Originally Posted by
mackie
Actually porousity is not caused by rust or pitting. When the metal is heated and poured into bars or sheets, sometimes they get the metal too hot and it is like when oatmeal is cooking. little bubbles come to the surface and then pop and leave craters. I used to be a jeweler and I saw this a lot in gold soldering joints. If the person were not experienced, they would heat the solder for too long and get it too hot and that would happen. In a casting it is sometimes microscopic. I do not know that much about it but if there are any blacksmiths here perhaps they could explain it better.