Code of Federal Regulations Title 30. Mineral Resources § 30.57.22219 Seals and stoppings (II–A mines)

(a) Exposed surfaces on the intake side of stoppings constructed of combustible materials, except brattice, shall be coated with at least one inch of construction plaster containing perlite and gypsum;  at least one inch of expanded vermiculite, Portland cement and limestone;  or other coatings with equivalent fire resistance.

(b) Seals shall be of substantial construction. Exposed surfaces on the fresh air side of seals constructed of combustible materials shall be coated with at least one inch of construction plaster containing perlite and gypsum;  at least one inch of expanded vermiculite, Portland cement and limestone;  or other coatings with equivalent fire resistance. Foam-type blocks shall not be used for seals.