SAMPLING AND TESTING HIGHWAY MATERIALS. 
43 
25. FLOAT TEST FOR CONSISTENCY. 
The float apparatus consists of two parts (see fig. 21), an aluminum float or saucer, 
a, and a conical brass collar, b. The tv/o parts are made separately, so that one float 
may be used with a number of brass collars. 
In making the test the brass collar is placed with the small end down on the brass 
plate, which has been previously amalgamated with mercury by first rubbing it with 
a dilute solution of mercuric chloride or nitrate and then with mercury. A small 
quantity of the material to be tested is heated in the metal spoon until quite fluid, 
with care that it suffers no appreciable loss by volatilization and that it is kept free 
from air bubbles. It is then poured into the collar in a thin stream until slightly 
more than level with the top. The surplus may be removed, after the material has 
cooled to room temperature, by means of a spatula or steel knife which has been slightly 
heated. The collar and plate are then placed in one of the tin cups containing ice 
D 
Fig. 21.— New York Testing Laboratory float apparatus. 
water maintained at 5° C, and left in this bath for at least 15 minutes. Meanwhile 
the other cup is filled about three-fourths full of water and placed on the tripod, and 
the water is heated to any desired temperature at which the test is to be made. This 
temperature should be accurately maintained, and should at no time throughout the 
entire test be allowed to vary more than one-half a degree centigrade from the temper- 
ature selected. After the material to be tested has been kept in the ice water for at 
least 15 minutes, the collar with its contents is removed from the plate and screwed 
into the aluminum float, which is then immediately floated in the warmed bath. As 
the plug of bituminous material becomes warm and fluid, it is gradually forced up- 
ward and out of the collar, until water gains entrance to the saucer and causes it to 
sink. 
The time in seconds between placing the apparatus on the water and when the 
water breaks through the bitumen is determined by means of a stop watch and is taken 
as a measure of the consistency of the material under examination. 
