15 U.S.C. § 296
In the case of articles of merchandise made in whole or in part of silver or any of its alloys so imported into or exported from the United States, or so deposited in the United States mails for transmission, or so delivered for transportation to any common carrier, or so transported or caused to be transported as specified in section 294 of this title, the actual fineness of the silver or alloy thereof of which such article is wholly or partly composed shall not be less by more than four one-thousandth parts than the actual fineness indicated by any mark (other than the word "sterling" or the word "coin") stamped, branded, engraved, or printed upon any part of such article, or upon any tag, card, or label attached thereto, or upon any box, package, cover, or wrapper in which such article is incased or inclosed; and that no such article or tag, card, or label attached thereto, or box, package, cover, or wrapper in which such article is incased or inclosed shall be marked, stamped, branded, engraved, or printed with the word "sterling" or "sterling silver" or any colorable imitation thereof, unless such article or parts thereof purporting to be silver contains nine hundred and twenty-five one-thousandth parts pure silver; and that no such article, tag, card, label, box, package, cover, or wrapper shall be marked, stamped, branded, engraved, or printed with the words "coin" or "coin silver" or colorable imitation thereof unless such article or parts thereof purporting to be silver contains nine hundred one-thousandth parts pure silver: Provided, That in the case of all such articles whose fineness is indicated by the word "sterling" or the word "coin" there shall be allowed a divergence in the fineness of four one-thousandth parts from the foregoing standards: Provided, That in any test for the ascertainment of the fineness of any such article mentioned in this section according to the foregoing standards the part of the article taken for the test, analysis, or assay shall be such part or portion as does not contain or have attached thereto any solder or alloy of inferior fineness used for brazing or uniting the parts of such article: Provided further, That in the case of any article mentioned in this section, in addition to the foregoing tests and standards, the actual fineness of the entire quantity of silver or of its alloys contained in such article, including all solder and alloy of inferior fineness used for brazing or uniting the parts of such article (all such silver, alloys, and solder being assayed as one piece), shall not be less by more than ten one-thousandth parts than the fineness indicated by the mark stamped, branded, engraved, or imprinted upon such article, or upon any tag, card, or label attached thereto, or upon any box, package, cover, or wrapper in which such article is incased or inclosed, it being intended that the standards of fineness and the tests or methods for ascertaining the same provided in this section for articles mentioned therein shall be concurrent and not alternative.
15 U.S.C. § 296