31 U.S.C. § 5121

(a) The Secretary of the Treasury shall— (1) melt and refine bullion; (2) as required, assay coins, metal, and bullion; (3) cast gold and silver bullion deposits into bars; and (4) cast alloys into bars for minting coins. (b) A person owning gold or silver bullion may deposit the bullion with the Secretary to be cast into fine, standard fineness, or unrefined bars weighing at least 5 troy ounces. When practicable, the Secretary shall weigh the bullion in front of the depositor. The Secretary shall give the depositor a receipt for the bullion stating the description and weight of the bullion. When the Secretary has to melt the bullion or remove base metals before the value of the bullion can be determined, the weight is the weight after the melting or removal of the metals. The Secretary may refuse a deposit of gold bullion if the deposit is less than $100 in value or the bullion is so base that it is unsuitable for the operations of the Bureau of the Mint. (c) When the gold and silver are combined in bullion that is deposited and either the gold or silver is so little that it cannot be separated economically, the Secretary may not pay the depositor for the gold or silver that cannot be separated. (d) (1) Under conditions prescribed by the Secretary, a person may exchange unrefined bullion for fine bars when— (A) gold and silver are combined in the bullion in proportions that cannot be economically refined; or (B) necessary supplies of acids cannot be procured at reasonable rates. (2) The charge for refining in an exchange under this subsection may be not more than the charge imposed in an exchange of unrefined bullion for refined bullion. (e) The Secretary shall prepare bars for payment of deposits. The Secretary shall stamp each bar with a designation of the weight and fineness of the bar and a symbol the Secretary considers suitable to prevent fraudulent imitation of the bar.