In any nonresidential zone, no smoke, the shade or appearance of which is darker than No. 1 on the Ringelmann Smoke Chart, shall be emitted into the open air from any fuel burning equipment; provided, however, that smoke emitted during the clearing of a fire box or the building of a new fire, the shade or appearance of which is not darker than No. 2 on the Ringelmann Smoke Chart, may be permitted for a period or periods aggregating no more than three minutes in any 15 consecutive minutes.