49 CFR § 172.325 - Elevated temperature materials.
(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.)
(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.: