44 
BULLETIN 949, U. S. DEPARTMENT OF AGRICULTURE. 
26. FIXED CARBON DETERMINATION. 
This determination is made in accordance with the method described for coal in 
the Journal of the American Chemical Society, 1899, volume 21, page 1116. One 
gram of the material is placed in a platinum crucible weighing from 20 to 30 grams 
and having a tightly fitting cover. It is then heated for seven minutes over the 
full flame of a Bunsen burner, as shown in figure 22. The crucible should be sup- 
ported on a platinum triangle with the bottom from 6 to 8 cm. above the top of the 
burner. The flame should be fully 20 cm. high when burning freely, and the deter- 
mination should be made in a place free from drafts. The upper surface of the cover 
should burn clear, but the under surface should remain covered with carbon, ex- 
cepting in the case of some of the more fluid bitumens, when the under surface of the 
cover may be quite clean. 
The crucible is removed to a desiccator and when cool is weighed, after which the 
cover is removed, and the crucible is placed in an inclined position over the Bunsen 
burner and ignited until nothing but ash remains. 
Any carbon deposited on the cover is also burned 
off. The weight of ash remaining is deducted from 
the weight of the residue after the first ignition of 
the sample. This gives the weight of the so-called 
fixed or residual carbon, which is calculated on a 
basis of the total weight of the sample, exclusive of 
mineral matter. If the presence of a carbonate min- 
eral is suspected, the percentage of mineral matter 
may be most accurately obtained by treating the ash 
with a few drops of ammonium carbonate solution, 
drying at 100° C. then heating for a few minutes at 
a dull red heat, cooling and weighing. 
An excellent form of crucible for this test has a 
cover with a flange 4 mm. wide, fitting tightly over 
the outside of the crucible, and weighs complete 
about 25 grams. Owing to sudden expansion in burn- 
ing some of the more fluid bitumens, it is well to 
hold the cover down with the end of the tongs until 
the most volatile products have burned off. 
Some products, particularly those derived from 
Mexican petroleum, show a tendency to suddenly 
expand and foam over the sides of the crucible in 
making this determination, and no method of ob- 
viating this trouble without vitiating the result has 
thus far been forthcoming. Recent experiments in the laboratory of the Bureau of 
Public Roads indicate that the difficulty may be overcome by placing a small piece 
of platinum gauze over the sample and about midway of the crucible. The gauze 
should be. so cut or bent as to touch the sides of the crucible at all points, and is of 
course weighed in place in the crucible before and after ignition. 
Fig. 22. 
-Apparatus for determining 
fixed carbon. 
27. SPECIFIC VISCOSITY DETERMINATION. 
The viscosity of fluid bituminous road materials may be determined at any suitable 
temperature by means of the Engler viscosimeter. This apparatus is shown in figure 
23, and may be described as follows: a, is a brass vessel for holding the material to 
be tested, and may be closed by the cover, b. To the conical bottom of a is fitted 
a conical outflow tube, c, exactly 20 mm. long, with a diameter at the top of 2.9 mm. 
and at the bottom of 2.8 mm. This tube can be closed and opened by the pointed 
hardwood stopper, d. Pointed metal projections are placed on the inside of a at 
