56 METALLURGY OF CAST IRON.
tfoe " down-comers." There is much study being" given in hopes to devise methods to overcome these difficulties. To help matters, a few have taken out their old bells and replaced them with smaller ones, and they report a very commendable improvement in preventing slips when using Messabi ores.
The reason for stock scaffolding in a furnace is often found in the irregularity of the lining. The constant friction of the stock in working downward cuts cavities into the lining, often forming regular shelves upon which the stock can easily hang up. The longer a furnace runs, the more favorable conditions become to scaffolding, and when it is stated that ore is a substance which becomes gummy and swollen before it is reduced to a fluid state, one can readily perceive why such trouble may be expected in a furnace, causing an irregularity in the product, and at times disarranging all calculations of the furnaceman by producing an tmdcsired character of iron. When furnacemen experience trouble with scaffolding, etc., not due to a hot furnace, as described in Chapter X., page 75, they often resort to the use of more fuel than when all is working well. The additional percentage of fuel causes a greater heat, making the stock more plastic, and causing it to give way more easily from the walls of a furnace. It .generally takes from five to ten hours for stock to work down from the top to be tapped out as iron.
A slip In a furnace often means the falling of from twenty-five to two hundred tons of stock from a height of one to fifteen, feet. The contemplation of this taking place within a furnace filled with combustible gases, heated stock, and liquid metal should enable can counteract the absorbing power ofee reactiony they are often composed of about equal parts of silica and alumina. Bricks should contain silica or alumina in proportion to the amount of heat or friction they are if j