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:
This casting shows a very common defect, flash. This is where the mold somehow seperated enough to allow metal between the halves, along the parting line. (See also the trivet for more flash.) You can see the inside circle here is nearly completely filled in with flash. Fixing flash is no problem as it's usually less than 1/8" thick (unless something really bad happened) so can be broken off with a hammer or pliers. A file will take it down to the parting line. Causes include letting the mold dry out; the clay in the sand shrinks resulting in a gap between the halves. In the pictured case, it was left out overnight. Another cause is much more serious: if your sprue is very tall and the casting covers a wide area of the mold face, it's very possible for the mold to actually be forced up by the hydrostatic pressure of the metal. The seriousness of this depends on the density of the metal (aluminum is very light, but be careful with a bronze pour!) and the weight of the mold fighting it. The solution here is very simple: weight down the mold!
This is due to operator error: not aligning the mold correctly. Most flasks have alignment pins to prevent this, but I never installed them on my 6x6 set so I have to guess at it. As you can see here, I got it pretty wrong ;) oh well.
This is an investment casting. Different from sand casting, but defects still happen all the same. In this case, it was either gas (which is unlikely IMHO, since the rest of the casting is fine) or slag (but the area doesn't have the right appearance for slag). Come to think of it, it could be gas from the mold, but that's just a thought. In any case, the area in question is on the right, where it looks rough (the area on the left appears to be a broken section of the mold (analogous to broken sand), which might've contributed to the next listed defect). There are actually a few pinholes which you can see light clear though in the porous area.
Still on the same casting, this is the bottom plate. What I suspect happened is the broken piece of investment (the hole can be seen in the above picture) fell to the bottom of the mold, coming to rest in the corner, forming a notch in the metal as it solidified around it. This is much more of a problem in sand molds, since loose sand can easily be abraded off and let loose in the mold cavity. Makes for a bad finish on (usually) the drag side of the casting.
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.
Another type of inclusion, this struck me first on my rammer. There was a big chunk of it, and today there's a divet out of the shaft section from it. It is easy to recognize due to its sulfury smell (at least in aluminum) and the void or soft spot it leaves. More recently I've cut off a gate which had some sort of void in it, not sure if it was slag or gas. It'll usually look dark. Cause is insufficient de-slagging, i.e. skimming of the melt, leaving slag in it, allowing it to be entrained in the pour and get trapped in the mold.
Gas pockets come from gas dissolving in the melt then coming out (like shaking a can of pop) when it solidifies. This usually manifests itself as a rough surface on areas exposed to air (such as the top of the sprue, riser or ingots) or pockets of varying size in the cross-section of the metal. Gas comes from melting too long or heating too hot, 'stewing' the metal (keeping it molten longer than needed), using an unusually oxidizing or reducing flame in the furnace, getting water (or pretty much anything else that has hydrogen in it, or will burn; painted scrap for instance) in the melt, and the alignment of the Moon with the Earth and Sun. A good idea is to recycle scrap into ingots as a first step since the scrap might be wet, oily or painted and will add gas to the melt. The gas comes out in the ingots, not your casting.
As you can see, the mold didn't fill all the way. This is usually caused by the metal solidifying before it fills the cavity - the metal was too cold. It could also be a restriction: too small a sprue, gate, or not enough venting keeping the metal from going in. The grinder (that's what it is) up above under porosity filled insufficiently, either due to small gates or the aluminum cooling too quick.
More defects to come as I make them! ;)