202 K L. Troxell— Plaster of Paris. 



The sienna curve is based on the tests of 48 briquets, the 

 bone-black curve on twenty-one. Since the change in color is 

 not in proportion to the amount of coloring matter used, in a 

 very dark mixture the addition of a considerable quantity of 

 pigment is necessary to make an appreciable change, and the 

 strength of the plaster has to be sacrificed. 



Methods of Coloring. 



Other devices are sometimes employed to produce the 

 results desired. A veneer of colored plaster mixed with gum 

 arabic may be spread evenly over the surface. A color may 

 be deepened by a final coat of gum arabic alone, for on drying 

 the water seems to bring the color to the surface where it is 

 deposited in the hard outer crust formed by the gum. A 

 liquid color such as an ink may well be used and can easily be 

 applied to the surface. It can be diluted to the desired intens- 

 ity and it does not destroy the detail of the surface by filling 

 the small depressions. Common paints and stains are frequently 

 used. The latter are especially good because they penetrate 

 more deeply and have but little luster. Brown shellac gives 

 plaster a glossy yellow color. All these have the disadvantage 

 of their limited penetration and if the plaster is cut or accident- 

 ally chipped it shows the white beneath. 



In a selection of colors it is commonly supposed that mineral 

 pigments are best since they do not deteriorate. Many colors 

 otherwise liable to alteration might be quite permanent in the 

 favorable conditions found in our museums. Lamp black is 

 almost useless because it is so insoluble. Nearly all the shades 

 necessary in the imitation of bone can be produced by the use 

 of bone black, burnt and raw sienna and Venetian red, com- 

 bined with the white of plaster of paris. 



Adhesives. 

 Dextrine. 



Dextrine is frequently used for hardening plaster; the yel- 

 low unbleached variety is a very cheap material costing but 

 fifteen cents per pound. When in solution, which is rather 

 difficult to form, it has a coffee color, and mixed with plaster 

 gives a soft yellow. It has a sweet odor and since it is a sugar 

 the solution deteriorates rapidly unless some preservative such 

 as corrosive sublimate or carbolic acid be used. 



The tests with briquets show that dextrine in the original 

 mixture does not give to plaster a great tensile strength, rather 

 detracting from it in direct proportion to the density of the 

 solution used in the mixing. Plaster, with a one-fourth solu- 

 tion of dextrine, gave for the highest test 175 pounds per 



