Pressure processes are the most permanent method around today in preserving timber life. Pressure processes are those in which the treatment is carried out in closed cylinders with applied pressure or vacuum. These processes have a number of advantages over the non-pressure methods. In most cases, a deeper and more uniform penetration and a higher absorption of preservative is achieved. Another advantage is that the treating conditions can be controlled so that retention and penetration can be varied. These pressure processes can be adapted to large-scale production. The high initial costs for equipment and the energy costs are the biggest disadvantages. These treatment methods are used to protect ties, poles and structural timbers and find use throughout the world today. The various pressure processes that are used today differ in details, but the general method is in all cases the same. The treatment is carried out in cylinders. The timbers are loaded onto special tram cars, so called "buggies" or "bogies", and into the cylinder. These cylinders are then set under pressure often with the addition of higher temperature. As final treatment, a vacuum is frequently used to extract excess preservatives. These cycles can be repeated to achieve better penetration.