TECHNIQUE 61 



individually can be sorted, and often sufficiently cleaned, 

 by placing them at the top of a pile of sieves (becoming 

 finer downwards), and directing a steady jet of water 

 through the whole. 



It often happens that shells in clay are so friable that 

 they cannot withstand brushing, however gentle. In 

 such cases, both fossil and matrix must be reinforced 

 with glue or shellac. The whole specimen 'should be 

 soaked in dilute solutions of one or the other reagent, 

 and allowed to set hard. The matrix can then be 

 painted locally with water or alcohol, and gradually 

 brushed away. 



When the matrix is too hard to yield to a brush, 

 it may either be softened chemically or chipped off 

 mechanically. Both methods are fraught with danger, 

 since in many cases rock and fossils are of similar 

 chemical composition, while even petrifying substances 

 are rarely harder than steel. 



In the case of fossils preserved in partly amorphous 

 limestone, " weathering " can be imitated by leaving the 

 lump for some weeks in water through which carbon 

 dioxide is slowly bubbled. This is preferable to 

 exposure to rain-water, since the latter often leads 

 to the growth of green stains, while its intermittent 

 application encourages precipitation of hard calcareous 

 crusts. Occasional bouts of brushing will slightly 

 accelerate this tedious process. Soaking in strong sugar- 

 solution has a similar effect, especially if the liquid is 

 hot ; but great care is necessary to remove all traces of 

 sugar before drying, lest its crystallization should 

 crumble the surface of the fossil. Hydrochloric acid 

 (or, preferably, acetic acid) may be painted on to the 

 matrix remote from the fossil ; but, unless the latter is 

 a siliceous petrifaction, it must never come in contact 

 with the shell. In the case of calcareous fossils in a 



