Interesting structure to WW pot when cooled.

[BRatigan]

I noticed this when my WW melt cooled. Interesting sinkhole and crystal structure. My other harder allows don't do this at all and seem flat with a minimal of shrinkage. Is this a normal look?

[Rickf1985]

Looks normal to me.

[lightman]

It looks normal to me.

[BRatigan]

Much thanks.

[Winger Ed.]

That's just what the alloy does.
Put a fresh ingot somewhere to cool slowly and it'll do the same.

[Recycled bullet]

It looks like a strange plumbus cornbread.

[BRatigan]

Doesn't taste as good though.

[Recycled bullet]

Fooled me with the delectable texture and jalapeno pepper in the center.

[kevin c]

My understanding is that the exposed top of the melt hardens first. As the rest of the metal underneath cools and shrinks, it makes that divot. Slower cooling in a hotter environment avoids this, but it really isn’t a problem in a casting pot. If, though, there’s such a void in an ingot and it gets submerged in the melt, the expansion of the trapped air can bubble. Much worse is any moisture in there, even just a tiny bit of condensation: the 1700+ fold volume expansion from water to steam causes a Mount St. Helens eruption of molten lead which could cover you, your bench, and the floor, ceiling and walls of your shop, so preheat your ingots (and any tools that have lead stuck on them, especially if they are stored where temperature changes can cause condensation) both for safety and casting reasons.

[dondiego]

It will do that to a bullet too unless you leave an ample sprue.