MIXED PAINTS. 39 



quantitative determinat ion of the blue, but its presenee ean lie detected by 

 means of hydrochloric acid, and the combined peiventa-e of ultramarine and 

 insoluble impurities thus be determined. 



(2) DETERMINATION OF YELLOW. 



Treat 1 gram of the dry powder with 25 cc strong ammonium hydroxid, stir 

 well, and dilute to about 100 cc with water. Stir frequently for al.oni ihiny 

 minutes, carefully breaking up all lumps, but do not heat. Filter, wash with 

 dilute ammonium hydroxid, and then with water; dry the residue at 105 C. 

 to constant weight, and weigh. The loss in weight gives the amount of zinc 

 yellow if no other soluble compounds are present. The filtrate may be tested 

 as in the examination of zinc yellow. 



(3) ULTBAMABINE AND INSOLUBLE IMPURITIES. 



The insoluble material remaining from the determination of yellow (2) may 

 contain both ultramarine and other insoluble substances, but its color strength 

 can be determined as compared with a standard pure ultramarine, and the 

 result calculated to percentage based on a standard ultramarine. 



MIXED PAINTS. 



Single pigments are generally sold either dry or ground in oil alone; rarely 

 do they contain thimiers or driers. Ready-mixed paints, however, contain, in 

 addition to oil, turpentine or gasoline and usually some drier. More compli- 

 cated yet are enamel paints, which are pigments ground in oil varnish, and con- 

 tain, beside the substances in the vehicle of mixed paints, varnish gums. 



The can of mixed paint should not be shaken, but should be weighed as a 

 whole, opened, and as much of the clear vehicle removed as possible ; this should 

 be weighed and kept in a stoppered flask (use cork, not rubber, stopper). After 

 removing the clear vehicle mix the contents of the can thoroughly and transfer 

 to another jar. Wipe and weigh the can. These figures will give the neces- 

 sary data for calculating the analytical results back to the original material. 



1. Extraction of the Vehicle. 



Weigh from 15 to 35 grams of the thoroughly mixed pigment in a tall, nar- 

 row Erlenmeyer flask of 300 cc capacity. Add about 10 cc of gasoline, mix 

 thoroughly, then add 150 cc of gasoline, mix well, and stopper with a cork; 

 allow to stand until the pigment has settled, and decant the liquid into a beaker; 

 some pigment will frequently go over with the gasoline, so it is well to allow 

 this to settle and decant into a second beaker. Repeat the treatment with gaso- 

 line; after the first treatment settling is generally much more rapid. Decant 

 the gasoline as completely as possible the second time. Then add 150 cc of 

 benzol (C 6 H a ), shake, and allow to settle. Decant the benzol and treat in same 

 way with 150 cc of ether. (If a large number of samples are examined at the 

 same time this method is as rapid as any ; but when working with a small num- 

 ber of samples much time is saved by extracting in tubes and throwing down 

 the pigment by whirling in a centrifuge.) This method of extraction is often 

 more satisfactory than any method using a continuous extraction apparatus, 

 for frequently the pigments can not be held by extraction thimbles. This 

 method of treatment will generally give an almost complete separation of the 

 vehicle, but in some enamel paints it is well to follow the gasoline treatment 

 by a treatment with turpentine, and then remove the turpentine with gasoline 



