30 CFR § 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.
