Defects, the scourge of the metalcaster. These can be as simple as broken or loose sand, or more complicated like gas bubbles. In any case, it doesn't look good, and it may make the casting useless.
Now, I'm pretty good at casting <g>, so I don't make many mistakes... but here's one that got by me:
For sand molds, the solution is to fastidiously blow out all loose sand, and make sure the sprue and gate areas are strong, since the metal will erode sand and wash it deeper into the mold. For investment molds, I'd have to say, use a good investment mix and handle it carefully to avoid jarring out a piece. I don't have much investment casting experience so I'm no encyclopedia on the subject.
More defects to come as I make them! ;)
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