Grease is made up of base oil, a gelling agent or soap thickener (sometimes called filler) and additives, which perform in much the same way as oil additives. The consistency of grease is controlled by the type and ratio of the gelling agent to the oil and its viscosity. Grease can harden or soften in service due to the effects of contamination, loss of oil or mechanical shearing.
The classic way in which the consistency of a grease is measured is using the cone penetration test (ASTM D217). In this test, the grease is heated to 25°C (77°F) and placed below the tip of the test cone, as shown in Figure 1.