198 P. L. Troxell — Plaster of Paris. 



Art. X. — Plaster of Paris and the Effect of Foreign Sub- 

 stances. From Experiments in the laboratory of Paleontol- 

 ogy, University of Michigan ; by Edward L. Troxell. 



Contents: 



Introduction. 

 Pure plaster briquets. 

 Colored plaster. 

 Adhesives— Dextrine. 



Gum arabic. 



Glue. 



Shellac. 



Waterglass. 

 Magnesium plaster. 

 Summary. 



Introduction. 



Plaster of Paris is a very essential article in the vertebrate 

 paleontologist's laboratory. It has two chief uses : first, for 

 making casts and restoring parts, and second, for mending 

 broken bones by cementing the pieces together. It is fre- 

 quently combined with other substances in order to increase 

 its hardness, strength and durability. Some of the materials 

 commonly used are glue, gum arabic, dextrine and shellac. 

 The first three, which are soluble in water, are sometimes put 

 into the original mixture, hardening with the plaster ; or they, 

 like the shellac, may be applied to the surface of the plaster 

 after it has set. 



Experiments have been carried on with these various com- 

 pounds to determine if possible the best material for each pur- 

 pose. Over four hundred briquets were made and tested for 

 their tensile strength. They were also examined as to their 

 physical characters, because the color, smoothness, surface 

 hardening, adhesive strength and permanency are at times as 

 important as the strength. The results derived from the tests 

 of strength can well be shown in many cases by the curves 

 which indicate the trend of the varying mixtures. The bri- 

 quets were made each with a cross section of one square inch, 

 therefore the force necessary to break a briquet is equivalent 

 to the strength of the plaster per square inch. 



I am indebted to Professor Charles W. Cook, and Doctor J. 

 S. Laird for valuable suggestions and to Professor E. C. Case, 

 who very kindly gave me the use of laboratory materials and 

 facilities. 



Pure Plaster Briquets. 



It was surprising to note the very high tensile strength of 

 pure plaster. It frequently held 500 pounds, and in one case 



