grease, mixture of lubricant and thickener. It is used to reduce friction between surfaces from which oils would leak away or cause damage by dripping, or where lubrication must be assured for extended periods. Many greases are mixtures of mineral oil and soap. The more common of them contain a calcium-base soap that withstands water but not high temperature, or a sodium-base soap that withstands higher temperatures and adheres well but dissolves in water. Other soaps used in greases have bases of lithium, aluminum, barium, or strontium. Nonsoap thickeners include carbon black, which is unaffected by temperature and is therefore used with extreme low-temperature lubricants; silica gel; and bentonite, a clay developed for universal greases. Solid lubricants are sometimes used for extreme bearing pressures and high temperatures. Synthetic oils are sometimes used for special conditions, generally temperature extremes.