49 CFR §172.325

1. (a)Except as provided in paragraph (b) of this section, a bulk packaging containing an elevated temperature material must be marked on two opposing sides with the word “HOT” in black or white Gothic lettering on a contrasting background. The marking must be displayed on the packaging itself or in black lettering on a plain white square-on-point configuration having the same outside dimensions as a placard. (See § 172.302(b) for size of markings on bulk packagings.)
2. (b)Bulk packagings containing molten aluminum or molten sulfur must be marked “MOLTEN ALUMINUM” or “MOLTEN SULFUR”, respectively, in the same manner as prescribed in paragraph (a) of this section.
3. (c)If the identification number is displayed on a white-square-on-point display configuration, as prescribed in § 172.336(b), the word “HOT” may be displayed in the upper corner of the same white-square-on-point display configuration. The word “HOT” must be in black letters having a height of at least 50 mm (2.0 inches). Except for size, these markings shall be as illustrated for an Elevated temperature material, liquid, n.o.s.: