtained. Obtained in this form the pulp is readily transported.
The woods commonly employed are white pine and aspen. The latter yields a pulp of a better colour, but of inferior strength than the former.
Paper containing mechanical wood pulp is very liable to become discoloured by the action of air and light, the ligno-celluloses being much more readily acted upon than the celluloses isolated from them. Such fibre is, moreover, devoid of much felting power; it has, in fact, little to recommend it but its comparative cheapness. It is nevertheless used in large quantities, some cheap papers being made entirely from it. {109}
| Specific Gravity. | Degrees Twaddle.[*] | Per cent. Na2O. | Equivalent Percentage of 60 per cent. Caustic Soda. |
|---|---|---|---|
| 1·0040 | 0·80 | 0·302 | 0·503 |
| 1·0081 | 1·62 | 0·601 | 1·001 |
| 1·0163 | 3·26 | 1·209 | 2·015 |
| 1·0246 | 4·92 | 1·813 | 3·021 |
| 1·0330 | 6·60 | 2·418 | 4·030 |
| 1·0414 | 8·28 | 3·022 | 5·037 |
| 1·0500 | 10·00 | 3·626 | 6·043 |
| 1·0587 | 11·74 | 4·231 | 7·051 |
| 1·0675 | 13·50 | 4·835 | 8·059 |
| 1·0764 | 15·28 | 5·440 | 9·067 |
| 1·0855 | 17·10 | 6·044 | 10·073 |
| 1·0948 | 18·96 | 6·648 | 11·080 |
| 1·1042 | 20·84 | 7·253 | 12·090 |
| 1·1137 | 22·74 | 7·857 | 13·095 |
| 1·1233 | 24·66 | 8·462 | 14·103 |
| 1·1330 | 26·60 | 9·066 | 15·110 |
| 1·1428 | 28·56 | 9·670 | 16·117 |
| 1·1528 | 30·56 | 10·275 | 17·125 |
| 1·1630 | 32·60 | 10·879 | 18·131 |
| 1·1734 | 34·68 | 11·484 | 19·140 |
| 1·1841 | 36·82 | 12·088 | 20·147 |
| 1·1948 | 38·96 | 12·692 | 21·153 |
| 1·2058 | 41·16 | 13·297 | 22·161 |
| 1·2178 | 43·56 | 13·901 | 23·170 |
| 1·2280 | 45·60 | 14·506 | 24·177 |
| 1·2392 | 47·84 | 15·110 | 25·170 |
NOTE.—It must be borne in mind that the above numbers refer only to solutions of pure caustic soda. With liquors containing sodium chloride, sulphate, &c., the specific gravity will give an erroneous view of the amount of alkali present, as these salts of course raise the gravity. For example, a liquor prepared by causticising a solution of recovered soda has a specific gravity of 1·05 (10° Tw.). According to the above table this corresponds to 6·043 per cent. of 60 per cent. caustic. Tested by means of standard acid it showed 4·520 per cent. Too much reliance should therefore not be placed upon determinations of specific gravity, but in important cases the actual amount of alkali should be determined by titration with standard acid.
[*] To convert degrees Twaddle into specific gravity, multiply by 5, add 1000 and divide by 1000: thus 7° Tw.,
{110}
CHAPTER VII. BLEACHING.
The methods of bleaching being essentially the same for all pulps, it will only be necessary to describe one in detail.