Drilling mud (or mud to the user) is a chemical mixture used while drilling a well for several purposes. The mud is circulated down the drill pipe, out through the drill bit, across the rock face being drilled, then back to the surface carrying debris from the bottom of the well. The mud is then cleaned, tested, and re-used in the well on a continuous basis. Drilling mud does several things: It cools the drill bit; it forms a seal against the drilled rocks to prevent the rocks from caving into the recently drilled hole; it holds the oil or gas in the ground while the rocks are drilled. There are many types of mud. Some are based on using fresh water, salt water, oil, diesel, or synthetic polymers and oils. The mud is engineered and designed to speed the process of drilling yet protect the rock for future production. The oil company hires a mud company to supply the chemicals and a man to measure, test and mix the chemicals as the well is being drilled. There are hundreds of mud companies as this is a very profitable business due to the mark-up in the pricing of the chemicals sold to the oil company. Major suppliers of mud chemicals and mud services are Baroid (owned by Halliburton), Baker Hughes, MI-Swaco, and many others.