After the white liquor leaves the char filters, the greatest care must be exercised to keep all the machinery, piping and apparatus scrupulously clean, for if any foreign matter becomes mixed with the liquor or sugar it can only be removed by refiltering or remelting. By means of vacuum, the liquor is drawn from the tanks into the vacuum pans, this being the last operation in which the sugar is handled in a liquid state. From this point on it drops by gravity from floor to floor in a solid or semi-solid form, until it reaches the packing room as a finished product. In a first-class refinery, the vacuum pans, as well as all the piping through which the liquor passes, are made of copper instead of iron and steel, which eliminates the possibility of rust or scale getting into the sugar. Refinery vacuum pans are usually from fourteen to sixteen feet in diameter and from sixteen to seventeen feet high, while in shape they appear almost spherical. The boiling takes from one hour and twenty-five minutes to one hour and forty-five minutes, and about forty-five tons of granulated sugar can be made at each boiling in a fourteen-foot pan. Before the liquor is drawn in, the pan is thoroughly cleansed with hot water and steam. All openings are then closed and the vacuum pump started. The air is quickly exhausted, a valve in the pipe line leading from the receiving tank is opened and the pan is given a charge of liquor. Steam is turned into the coils of the pan and the boiling process begins. Soon sufficient moisture is driven off to cause the sugar to “grain.” Shortly after the grain forms, another charge of liquor is drawn into the pan and the operation is repeated until the pan is full of a thick, white, mushy substance called massecuite, that looks very much like half-formed ice. The vapor driven off in the boiling passes out through a large pipe at the top of the pan and is condensed by being sprayed with cold water. On account of the high vacuum, the liquor boils violently at temperatures ranging from 140 degrees to 195 degrees Fahrenheit; thus the risk of scorching, discoloration or caramelization of the sugar is minimized.