E. L. Troxell— Plaster of Paris. 209 



The glossy finish of shellac makes it unsuitable foi* coating 

 the mounted specimen except where a very thin solution is 

 used ; even this should be wiped with alcohol to remove the 

 excess after it has had an opportunity to soak in. It takes 

 shellac quite a while to dry out thoroughly. One briquet 

 while green had a decided alcohol odor and gave a test of but 

 little more than half that of its fellows, yet it had dried for 

 two or three days. 



Shellac gives by far the strongest support to plaster, of all 

 the hardening compounds. It is necessarily limited to the 

 surface but the coating it forms is almost brittle in its 

 hardness. Briquets so treated give the maximum test for 

 plaster. Seven of them soaked in shellac for forty -five minutes 

 or more each showed a strength of over 500 pounds per square 

 inch ; while two were recorded registering 621 and 632 pounds. 

 The average for the seven was 572. Those soaked for four or 

 six hours gave but little better results than those soaked about 

 an hour. The only other briquet which showed a strength of 

 over six hundred pounds was one of pure plaster which broke 

 at 619. 



Two briquets dipped twice for a period of one half minute 

 each, registered 422 and 442 ; two others, dipped once for one 

 half minute gave a strength of 413 and 421. The extra half 

 minute, if the results are to be relied upon, gave the first two 

 an average of 13 pounds more strength. 



Shellac serves as a very good glue for uniting broken parts. 

 Its strength, adhesiveness and stability are strong points in its 

 favor. 



Water Glass. 



Experiments to determine the effect of water glass in harden- 

 ing bones and plaster gave unfavorable results. If a thick 

 solution be applied it gives a smooth glossy surface which, 

 however, soon peels off. Its effect on plaster is to produce an 

 insoluble calcium silicate, at the same time giving off a soluble 

 sodium sulphate which forms fine hair-like crystals on the 

 surface. It detracts from the strength of plaster as shown by 

 tests on briquets, from forty to eighty pounds, and it does not 

 produce a good surface. 



Magnesium Plaster. 



Magnesium oxychloride plaster has been suggested as a good 

 substitute for plaster of paris. Its crushing strength and 

 tensile strength are high and it has very great adhesive 

 properties: one part of the plaster will hold firmly ten 

 parts of sand. It is somewhat more complicated in its 



