30 CFR § 18.27
A gasket(s) shall not be used between any two surfaces forming a
flame-arresting path except as follows:
(a) A gasket of lead, elastomer, or equivalent will be acceptable
provided the gasket does not interfere with an acceptable metal-to-metal
(b) A lead gasket(s) or equivalent will be acceptable between glass
and a hard metal to form all or a part of a flame-arresting path.