these black dots on magnets annoy my mind

[vitaliikit]

guys .. thanks for the great comments. a storehouse of useful thoughts and valuable answers. informatively

[Sweetfinger]

Interested parties could pull one of the slugs in question, and start grinding one or both faces down and simply see what shows up. If the slugs were cast in a long rod which was sectioned, I'd expect to quickly find a void of some kind behind the "good" face. I don't think you would, though, until you were well into it.
Smart and/or industrious people have figured out how best to quickly and cheaply make whatever magnets needed for a particular use.
Chopping a long rod down trying to find good sections seems like a hassle from a fabrication standpoint. I suspect that the rods are cast close to final length and the defects show primarily on the open end. Some get ground enough that they're smooth and some still have some rough surface or a pit.
Like many technologies, magnet casting has gotten better with more uniform and consistent product which is evident in the super clean mags I see in even the cheapest pickups. Have a look at magnets from 1940s, 50s and 60s pickups and they're quite funky in many cases. No doubt you can buy magnets today made at every level of technological innovation, from pouring alloy right out of the furnace into sand in small numbers, up to inductively heated, vacuum cooled, laser/plasma/water shaped high tolerance pieces made at high capacity.

[ken]

If I remember right, there is a tolerance for casting flaws in Alnico magnets, and a magnet can have a face or side void up to 15% of the actual area of the surface it appears on. Frankly, I'd be much more worried about a rod magnet with a side void than one with a top one. Top ones are visually ugly, but side voids can destroy windings in time.

A magnet with voids is still a viable magnet, but you're right - it doesn't look good. When I get them, I don't use them for sale pickups - I save them for pickup testing where I know I'm going to destroy the pickup anyway.

I have original Gibson bar magnets here that must have been cut from the far outside edges of the cast Alnico block, including one from a P90 that actually tapers in thickness about .002 in one spot and one wide side is about 80% rough casting. It makes a killer nail file.

Ken