The walls of a furnace are made of 1.2-ft-thick concrete (k = 0.64 Btu/h·ft·°F and α = 0.023 ft²/h). Initially, the furnace and the surrounding air are in thermal equilibrium at 70°F. The furnace is then fired, and the inner surfaces of the furnace are subjected to hot gases at 1800°F with a very large heat transfer coefficient. Determine how long it will take for the temperature of the outer surface of the furnace walls to rise to 70.1°F.