68 BULLETIN 949, U. S. DEPARTMENT OF AGRICULTURE. 



50. DETERMINATION OF PERCENTAGE OF PIGMENT IN PAINTS. 



The per cent of pigment shall be determined by extracting a weighed amount of 

 from 10 to 50 grams of the paint, or from 5 to 30 grams of the paste, several times 

 with light paraffin naphtha (preferably 86° B. ), using approximately 50 c. c. of fresh 

 naphtha for each extraction, and repeating the operation until the liquid of extrac- 

 tion remains clear and colorless. The amount taken for analysis may be varied 

 according to the nature of the material. The extraction and washing of the pigment 

 shall be completed by the use of two or three portions of about 50 c. c. of ether, the 

 pigment finally dried to constant weight at 105° C, and the per cent of pigment by 

 weight calculated. One hundred less the per cent of pigment found shall be con- 

 sidered as the per cent of vehicle. 



51. TESTS FOR THE AMOUNT OF SPELTER COATING ON CULVERT 



METAL. 



The amount of spelter coating shall be determined by one of the following methods: 



(a) Lead acetate method. — The solution used for making this test is prepared by 

 dissolving 400 grams of crystallized lead acetate in 1 liter of water. When dissolved, 

 add 4 grams of finely powdered litharge and agitate until most of it has dissolved. 

 The solution is allowed to settle and the clear portion decanted for use. 



Ordinary glass tumblers have been found very satisfactory to use in making this 

 test, as they are the right diameter to enable the sample to be maintained in an up- 

 right position without supports. 



Use several 2|-inch by 2|-inch pieces cut accurately to -^ inch and weighed to three 

 decimal places. Weigh and submerge separately, for 3 minutes, in tumblers contain- 

 ing solution of lead acetate. The samples are then taken out and the adherent lead, 

 removed with a stiff brush or steel spatula. A burnishing action should be avoided, 

 as under some conditions closely adherent lead will be plated out on the iron. Repeat 

 the 3-minute immersions in the lead acetate solution until a bright surface is exposed. 

 Four 3-minute immersions are usually sufficient. Wash specimens in water, dry, 

 and weigh. The loss in grams will also be the loss in ounces per square foot. 



(b) Antimony hydrochloric acid method. 12 — Use several 2J-inch by 2^-inch pieces, 

 weighed to three decimal places. They are then immersed separately for one-half 

 minute in hydrochloric acid of specific gravity 1.20 to which has been added 5 c. c 

 of antimony chloride solution prepared by dissolving 20 grams of antimony trioxide 

 in 1,000 c. c. of hydrochloric acid of specific gravity 1.20. The pieces are scrubbed 

 with a brush under running water, dried, and weighed again. About 100 c. c. of the 

 hydrochloric acid will usually be sufficient for immersing the test pieces if a 200 c. c. 

 beaker is used. The same portion of hydrochloric acid may be used for at least 5 

 test pieces. Five cubic centimeters of the antimony chloride solution, however, 

 should be added for each sample on account of the antimony being removed from the 

 solution by the iron. The test pieces being exactly 2\ inches by 2\ inches, the loss 

 in grams will also be the loss in ounces per square foot. 



i 2 This method is discussed at length in the Proceedings of A. T. M. 1915, p. 120. 



