Cardboard

From PyroGuide

Cardboard or chipboard, as it is frequently called in the U.S., is a thick brown or grey paper-like material produced by pressing or sticking together pulp or waste paper. In fireworks manufacture solid boards are employed (not corrugated ones), serving as a raw material for various kinds of disks (as used e.g. as end-disks in shell manufacture, as perforated disks used to propell cake items etc.), end plugs, etc. and - in case of the lighter grades - e.g. for case liners. Chipboard is bought in large single sheets or in bundles and its thickness (caliper) is usually given in inches (e.g. 1/16").