Because a thin coating of soot acts as a good absorber of heat, and overcomes the non-absorbing quality of the bright surface.
"And the foolish said unto the wise, Give us of your oil, for our lamps are gone out."
205. But why should soot be prevented from accumulating in flakes at the bottom and sides of kettles and saucepans?
Because, although soot is a good absorber of heat, it is a very bad conductor; an accumulation of it, therefore, would cause a waste of fuel, by retarding the effects of heat.
206. Why should the lids and fronts of kettles and saucepans be kept bright?
Because bright metal will not radiate heat; therefore, the heat which is taken up readily through the absorbing and conducting power of the bottom of the vessel, is kept in and economised by the non-radiating property of the bright top and front.