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Brownell's wrote:Cerrosafe™ casting alloy is used any time a “positive” copy must be made
of a rifle, shotgun or handgun chamber, neck, throat or bore. Chamber casts
are used to determine the caliber of an unknown or unmarked firearm, and
to verify the marked caliber if it is suspected that the original chamber has
been altered. It is also used to check dimensions and condition of the neck,
throat, and bore to help determine bullet fit and case neck turning requirements.
Cerrosafe shrinks slightly during initial cooling. It then expands to the
chamber’s original size about one hour after cooling to room temperature.
After cooling for about 200 hours, the chamber cast will expand about .0025"
over the actual chamber size. Cerrosafe is completely reusable; the chamber
cast can be remelted and reused after all necessary measurements have
been taken.

Brownell's wrote:HOW TO USE
Cerrosafe has a melting range from 158° to 190° F.
Note: Overheating may cause separation of the components of Cerrosafe alloy.
Melt Cerrosafe in a double boiler or other means of clean, indirect heat.
It should be poured from a clean ladle. If a heavy iron bullet caster’s or
plumber’s ladle is used, a propane torch with a low flame can be used to heat
the ladle from the bottom until the Cerrosafe is completely molten. A bent
tin can may be used as a ladle, with the lip bent to form a pouring spout, and
locking pliers (such as Visegrip brand) used as a handle for the can. If a small
can is used, it should be set on a piece of heavy sheet steel and the heat applied
to the steel from underneath with a propane torch. An industrial hot
air gun can also be used to heat the Cerrosafe in a pouring ladle.