I mentioned in blog 4 that contamination is a topic in itself.  Well, why not follow that train of thought?

Anything and everything that is not the “stuff” being welded is a potential contaminant.  Whether it be oil from machining, paper fibers, soapy water from a wash cycle, a sliver of plastic from your expensive non-marring table top, or even brown mustard from that London broil sandwich you had for lunch, these are going to put something unexpected into your end product.