 Metal marks are dark lines, often accompanied by damage in the glaze, caused by the deposition of metal during the use of metal utensils.The cutlery, or other relatively soft metal, will leave a very thin smear of metal on pottery ware if the glaze is minutely pitted.A glaze may have this defective surface as it leaves the glost kiln, or it may subsequently develop such a surface as a result of inadequate chemical durability.