The first bronze barrel I cast was a 1/2 scale Coehorn mortar in a block investment. It came out with a long flaw that I attributed to gassing although in retrospect, I'm not sure what was the cause.
The first three mountain howitzers were cast in petrobond moulds; I had taken a wire and pushed it into the backs of both cope and drag in a grid pattern to within an inch of the parting line and a half inch of the pattern to allow gas to escape through the mould instead of into the metal. All three seem to be OK; including the one which is fully machined.
Bronze/brass casts at a much higher temperature than aluminum and burns the oil in the sand around the casting. I always toss the burned sand out since it will no longer clump and, if mixed back with the good sand, dilutes the good sand which eventually will not clump properly and must be tossed anyway. By keeping the burned sand out of the mix, it still works normally even though the quantity gets smaller with each pour.
One thing to consider is that the elements with the lower melting points get burned out of the alloy each time it is melted, so don't melt stuff several times before using and use a mixture of old sprues and new metal, not all old already melted stuff only. This is not so much a problem with aluminum as the aluminum is the low melting point metal and a little loss of Al doesn't make much difference, but with copper based alloys, the copper has the high melting point (silicon bronze being an exception) and the zinc (of brass) or the tin (of traditional bronze) oxidizes more quickly. In fact, the usual way of making tin bronze is to melt copper by itself, and then after it is melted, let it cool almost to the solidification point before adding the tin. When the tin is added the alloy melting point goes down and the metal becomes liquid again.
