From and after the passage of this act, each and every ton of anthracite coal, of the weight of two thousand two hundred and forty (2,240) pounds avoirdupois, mined, washed, screened, or otherwise prepared for market in this Commonwealth, shall be made subject to a tax of one and one-half per centum (1 ½ ), to the thirty-first day of May, one thousand nine hundred twenty-nine, one per centum (1) for the period beginning on the first day of June, one thousand nine hundred twenty-nine, and ending on the thirty-first day of May, one thousand nine hundred thirty, and one-half ( ½ ) of one per centum for the period beginning on the first day of June, one thousand nine hundred thirty, and ending on the thirty-first day of May, one thousand nine hundred thirty-one, of the value thereof when prepared for market, which said tax shall be assessed at the time when said coal has been mined, washed or screened, and is ready for shipment or market. After the thirty-first day of May, one thousand nine hundred thirty-one, no tax shall be assessed on anthracite coal under the provisions of this act, saving to the Commonwealth, however, the right to collect thereafter all taxes imposed or due and owing under the provisions of this act.