Alum to remove broken tap

I have to get some alum to try this, stores aren't open yet.  From what I understand this can only be done on broken taps in NON-FERROUS metal.  I just so happen to have broken off a 2.5 mm tap in silica bronze yesterday.  Wifey will get me some when she goes shopping today.
The process is to saturate alum in tap water and use the solution to soak the part over low heat for several hours.  This turns ferrous metal to a powdery substance?
Any you folks done this?  Would be interested in your feedback?

There's more than one chemical called "Alum" - make sure you get the right one.  I'll try and find it...

Wilf

Edit - OK - you want "Aluminium potassium sulphate".  You don't want "Potassium sulphate", which can also be called Alum.

Yes I have done this to remove a broken tap in bronze. Warmed the solution over a lamp and occasionally had a dig into the broken tap to remove bits. 100% successful.

soaking now, not sure of chemical composition

Quote from: uuuThere's more than one chemical called "Alum" - make sure you get the right one.  I'll try and find it...

Wilf

Edit - OK - you want "Aluminium potassium sulphate".  You don't want "Potassium sulphate", which can also be called Alum.

Thats probably why the one time I tried it, nothing happened!

See this post for an example of a product that did work.
https://www.modelenginemaker.com/index.php?topic=11952.msg283009#msg283009

Had Wifey pick up some at WinCo and it took 2 days but it got it out.  I am one happy camper.

First time I tried Alum, no luck. Next time I kept the mix warm using a light bulb 60 watts I think. Made a box from Styrofoam , placed jar and bulb inside. After first day I could see the tap was starting to crumble. Left it in another day. Then I could use a pic to remove the broken tap.