19 CFR § 151.52 - Sampling procedures.
(a) Commercial samples taken under Customs supervision. Representative commercial moisture and assay samples shall be taken under Customs supervision for testing by the Customs laboratory. The samples used for the moisture test shall be representative of the shipment at the time the shipment is weighed for Customs purposes. When a shipment is made up of a number of lots a composite sample of the shipment shall be drawn for assay, providing composite sampling is feasible and assays of the individual lots are not required for tariff classification or other Customs purposes. The composite sample shall consist of proportional parts by weight of the prepared sample drawn from the various lots represented and shall be thoroughly mixed.
(b) Commercial samples furnished by importer. When commercial samples cannot be taken under Customs supervision, the importer shall be required to furnish a verified commercial moisture sample and prepared assay sample certified to be representative of the shipment at the time the shipment was weighed for Customs purposes. The samples shall be in appropriate containers, properly labeled, and shall be accompanied by a statement including:
(1) Entry number,
(2) Lots represented,
(3) Kind of ore or material,
(4) Date and place where sampling occurred, and
(5) The name and address of the sampling concern.
(c) Samples taken by Customs. Where no commercial samples have been taken, an authorized CBP official shall take representative samples from different parts of the shipment.