In the 14th century AD it was found that the addition of salt to the kiln gases during the firing of stoneware meant that the salt volatilized and the resultant sodium chloride vapor fluxed with the silicas in the body of the vessels to form a soda-glass glaze. As a further refinement, a brown-colored surface could be achieved by coating the vessels in a thin iron wash before firing. A patent was granted for the manufacture of such salt-glazed wares in England in 1671.