The materials usually employed for this purpose are ice, and coarse salt, or freezing salt, the correct proportions being one pound of salt to seven or eight pounds of ice. More salt than this is often added with a view to making the mixture freeze more quickly, which it does for a short time, but the large proportion of salt causes the ice to speedily melt, and the freezing operation comes to a standstill unless the ice is frequently renewed. The ice tub, or outer compartment of the freezing machine, must be filled with alternate layers of crushed ice and salt. A good layer of ice at the bottom of the tub enables the freezing pot to turn more easily and more quickly than if it were placed on the bare wood.