COLORED PIGMENTS. 



33 



v;i!nc, though other colors may be employed, which makes the table of only 

 limited use. 



Determination of red colors in organic lake. 



It is well also to try the action of reducing and oxidizing agents such as 

 stannous chlorid, ferric chlorid, etc. (See Zerr, Bestimmung von Teerfarb- 

 stoffen in Farblacken ; also Schultz and Julius, A Systematic Survey of the 

 Organic Coloring Matters.) 



Paranitranilin red is soluble in chloroform. It is also well to try fhe solvent 

 action on different reds of sodium carbonate, etc. The amount of organic pig- 

 ment present in such reds is generally very small, and when it can not be 

 determined by ignition owing to the presence of lead, zinc, or carbonate, it is 

 best determined by difference. 



2. Oxid and Earth Pigments. 



Iron oxid is extensively used as a paint. The native oxid naturally varies 

 very much in its composition. In general, however, only the poorer grades of 

 native hematite are used as paints. Artificial iron oxid pigments, made by 

 calcining copperas, may be practically pure ferric oxid. Umbers, ochers, and 

 siennas are earthy substances containing iron and manganese oxids and more 

 or, less organic matter. 



The methods of analysis are very much the same as those for iron ores. It 

 is generally sufficient to determine moisture, loss on ignition, insoluble in hydro- 

 chloric acid, ferric oxid, and manganese dioxid. If much organic matter is 

 present, roast 2.5 grams in a porcelain dish at a low temperature until all 

 organic matter is destroyed, add 25 cc of hydrochloric acid, cover with a watch 

 glass, and heat on the steam bath for two hours ; then add 10 cc of sulphuric 

 acid (1:1), evaporate, and heat until fumes of sulphur trioxid are evolved and 

 all the hydrochloric acid is driven off. Cool, add 50 cc of water, boil until all 

 of the iron sulphate is dissolved, filter into a 250 cc graduated flask, fill to the 

 mark, mix, and take out a portion of 50 cc for iron determinations. For the 



21694 Bull. 10912 5 



