COLORED PIGMENTS. 37 



bonate, fuse, using an aluminum or asbestos shield to prevent the sulphur 

 from being taken up from the gas. Dissolve the fused mass in water, making 

 acid with hydrochloric acid ; there is no insoluble residue. Precipitate with 

 barium chlorid and determine total sulphur by weighing the barium sulphate. 



(6) SULPHUR AS SULPHATE. 



Weigh 1 gram in a beaker, add water, then hydrochloric acid, boil until the 

 hydrogen sulphid is expelled, then add barium chlorid, and determine the sul- 

 phur as usual by weighing the barium sulphate. 



(7) SULPHUR AS SULPHID. 



Subtract sulphur as sulphate from the total sulphur. Pure ultramarine 

 will generally have about the following composition : 



Per cent. 



Silicon dioxid (SiO 2 ) 39 to 43 



Alumina (A1 2 O 3 ) 24 to 30 



Sodium monoxid (Na 2 O) 17 to 21 



Sulphur (S) 10 to 16 



4. Green Pigments. 



The most important green pigment is chrome green, which is a mixture of 

 Prussian blue and lead chromate. A green made of ultramarine and zinc 

 yellow is occasionally encountered. If absolute permanence is required chro- 

 mium oxid (Cr 2 O 3 ) is sometimes used, but this pigment is not common. The 

 very brilliant copper arsenic compounds (Paris green, etc.) are little used in 

 paints. 



(a) CHROME GREEN. 



(1) GENERAL DISCUSSION. 



A pure chrome green should contain only Prussian blue, lead chromate, and 

 lead sulphate. Any other substances should be considered adulterants. Greens 

 made by precipitating the ingredients together are superior to those made by 

 mixing the blue and yellow after they are precipitated separately. To dis- 

 tinguish the two, rub up the pigment in oil and allow to stand. A badly made 

 green will show blue, but one properly made by precipitation will not. A micro- 

 scopic examination of the dry pigment should also be made. A badly made 

 green will show yellow and blue particles, as well as green. A well-made green 

 will show green and some blue particles, but no yellow. 



As different grades of lead chromate vary greatly in shade, and as the same is 

 true to a somewhat less extent with Prussian blue, actual determinations of the 

 percentages of Prussian blue, lead chromate, and lead sulphate are of little real 

 value, and the analysis of chrome green is made with difficulty. It is, however, 

 important to determine impurities, which may be barium sulphate, silica and 

 silicates (clay), or calcium and magnesium compounds. 



(2) IMPURITIES. 



To examine for impurities proceed as follows: Place 1 gram in a small cas- 

 serole, add 10 cc of water and sodium peroxid, a little at a time (about 1 gram 



For methods of analysis see U. S. Dept. Agr., Bureau of Chemistry, Bui. 107, 

 Revised, pp. 25-30; also, Lunge, Chemisch-techuische Untersuchungsmethoden, 

 2:807-810. 



