Outer linings are fitted in crucible furnaces with wall thicknesses of over approx. 150 mm. As a rule, these consist of casting compounds with a high clay content, which are cast with the aid of a form which is conical toward the top. The conicity is approx. 0.8%, as for crucible removal devices. For technical casting reasons, the thickness at the upper edge should not exceed 25 mm.
For a 30-t furnace, the thickness at the top is 30 mm, and therefore 46 mm at the bottom. These compounds are prepared with approx. 6% water. Although larger quantities of water produce better casting characteristics, this has the disadvantage of greater porosity, and consequently significantly lower strength. The water also has to be removed by means of various, laborious heating methods. Drying can be carried out with gas- or oil-fired burners, or inductively. The maximum temperature at the lower edge of the outer lining should not exceed 400 °C, in order to prevent any damage to the water-cooled induction furnace coil. Otherwise the drying process should be carried out in the same way as for casting compounds. An outer lining that has been dried
slowly will not tend to form cracks as quickly as an outer lining that has been dried too rapidly.