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What will dissolve a miniature steel screw in brass clock plate

April 3, 2010

I've just come across a broken-off miniature screw in a brass clock plate.
The screw is hardened!
I am happy to say that I am not the guy who twisted it off.

I have been a full-time professional clock restorer for 35 years and have often wondered what can be done to remove the steel shank, short of mechanical extraction (sawing, drilling, etc.).

I have heard of using alum; letting the item soak in a saturated alum/water solution, allowing the steel to dissolve away. However, I have never been successful with this method. Should it be heated?

I have also heard of the use of muriatic acid (easily available at pool stores). I have never performed any method using this chemical.
It is essential that the brass clock plate (or sometimes watch part), which is often highly decorated, not suffer any consequences in the way of staining, etching, dulling, etc.

Sure hope you can suggest something without my having to create a laboratory of hazardous chemicals.
Thank you all for your time.

David Roberts
Certified Master Clockmaker - Reading, Massachusetts, USA

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This is from David. I discovered the answer I was looking for regarding the effectiveness of using alum to remove a steel screw which broke off in a brass clock plate. Hardened or not, the steel will slowly dissolve when.

My alum ingredients is aluminum ammonium sulphate powder.
It is made by a firm called HUMCO and easily obtained in a pharmacy.

Put the brass part submerged in water, making sure there are no other steel parts in the brass. You need only to cover the part with the water. Dump lots of alum onto the part mixing it with the water. You cannot overdo the amount of alum. Create a slurry mix. Warm water may help speed the process, but there is no point in boiling or even simmering the water. It will only crystallize the alum in the water with no apparent benefit.

Using a small paint brush to poke the liquid into the screw hole may help remove air pockets which may prevent the alum from contacting the steel. I have found it helpful to wash the location of the steel to remove dissolved steel at least once a day. Then re-submerge in the same original liquid. It may take 2 or 3 days.

This works miraculously and the only thing that happens to the brass plate is it gets clean and shiny.

David B. Roberts
clock restoration - Reading, Massachusetts, USA
April 18, 2010