I am attempting to cast a bronze sword. I am made the bronze using copper grounding wire bought from the hardware and 99.9% pure tin. The bronze is a 90/10 mix. The mold is carved into gray soapstone. I am heating the mold and then planting it into a pit vertically and surrounding the mold with hot coals to keep it heated for an even flow. After casting the sword and leaving it to cool slowly over night, I quench it in a vat of water and it cracked. I tried to let the next one go cold in the mold and when I took it out it was cracked.
Any ideas as to what I could be doing wrong? Am I not getting the metal hot enough or could I be getting it to hot? Should I not keep the mold heated?
Any ideas or suggestions would be appreaciated.
Any ideas as to what I could be doing wrong? Am I not getting the metal hot enough or could I be getting it to hot? Should I not keep the mold heated?
Any ideas or suggestions would be appreaciated.
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Re: cracking bronze
Tue, November 6, 2007 - 9:57 PMWeird. I have cast a couple bronze swords and daggers and never had a problem with the metal cracking. 90/10 tin bronze ought to cast just fine. If the metal is flowing and filling the mold, you are not pouring it too cool. If it was too cool it likely would chill and not fill the mold completely. Perhaps you are cooling it too slowly. Copper alloys if quenched are usually soft. They age harden when cooled slowly. If memory serves though, tin bronzes don't really age harden enough to become brittle and crack. This one really beats me. -
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Re: cracking bronze
Wed, November 7, 2007 - 5:40 AMI am stumped myself. My first couple cast had inclusions and air pockets. I carved in larger vent channels and the pour appears even but the last two have had cracks in the thicker part of the cast. Since it is a stone mold I can not just quench it quickly, I have to wait until the bronze is solid enough to remove from the mold. I figure I only have a couple tries left in the mold. It is showing signs of stress from the heating. I have not had any problems with smaller items in soapstone molds but most of those were cast from brass pipe or old bronze plaques I got from the scrap yard. I wonder if it could be the alloy I made. Can the tin or copper being to pure cause problems? I noticed when reading the metal content of the Celt and Roman bronze they contained some amount of lead and other impurities.
Maybe I will try a clay mold process in the spring. It is getting to cold for casting outside. -
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Re: cracking bronze
Wed, November 7, 2007 - 6:55 AMAdding a small amount of zinc will prevent the copper from oxidizing. However, the tin ought to be sufficient and you are already getting a solid pour. Copper grounding rod is probably pure electrolytic copper. The tin as you stated is pure. The only thing that I can think of is the mold design. The sword may be long enough that as it cools and contracts the two ends are held in place and the sword cracks. The only solution would be to open the mold carefully when still very hot and pry the casting loose from both halves. -
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Re: cracking bronze
Wed, November 7, 2007 - 9:42 AMI never thought of the shrinkage being a problem. That might be it since the crack forms just below where the leaf spreads. The hilt shape would keep the end from moving. I think you are on to something. time for a redesign or a clay mold.
Thanks! -
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Re: cracking bronze
Sun, December 23, 2007 - 10:06 AMI agree with Jim in the thought that shrinkage is likely the issue. If the casting bites the mold at all, as it cools and shrinks, it pulls apart. With sand, if you ram it up too hard, the same thing shows up. You only ram the facing sand up hard, and leave the bulk of the sand only tight enough to provide support. Saw it mostly with large ring shapes, where the casting shrank around the circumference.
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