208 E. L. Troxell— Plaster of Paris. 



In contrast to this, glue has some very good qualities: it 

 adheres with great tenacity to wood, or bone, and whether 

 mixed witli plaster or not it is very strong and is always ready 

 for use. It is sometimes used as a retarder to the setting of 

 the plaster. Glue if used in mending clean breaks with a thin 

 seam exposing only a narrow edge to the surface, should be 

 very permanent. We have instances of this in mended 

 crockery which has lasted for many decades. In cementing 

 broken bones, if an accurate fit is desired glue is almost 

 essential because it permits a close approximation of the pieces, 

 — and the closer they are the stronger is the seam of glue. 

 Plaster, on the other hand, necessarily occupies a greater space 

 and does not allow the parts of the bone to assume the proper 

 relative position. A coating of shellac over a bone cemented 

 with glue would render it moisture proof, in which case 

 the glue would be the very best adhesive possible. 



Experiments in making briquets with glue quite confirm 

 the statement tbat it cannot be used in large quantities even 

 though mixed with plaster. It requires an excess of water in 

 order to dissolve the plaster ; because of this and by reason of 

 the glue itself the chemical action of the setting is not only 

 retarded but the normal formation of the crystals is hindered. 

 The glue makes a very hard crust on the exposed surfaces of 

 the plaster while the inner part shrinks and cracks and is very 

 greatly weakened. 



Briquets made with a glue-water solution of one fourth or 

 even down to 1/18 gave very poor results. The highest test 

 showed a strength of 226 pounds while many broke in hand- 

 ling. Other briquets were made using a smaller quantity of 

 glue, one gram to one quart of w r ater. Of sixteen ot these 

 twelve were subjected to changes of temperature and humidity ; 

 the other four were dried slowly. Of the four the average 

 was 359 ; two gave a test of 400 pounds each. In contrast 

 none of the other twelve which had been weathered, registered 

 over 277 pounds. Though not an extensive test, this indicates 

 a loss of one third in strength due to the weathering of the 

 glue. All of the briquets were dried thoroughly before testing. 



Shellac. 



Two kinds of shellac, the white and the yellow, are commonly 

 used by collectors and preparators. The former has the good 

 quality of giving no color but unfortunately it is less soluble. 

 For this it is necessary to have wood alcohol. In the field it 

 is sometimes difficult to secure alcohol, so that there the gum 

 arabic has an advantage over shellac. However, bones which 

 are to be wrapt in plaster bandages should be shellaced rather 

 than gummed in order to make them waterproof. 



