32 TESTING MISCELLANEOUS SUPPLIES. 



These colored lead pigments may have their color modified by the addition of 

 organic coloring matters. As a general rule, such adulteration may be detected 

 by adding 20 cc of 95 per cent alcohol to 2 grams of the pigment, heating to a 

 boil, and allowing to settle. Pour off the alcohol, boil with water, and allow to 

 settle, then use very dilute ammonium hydroxid. If either the alcohol, water, or 

 ammonium hydroxid is colored, it indicates organic coloring matter. The quan- 

 titative determination of such adulteration is difficult and must generally be 

 estimated by difference. 



(d) VERMILION. 



True vermilion, or, as it is generally called, English vermilion, is snlphid of 

 mercury. On account of its cost it is rarely used in paints, and is liable to gross 

 adulteration. It should show no bleeding on boiling with alcohol and water and 

 no free sulphur by extraction with carbon disulphid. A small quantity mixed 

 with five or six times its weight of dry sodium carbonate and heated in a tube 

 should show globules of mercury on the cooler portion of the tube. The best 

 test for purity is the ash, which should be not more than one-half of 1 per 

 cent. Make the determination in a porcelain dish or crucible, using 2 grams of 

 the sample. Ash in a muffle or in a hood with a very good draft, as the mercury 

 fumes are very poisonous. It is seldom necessary to make a determination of 

 the mercury ; but if this is required, it may be determined by mixing 0.2 gram 

 of the vermilion with 0.1 gram of very fine iron filings, or better " iron by hy- 

 drogen." Mix in a porcelain crucible and cover with a layer 10 mm thick of 

 the iron filings, place the crucible in a hole in an asbestos sheet so that it goes 

 about half way through, cover with a weighed, well fitting, gold lid which is 

 hollow at the top,*fill this cavity with water, heat the crucible for fifteen minutes 

 with a small fiame, keep the cover filled with water, cool, remove the cover, 

 dry for three minutes at 100 C., and thirty minutes in a desiccator, and weigh. 

 The increase in weight is due to mercury. The mercury can be driven off from 

 the gold by heating to about 450 C. A silver lid may be used, but gold is much 

 better. 



Another method is to place in the closed end of a combustion tube, 45 cm 

 long and 10 to 15 mm in diameter, a layer of 25 to 50 mm of roughly pulver- 

 ized magnesite, then a mixture of 10 to 15 grams of the vermilion with four 

 .r I'm* times its weight of lime, followed by 5 cm of lime, and plug the tube 

 with asbestos. Draw out the end of the tube and bend it over at an angle of 

 about 00. Tap the tube so as to make a channel along the top, and place it 

 in n combustion furnace with the bent neck down, resting with its end a little 

 below some water in a small flask or beaker. Heat first the lime layer, and 

 carry the heat back to the mixture of lime and pigment. When all the mer- 

 -iiry has been driven off, heat the magnesite, and the evolved carbon dioxid 

 will drive out the last of the mercury vapors. Collect the mercury in a globule, 

 wash, dry, and weigh. 



<;-tniin,. \.-niiilii>n is at the present time little used in paints. Organic 

 lakes are used for most of the brilliant red, scarlet, and vermilion shades. 

 These organic coloring matters are sometimes precipitated on red lead, orange 

 mint-nil. <>r /i n ,- nxid: but as a usual thing the base is barytes, whiting, or 

 china clay. I'lininiinmllin red, a compound of diazotized paranitranilin and 

 beta-naphthol is largely employed; but a number of colors may be used. To 

 test for red colors in such a lake the following method from Hall may be of 



The Chemistry of Paints and Paint Vehicles, p. 29. 



