Soap thickeners form microscopic fibers in lubricating grease. Some soaps crystallize in the form of threads, which are 20 or more times as long as they are thick. (Most soap fibers are microscopic in size, so that the grease appears smooth to the eye). Some greases have a fibrous appearance when fiber bundles are large enough to be seen by the unaided eye. The most common fibrous lubricating greases contain sodium based thickeners, although not all sodium base greases are fibrous. (See also Appearance and Texture).