92 



BULLETIN 1036, U. S. DEPARTMENT OF AGEICULTUKE. 



COKE TEST. 



The coke test was proposed by the National Electric Light Asso- 

 ciation (45) for the determination of the amount of admixed tar, 

 and was later adopted by the American Wood Preservers' Associa- 

 tion and the American Society for Testing Materials. It depends 

 upon the amount of coke residue left after all the volatile matter has 

 beer driven off. The coke residue in itself gives only a rough 

 'ipproximation of the amount of tar that has been added, for the coke 

 residues of coal tars may vary anywhere from 26 to 57 per cent, while 

 those of water-gas tars vary from 18 to 28 per cent. As usually 

 applied, this test favors the use of water-gas tar in tar solutions iiL 

 preference to coal tar. If, however, the free-carbon content (matter 

 insoluble in benzine) is subtracted from the coke residue the resulting 

 figure, which may be termed the ''bituminous-coke residue," is 

 fairly constant for both water-gas tar and coal tar, as is shown in 

 Table 33. 



Table 33. — Free-carbon content, coJce residues, and bituminous-cohe residues of various 



tars {49). 



Kind of tar. 



High or medium free-carbon tar. 



Low-carbon coal tar. 



Water-gas tar. 



Tar No. 



Free- 

 carbon 

 content. 



Per cent. 



37.1 



18.6 



16.2 



15.4 



7.6 



. 6.7 



5.4 



5.2 



4.4 



1.3 



1.1 



.1 



Coke resi- 

 due. 



Per cent. 

 SI. 2 

 43.5 

 42.9 

 34.4 

 26.8 

 27.0 

 28.4 

 30.6 

 29.4 

 25.3 

 28.1 

 23.2 

 18.2 



Bitumi- 

 nous-coke: 

 residue. 



Per 



cent. 

 20.1 

 24.9 

 26.7 

 19.0 

 19.2 

 20.3 

 23.0 

 25.4 

 25.0 

 24.0 

 27.0 

 23.1 

 18.2 



Fig. 44.— Bulb used in 

 making coke test. 



The method of conducting the coke test, as 

 recommended by the three societies, is as follows: 



The bulb shall be of hard glass, as shown in figure 44, and 

 shall have the following approximate dimensions: 



Mm. 



Diameter of bulb 15 



Length of vertical neck 10 



Length of horizontal neck 20 



Diameter of orifice 1 



Warm the bulb shghtly to drive off all mositm-e, cool in a 



desiccator, and weigh. Again heat the bulb by placing it 



momentarily in an open Bunsen flame, and place the tubular underneath the surface 



of the oil to be tested, and allow the bulb to cool until sufficient oil is sucked in to 



fill the bulb about two-thirds full. 



Any globules of oil sticking to the inside of the tubular should be drawn into the 

 bulb by shaking or expelled by slightly heating it, and the outer siuface should be 



