COLLECTING AND PREPARING. 813 



protective solution and the whole again suspended in the acid. 

 The process is repeated indefinitely until the whole of the matrix 

 is dissolved and brushed away and the complete fossil exposed. 

 The protecting collodion may then be removed by acetine or ether- 

 alcohol."^ 



Delicate pyritized fossils with hard shale or slate matrix adher- 

 ing, may be cleaned according to Bather by the use of hydrofluoric 

 acid. This will attack the slate but not the pyrites. The speci- 

 men may be exposed to the fumes or better since these are 

 obnoxious, the specimen may be washed in either a strong or dilute 

 solution, according to the character of the matrix. Any exposed 

 portion may be covered with a protective solution^ or with melted 

 wax and the process repeated until the whole fossil is exposed. 

 There is danger of carrying the process too far and loosening the 

 entire specimen. 



When the fossils are silicified and enclosed in a calcareous 

 matrix they may be placed in hydrochloric acid of moderate 

 strength and dissolved out. It should be noted, however, that the 

 fossil may not always be entirely silicified. If the acid appears to 

 attack the fossil, it should be washed, dried and the exposed part 

 covered with a protective solution. It is well in that case to 

 weaken the acid. Acetic acid or pure vinegar may be used for 

 purposes of delicate etching. 



3. Preservation of Fossils. 

 Many fossils are so soft and fragile that they must be hardened 

 sometimes even before cleaning away the matrix. Bather recom- 

 mends a thin alcoholic solution of "brillac" or white shellac. A 

 good formula is 6 dessert-spoonsful of dry white shellac, dissolved 

 in one quart of 95 per cent, alcohol. This has also been used to 

 advantage in hardening tests of recent echinoids. In the case of 

 fossils, the solution should be repeatedly applied, until the mass is 

 sufficiently hardened. Carbon-tetrachloride^ has also been recom- 

 mended as a solvent for finely powdered shellac, copal or other 

 hardening substance. Its non-inflammability especially recom- 

 mends it. Silicate of potash or water glass has also been used as 



' Bather, p. 83. 



^ Shellac dissolved in alcohol or other solvent, or hot beeswax. 



"^ Carbona of the market will serve. 



