92 
BULLETIN 1036, IT. S. DEPARTMENT OF AGRICULTURE. 
COKE TEST. 
The coke test was proposed by the National Electric Light Asso- 
ciation (48) 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 
been driven off. The coke residue in itself gives only a rough 
approximation 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 IS to 28 per cent. As usually 
applied, this test favors the use of water-gas tar in tar solutions in 
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 u : 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, cole residues, and bitummous-coJ:e residues of various 
tars (49). 
Kind of tar. 
Tar No. 
Free- 
carbon 
Coke resi- 
due. 
content. 
Per cent. 
Per cent. 
1 
37.1 
57.2 
2 
18.6 
43.5 
3 
16.2 
42.9 
4 
15.4 
34.4 

7.6 
26.8 
6 
6.7 
27.0 
/ 
5.4 
28.4 
8 
5.2 
30.6 
9 
4.4 
29.4 
10 
1.3 
25.3 
11 
1.1 
28.1 
12 
.1 
23.2 
13 
18.2 
Bitumi- 
nous-coke 
residue. 
High or medium free-carbon tar. 
Low-carbon coal tar. 
Water-gas tar. 
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 
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 folio-wing approximate dimensions: 
Mm. 
Diameter of bulb 15 
Length of vertical neck 10 
Length of horizontal neck 20 
Diameter of orifice 1 
Fig. 44.— Bulb used in 
making coke test. 
Warm the bulb slightly to drive off all mositure, 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 surface should be 
