Wood 



SECTIONS OF FREE MATERIAL 



93 



longer floats, a state showing that the 

 greater part of air has been removed. 



The blocks are now transferred to 50 % 

 commercial hydrofluoric acid, and a word 

 of warning must be issued as to the danger 

 of this material. Since it dissolves sihca, 

 it cannot be handled in glass vessels, and 

 the choice lies between hard rubber and 

 lead. Not only is the vapor extremely 

 corrosive, but a burn from hydrofluoric 

 acid on the skin is worse than that from 

 any other chemical known to the writer. 

 Extreme care should be taken, therefore, 

 in transferring blocks to hydrofluoric acid, 

 where they may remain until the silica 

 has been dissolved from them. A block of 

 1.^-inch oak will be satisfactorily desihci- 

 fied in one day, but teak should remain 

 for at least three or four. Observing the 

 same precautions as before, the blocks of 

 wood are removed from the hydrofluoric 

 acid to running water and must be washed 

 for at least three or four hours before thej' 

 are safe to handle with the hand. 



A block ma}' now be removed from the 

 water and mounted in any convenient mi- 

 crotome for cutting. The harder woods 

 can never be satisfactorily cut on a hand 

 microtome and the best mechanism is un- 

 doubtedly the sliding microtome described 

 in Chapter 12. Exactly the same precau- 

 tions in cutting a wood block on this mi- 

 crotome should be taken as in cutting a 

 block of anything else. That is, the knife 

 must be sloped at an angle towards the 



block so that the greatest possible length 

 of knife is used to cut a single section, and 

 the block must be so orientated that the 

 knife enters at one corner rather than 

 flat on the side. The sections as they are 

 cut ma}'- be removed to water, and any 

 tendency which they have to curl may be 

 counteracted by warming the water. 



An interesting variation of this stand- 

 ard technique was proposed at the same 

 time by Crowell 1928 (20540b, 5:149) and 

 b}' Kisser (cited from Crowell). This 

 method consists of mounting a dry block 

 of wood in any tj^pe of microtome and di- 

 recting onto its surface a jet of high-pres- 

 sure steam. After the steam has acted for a 

 moment or two, a single section is cut and 

 the steam again apphed for another few 

 moments before cutting the next section, 

 and so on. It is stated that by this means 

 sections of the hardest material may be 

 taken without the use of hydrofluoric acid. 



Sections of wood are usuallj- mounted in 

 balsam, dehydrated, and cleared as de- 

 scribed in Chapter 6. Sections which tend 

 to curl may be tied between sUdes. 



Sections of wood containing some natu- 

 ral color, as oak and mahoganj', are best 

 mounted unstained, but thin sections of 

 colorless wood may become too transpar- 

 ent under this treatment. Almost any dj'e 

 may be used, since the purpose is not to 

 differentiate the parts but only to render 

 them Wsible. 



