these black dots on magnets annoy my mind

[vitaliikit]

please, can someone explain to me what these dot do on pickup magnets?
I thought they are present only on Fender pickups, but I also meet them on other modern brands.
It starts to annoy me because I don’t understand.
these are not just dot from a pencil .. these are shock dot

[LtKojak]

It starts to annoy me because I don’t understand. ]

Man, you've got issues...

[big_teee]

The dot appears from the photo to be an air bubble.
When the end of the magnet was ground, it exposed the air bubble.
If the black dot is a pit, or indention, then the bubble is the obvious answer.
T

[vitaliikit]

The dot appears from the photo to be an air bubble.
When the end of the magnet was ground, it exposed the air bubble.
If the black dot is a pit, or indention, then the bubble is the obvious answer.
T

bubble ? no . here is a bit more pictures



as if hit spike and got a dent dot

[big_teee]

I still say it is due to the magnet casting process?
Beyond that, just be annoyed!
GL,
T

[Lollar Jason]

20 years ago about 1/3rd of the alnico slugs had voids due to air bubbles or chips around the perimeter where they were cut on each end. Probably around 15 years ago they either started casting them differently or stepped up thier quality control. We rarely get anything more than an occasional pin hole anymore. Those are voids due to air pockets in the photos- no question.

[vitaliikit]

Thank you for idea Jason .
I worked on this idea. but something does not fit.
These points are always bottom side
it is striking that these dots are mainly located in the center (or quite a bit from it) and very very rarely closer to the edges
and they are always of shallow depth, almost the same. and almost always the right shape, like from a core hit
I'm still in thoughts

[eschertron]

I'm still in thoughts

It's the nature of mind to find patterns - often seeing them where there is none. I might see unicorns or spaceships when I look at the clouds. Are they there by design? I can't deny that I see them.

[Mick Bailey]

it is striking that these dots are mainly located in the center (or quite a bit from it) and very very rarely closer to the edges

Castings cool from the outside and this increases the effective surface tension and the bubbles get pushed to the inside of the skin that first forms. That's why voids often only show up when machining and a casting can look perfect initially. This can be an expensive failure on (say) an engine cylinder head.

The magnets are perhaps gravity cast, which worsens the problem with gas inclusions - especially with small parts where the casting cools off very quickly and the gas doesn't have chance to escape upwards in time to clear the body of the casting.

[Chuck H]

And it's important to note that aren't NEVER off center or ALWAYS uniform. Really, how large a control sample for consistency are we dealing with here?

If you examine the grainy nature of most cast metals it's pretty clear to me that these are inclusions from either air bubbles or somehow evaporated impurities. Mick's explanation of why they're typically located near center makes sense too. As to why they're usually located bottom side, well, if you're making pickups and you don't want your product to look like @$$ because of visible inclusions in the finished surfaces wouldn't you choose to put the inclusions on the bottom?

[Lollar Jason]

you don't want your product to look like @$$ because of visible inclusions in the finished surfaces wouldn't you choose to put the inclusions on the bottom?

Tell em like it is

[vitaliikit]

As to why they're usually located bottom side, well, if you're making pickups and you don't want your product to look like @$$ because of visible inclusions in the finished surfaces wouldn't you choose to put the inclusions on the bottom?

I did not mean the location of the points, but I meant that only on one side. if these bubbles are often on the one hand, then why are they not on the other? is it a careful selection of specimens?

[vitaliikit]

20 years ago about 1/3rd of the alnico slugs had voids due to air bubbles or chips around the perimeter where they were cut on each end. Probably around 15 years ago they either started casting them differently or stepped up thier quality control. We rarely get anything more than an occasional pin hole anymore. Those are voids due to air pockets in the photos- no question.

But how to consider modern expensive pickups?

[big_teee]

But how to consider modern expensive pickups?
[ATTACH=CONFIG]56746[/ATTACH]

Have you asked the manufacturer of the pickups that have this birth mark on the bottom?
The guy that made the pickups, would be the one to answer these questions.
It may be something as simple, as a particular run of magnets.
If he bought them by the thousand, or more, it may take a while to work through them.
So I would go back to the pickup builder!
If I built the pickups, I could answer the questions.
T

[WolfeMacleod]

They are 1000% caused by air bubbles in the castings.

If you've ever worked with casting metals before, this becomes very obvious very quickly.