210 E. L. Troxell— Plaster of Paris. 



mixing-, being formed from magnesium oxide and magnesium 

 chloride crystals dissolved. It takes several hours to set and 

 is more expensive. In making valuable casts and in restoring 

 the parts of a rare skeleton these objections would be minor 

 matters. In the years to come this may take a very important 

 place in the preparator's work-room. 



Summary. 



Of all the substances used as a cement, glue is probably the 

 strongest but its usefulness is limited to clean breaks and thin 

 seams. When mixed with plaster it is more stable but should 

 never be used in places where a large space is to be filled. 



Plaster and gum arabic make a good cement, but the chief 

 strength is in the crust formed on the surface, while within 

 the plaster is weakened by the gum. 



Shellac serves well as a glue, it does not crack and adheres 

 well ; it is really quite strong if given time to dry out thoroughly. 



The strength of the plaster of'paris in nearly every case 

 is reduced by the addition of some foreign substance as 

 a retarder, a pigment or an adhesive. Shellac alone, applied to 

 the plaster, gave it greater strength. 



The pure plaster was strongest when mixed with a minimum 

 of water ; this can best be done by using hot water. 



Probably the most convenient colors to mix in with plaster 

 are boue black, raw and burnt sienna and Venetian red. 

 These pigments satisfy nearly eveiw demand. Liquid colors, as 

 inks and stains, are very convenient but do not penetrate 

 deeply. 



Dextrine proved very inefficient but gum arabic is found to 

 satisfy a great many needs of the paleontologist. It can be 

 used to harden fragile bones and the matrix about them ; it 

 gives a hard smooth finish to plaster and makes plaster adhere 

 more firmly to bone. It is conveniently soluble in water. In 

 cases where this constitutes a fault shellac may be substituted. 



Magnesium oxychloride promises to be more efficient in 

 some special uses than plaster of paris. Experiments with 

 this were not carried out extensively but it might profitably be 

 given a thorough test. 



University of Michigan, Ann Arbor, Mich. 



