204 ASPHALT. 



Trinidad Asphalts. — This deposit, known on the Island of 

 Trinidad as the Pitch Lake, is owned by the British Government, 



• and is mined by companies who pay a royalty to the Island 

 Government on every ton of crude asphalt removed from the 



■ deposit. This royalty amounts annually to more than one-quarter 

 million dollars (£52,000). 



The Pitch Lake is 114 acres in area, and over 150 feet deep 

 near the centre. The actual depth has never been ascertained 

 because the deposit is in constant motion, so the boring drill is 



•eventually broken off, or the hole becomes crooked so that the 

 operation stops. More than 170 million square yards of city pave- 

 ments have been laid with this asphalt since 1870 in the United 

 States alone, not to mention many miles in other parts of the 

 world. 



The asphalt in this deposit contains about 29 per cent, of its 

 weight of water, the removal of which constitutes the refining process. 

 This is done at about 325° F. by means of steam coils, closed coils 

 .to heat it, and open coils to agitate. Asphalt is a poor conductor 

 of heat, so that it is necessary to get all of the asphalt in a molten 

 condition to facilitate the evaporation of the water. For the same 

 reason it is best to get the heat disseminated throughout the mass 

 as quickly as possible. A kettle or still heated by direct fire would 

 be a dangerous procedure as it could not be agitated in the early 

 process of heating, and the material would tend to cake on the 

 bottom. The kettles (called steam stills) are square tanks capable 

 of holding 70 tons, the lower half being funnel shaped. The 

 lower portion is filled with the closed steam coils conveying steam 

 at about 1251bs. pressure, giving a temperature of about 325° F., 

 which naturally is never exceeded. Trinidad asphalt melts at 

 about 180° F., so that this temperature is sufficient to bring the 

 material to a state of fluidity, and when in this state steam at the 

 same temperature and pressure is admitted through the open coils, 

 which are perforated pipes placed at the bottom of the still under 

 each individual heating coil. The effect is to raise the temperature 

 of the portions lying away from the vicinity of the heating coils, 

 and violently agitate the mass, thus assisting in the removal of the 

 water from the crude asphalt. This operation takes about 8 hours. 

 When the water is all removed, the live steam is shut off and the 

 mass becomes quiescent; then the sticks and other refuse rise to 

 the surface and are skimmed off. The heating steam is kept on 

 until the operation is complete, when the hot material is run off 

 into barrels, which, when cool, are ready for shipment. 



When refined, the asphalt contains 56 per cent, of bitumen, 

 the remainder being a very fine clay. It is the presence of this 

 clay in a colloidal state that gives the stability of mixtures made 

 with Trinidad asphalt. The difference in stability exhibited by 

 the method of incorporating a colloidal clay in a liquid asphalt 

 can be demonstrated. In the one case it is incorporated by a 

 method similar to the manner in which it is introduced in Trinidad 

 asphalt, and in the other the wav it is mixed in the paving plant. 

 To illustrate: a sample that still retains the spiral form can be 



