One of the most influential T'ang ceramic achievements was the creation of underglaze decor. By painting their designs directly onto the clay body or slip coat and then covering them with a clear glaze before firing, potters could protect their designs in a way not previously possible. Potters working near Ch'ang-sha in southern China seem to have been the first to experiment with and perfect underglaze decor. Understandably their early attempts were somewhat limited. The colors used in the underglaze designs seldom varied from the brown and green hues seen in this ewer and bowl. The durability of underglaze decoration, however, became readily appreciated. And, in the fourteenth century, when these techniques began to be used with blue underglaze, the practice spread worldwide.