The char filters are cast-iron cylinders, usually ten feet in diameter and twenty feet high, with doors at the top for entrance of the char and openings at the bottom through which it is removed. There are also many pipe connections for the introduction and outlet of liquors, steam, hot water and compressed air. The filters are insulated on the outside with asbestos or some other non-conductor of heat to prevent the temperature of the liquor from being lowered as it passes through. Each filter has a capacity of from sixty thousand to eighty thousand pounds of bone-char. At the bottom of the filter is a perforated iron plate. Over this is placed a coarsely woven cotton blanket, through which the liquor will pass, but which prevents the char from escaping from the filter with the liquor or wash water. After the blanket is set in place, the char is delivered by gravity through an overhead pipe into the filter, until it is entirely full. The char, as it goes in, has a temperature of from 170 to 180 degrees Fahrenheit, and the bag-filtered liquor which is then run on has a slightly higher temperature. [Illustration: CHAR FILTERS] [Illustration: CHAR FILTERS—SHOWING OUTLET PIPES] When the liquor in the filter reaches the top and the char has settled in a compact mass, the cover is put on and fastened securely to prevent leakage. The liquor is again allowed to run into the filter by gravity, from the tanks about fifteen feet overhead. The valve on the bottom of the filter is then opened and the liquor, as it filters slowly through the char, is led through a copper pipe to the liquor gallery, to which station all the char-filtered liquor is delivered. This pipe, instead of leading downward from the filter, leads upward and nearly to the top, so that the flow of liquor through the char will be slow and uniform and the filter will always remain full of liquor. The diameter of the filter is ten feet, while that of the outlet pipe is two inches, so that the flow of liquor through the char is necessarily very slow. The reason for this is that the liquor must remain in contact with char a certain time to enable the char to absorb the coloring matter and soluble salts. The first liquor from the filter appears cloudy and is sent back for refiltration, but it soon becomes bright, perfectly colorless and transparent as plate glass. This white liquor is pumped from the liquor gallery into the tanks on the top floor in the pan house, ready for the next process, which will be dealt with presently.