If someone tells you an aluminium seat post is forever stuck in a steel frame, that person is wrong. Pipe up and say "Lye!"

Lye, also known as caustic soda, is a common name for sodium hydroxide, a chemical compound that people use to use to make soap, tan hides and cure foods. Mixed with water, it dissolves some types of organic material -- drain clearing products contain a lot of sodium hydroxide.

It also dissolves aluminium, while having no adverse effect on steel. That seized post is as good as gone. 

You can watch video of parts of the process here.

My awesome new polo bike

I acquired an early-model Fleet Velo Joust from a friend who had switched to a 700 frame. The seized post wasn't his fault. He bought it, already seized at the right height, from someone else. He's the one who told me about the possible use of lye to get the seat post out. I figured it would be fun to try, and started bugging him to sell the frame to me. 

I got the lye I needed from my soap-making friend Sarah at Flatlanders Soap Co. (Her felted soaps are especially awesome -- go buy some.) I needed about 800g (27 oz) in all. For your own lye, check for local soap-making supply shops. I've also read about success using drain cleaners. I tried a liquid drain cleaner at first, but not much happened. If you can get straight sodium hydroxide, get it. It comes as pellets, powder or flakes.

Hydrogen gas means it's working

You want to fill the seat tube with the lye/water solution, with no leaks. I wrapped inner tube over and around the top of the seat post, and used a pipe clamp to seal it. Then, so I could fill it from the bottom bracket, I set the frame upside down in the top of a steel barrel, just in case it leaked. It did leak a few times. You could probably use a big garbage bucket; lye doesn't seem to have an effect on plastic or rubber.

I mixed the lye (about 200 g or 7 oz) and water (about 500 ml or 1 pint) in a glass jar and stirred it gently with a screwdriver, before transferring it to a plastic squeeze bottle. (nb: Water first, then add the lye.) I used the squeeze bottle and two short lengths of narrow plastic tubing to get the lye into the seat tube via a small hole in the bottom bracket. It took not quite 250 ml (half pint) to fill the seat tube.

After a few minutes I heard bubbling. After a few more, the seat tube started giving off hydrogen infused smoke -- lots of it. Tipping the frame now and then seemed to invigorate the process when it slowed down. If you do this, take care moving the frame around -- the chemical reaction going on is "exothermic" and the frame is going to get almost too hot to hold.

After about 90 minutes, I removed the pipe clamp to drain the frame and apply fresh lye water. A dark and thick sludge came out, a mixture of water, lye and what used to be aluminium.

The whole job took eight applications of lye water, bubbling for about an hour each time, over four sessions. Say nine hours in all with refills. During the final round, I tipped the frame to mix up the lye inside when I head a faint "clunk" noise.

Quickly, I unclamped the stop and gave the frame a shake. Half of an aluminium seat post slid out, slowly, on a bed of sludge. The other half came loose with a nudge from a screwdriver. 

The sludge did make its way into the downtube and top tube, so I gave the frame a thorough rinse with a handheld shower head. A day later, a dusty white residue remained. I wiped it off with a damp cloth and admired my work.

A few cautionary notes:

- "Caustic" means don't get it on you. Lye can cause chemical burns. Maybe not as bad as Fight Club might make you think, but be safe anyway. Wear safety glasses and rubber gloves (elbow-length industrial if you can get them, though the Princess brand I got from a dollar store worked fine.) 

- Do the work outside or in well-ventilated area -- the smoke coming off includes flammable hydrogen gas.

- Always add lye to the water; never add water to the lye. Pouring water on lye can cause the reaction to be "explosive" according to various things I read and heard.

WTF?

It definitely works. How? I don't know. Chemical magic. I took the following from a forum discussion. I can't follow the chemical notion, but as I understand it the lye basically makes aluminum eat itself, forcing it to continually produce aluminium oxide. Here it is:

So that's what happens, I guess. I know that it got hot -- almost too hot to touch while moving the frame at one point. If you're interested in more chemical notation of the process, or a demonstration of aluminium foil dissolving in lye: https://www.youtube.com/watch?v=GESJ2YUWRx8