Mixing in some (20 to 30) percent graphite with the sulfur will make release easier, thus reducing the likelihood of breakage.
An alternative to using Cerosafe or the pounded slug method is to make a sulphur or sulphur/graphite cast. To do this, make up a mixture containing a ratio of approximately two-thirds sulphur to one-third powdered graphite. Sulphur requires a much higher melting temperature than Cerosafe but it has the advantage of neither growing nor shrinking once the cast has set. As noted, a chamber cast can be made using just sulphur but these casts tend to be somewhat hard to remove and they tend to be brittle. The addition of graphite in the casting makes it less brittle and easier to remove once it has hardened.
A container of sulphur can be had cheaply at the garden centre. Clean and degrease the chamber, put a paper plug about one half inch into the rifling, drop a piece of cotton string into the chamber, do not use synthetics, have a wood dowel longer than the barrel and of smaller but as close as possible to the inside diameter on hand. Alternately, you can make a barrel plug out of cork or other stiff material, attach the required cotton string through the plug and push it into the chamber/barrel. Place some sulphur in a small pan with a pour spout, using, preferably an electric hot plate, slowly heat the sulphur until it melts, do not let it catch fire, the smoke is (N) poisonous. Stir in the powdered graphite and pour in enough of the liquid mix to just fill the chamber, wait until the cast is completely hard, this will depend on how large the chamber is, which governs the mass that has to cool down. Carefully push on the cast with the dowel while firmly pulling on the string; it should come out easily once it starts to move.
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As noted, unlike Cerosafe, sulphur cast dimensions of the chamber do not change at all over time, even after many years, so how soon after casting you measure the cast is not important. Sulphur is one of the few elements that do not change dimensions when going from a liquid to a solid state. Molten sulphur can also be used to set machinery anchor bolts in concrete.
The inclusion of graphite allows the casting to release very easily after it cools. While not critical, the best ratio, is somewhere between ten and thirty percent graphite. Use a piece of poly tube through the receiver, pressed firmly against the rear of the chamber to keep from spilling it all over. Stick a funnel in it and pour until you see it in the tube. These castings can be remelted for re-use or they can be marked for identification and kept for future reference.