Alum and broken taps

[Deleted]

Hi folks,
During the disassembly of the cylinders of my Maisie restoration project I found that a tap had been broken in the block in one of the holes for the valve chest studs.
After reading through some of the old threads on this site I found that some folk had used a solution of Alum to dissolve the broken tap from non ferrous castings.
I have to admit I was a little sceptical about this but thought it worth a try. Guess what? it works!!! .
After boiling the offending block for about 12 hours total, (over several days) in the alum the broken tap has vanished.
I'm dead chuffed about this and thought it worth mentioning as others might not think to look back into the archive threads, there are a lot of useful gems tucked away in there for beginners such as myself.

[Myford Matt]

Nice one - what was the valve chest made from – gun metal?

MM

[lancelot]

Hi Valcook, it's slow and sure...but it works and probably saves you a fortune on new castings, I know...
All the best,
John.

[simonwass]

It does indeed work, I broke a 10ba tap in an almost complete Clayton lorry built up brass regulator body. I left it in a jamjar of alum solution on a tubular heater over night and next morning I could fish out the almost dissolved away remains of the tap. A bigger tap would have taken longer, the 10ba one was disolving maybe 16 hours and the solution kept hot.

[jackrae]

Not quite the same subject - in fact nothing like it - but may be of use to newcomers
The electolytic method of removing rust and paint from steel work, using washing soda and a battery charger also works a treat. details of this can also be found in the depths of the archives
Jack

[the_viffer]

www.bhi.co.uk/hints/rust.htm is a link for electrolytic rust removal.

I have recently had a lot of success by boiling the rusty items in tap water.

[Myford Matt]

So that's why they call it tap water

MM

[the_viffer]

Sheesh. And I thought my puns were bad. Count yourself lucky smiting has gone!

[ron]

Where does one purchase alum nowadays? sounds like a good idea to have some in stock, almost guarantees a tap will never break.
Ron

[stantheman]

Just reading through this thread a question came to mind. What, if any, problems would this process cause to the working faces of a gun-metal / bronze cylinder block. Seems to have a decomposition effect on the steel taps so it must have some effect I would have thought. Would it be safe to use on a cast iron cylinder block etc?
Stan.

[the_viffer]

You can get it from clockmakers' suppliers. Try a google for Walsh or Meadows and Passmore. I wouldn't risk CI with it without a trial first. Apparently leaves gunmetal and brass untouched.

[hat]

I have no doubts about it working, but how does it work - any chemists amongst the group?

[Deleted]

In my state of euphoria I maybe didn't make it clear that the broken tap was in a gunmetal cylinder block.
The Alum itself came from a firm called Fibrecrafts in Guildford.
www.fibrecrafts.com (search for Alum mordant).
The stuff itself was around £5 for a 500g bag, the handling and postage added another £5 so it may be worth several members of a club joining forces to order in one lot rather than as individuals. I used around half the bag, just enough to immerse the offending screwhole and broken tap.
As lancelot points out this was still a lot cheaper and easier than buying and machining a new casting.
The gunmetal (and brass drain cocks) appears to be completely unharmed by the Alum, I didn't pay much attention at chemistry at school but I would hazzard a guess that it works by a chemical reaction with the iron content of the tap. This would make it unsuitable for cast iron or steel. and of course, all steel screws, fittings etc. must be removed from the item being worked on.
Now does anyone have a similar formula for turning lead to gold? ;D

[the_viffer]

hat said:

I have no doubts about it working, but how does it work - any chemists amongst the group?

I have more chemistry degrees than you can shake a stick at and have had a professional interest in chemistry (and law) for more years than I care to be reminded of.

I was more than a little surprised the first time I heard from a reliable source that alum worked.

The best I can suggest is that it is mildly acidic and will therefore slowly attack the steel leaving the GM untouched. There might also be some electrolytic action ie the GM, steel and alum sol'n form a battery with the iron of the steel being converted into iron ions (no more punes eh Myford Matt? ).

I would be most interested if anyone has a proper chemism for what is happening.

[Noddy]

I'm certainly not a chemist, but here goes for part of the answers. True alum is aluminium sulphate (in chrome alum the S in the formula is replaced with Cr, the green colour is about the only noticable difference).

I guess that part of the corrosion of steel is based on ferrous sulphate being soluble in water, but as soon as it comes into contact with oxygen in the air, it precipetates Ferric oxide, and leaves behind sulphuric acid in the solution.

(Think of the ochre in streams that receive water from mines and coal tips, it is the rust left after oxidation of ferrous sulphate)

The sulphuric acid would then be free to attack yet more of the iron.


Boiling rusted parts converts the rust to black magnetic iron oxide, which is none aggressive (in effect you are bluing the part). the change also breaks up the rust allowing it to be removed more easily.

Using hot washing soda (or any other strong alkali, such as caustic soda) in an aluminium or zinc container (actually in any container as long as the aluminium or zinc and steel are touching) gives pretty good de rusting

With copper or silver it also removes sulphide tarnish as hydrogen sulphide.

[ron]

Would alum work with a broken tap in stainless steel?
Ron

[the_viffer]

Noddy,

Save that alum is potassium aluminium sulphate rather than aluminium sulphate I could buy pretty well all you say.

As you know but didn't mention a great way of cleaning silver is to stand it on some aluminium foil and add hot water and washing soda.

Ron

I'd be a little doubtful but if it is otherwise a scrapper give it a go or do a little experiment with some thing not important


oops sorry Noddy you did say it is good for removing tarnish. Sorry