A furnace wall is to consist in series of 7 in. of kaolin firebrick, 6 in.of kaolin insulating brick, and sufficient fireclay brick to reduce the heat loss to 100 Btu/(hr)(ft^2) when the face temperatures are 1500 F and 100 F, Respectively. What thickness of fireclay brick should be used ? If an effective air gap of 1/8 in. can be incorporated between the fireclay and insulating brick when erecting the wall without impairing its structural support, what thickness of insulating brick will be required ?