Pictures are worth a Billion words!! First link below (open them in a new window if you want, it is easier). That weld has the right Caterpillar look, but on the round pipe side is the dark smoke look - to ME (and without brushing the weld) it looks "Cold" on that side. That "Caterpillar" look is what you are looking for in Any Arc weld - and Mig's as well unless you do a 'one pass weld' they are NOT strong. A "One Pass" weld is where you just lay the rod down where you want the weld to go, and walk away - I have seen it done, the weld works for light stress but it is not strong and won't pass x-ray tests. The second link has a photo album of welds, all dam good welds IMO / they are brushed so you can see them better. The "Slug" look your talking about 'sounds' like a ground weld or a "One Pass Weld"- all welds will look basically the same (even computer welds - because the physics are the same, the human element is just removed) To "Weld" you do 3 things, heat part 1 to the point of melting, but not running, heat part 2 to the point of melting but not running, insert a higher heat (stronger) metal to the two and cause them to flow together - it is an art - when you are pushing that metal into the weld you normally move it in very small circles - that gives you the stacked dimes or what we always called a Caterpillar look