After removing particles larger than the No. 40 sieve size, a specimen of soil about 1/2 cube in size as molded to the consistency of putty.  If too dry, water must be added and if sticky, the specimen should be spread out in a thin layer and allowed to lose some moisture by evaporation.  Then the specimen is rolled out by hand on a smooth surface or between the palms into a thread about 1/8 inch in diameter.  The thread is then folded and re-rolled repeatedly.  During this manipulation the moisture content is gradually reduced and the specimen stiffens, finally loses its plasticity, and crumbles when the plastic limit is reached.  After the thread crumbles, the pieces should be lumped together and a slight kneading action continued until the lump crumbles. The tougher the thread near the plastic limit and the stiffer the lump when it finally crumbles, the more potent is the colloidal clay fraction in the soil.  Weakness of the thread at the plastic limit and quick loss of coherence of the lump below the plastic limit indicate either inorganic clay of low plasticity, or materials such as kaolin-type clays and inorganic clays which occur below the A-line.