Metal composting "bins" - Good, Bad or Indifferent?

One of my students asked a question the other day - would it be worth making compost pens from corrugated iron, on the grounds that they already had the metal, and it would save having to buy new components. 

What an interesting question: my instant reaction was to say no: they are better made out of wood. Or even - dare I say it - plastic. I'm not at all a fan of plastic compost pens, and if you would like to know why, just type "compost" into the search box up there at the top left of the page:

There, can you see it? Super-useful: so if there's a particular topic which interests you, type it in there, and see what you get - I would almost put money, that I have written something about it...
 
And if by chance I haven't - then drop me an email, the address is in the right-hand pane, just below the big blue advert for my book.

So - no, I'm not a fan of plastic bins, but I would prefer them -  just - to a metal bin.

But why?

Why this automatic rejection of a metal compost pen?

My first thought was obviously, rust-pain-blood-tetanus injection. I catch myself often enough on wooden bins: the thought of damaging myself on metal ones is almost not to be considered.

But maybe that's just me being a sissy, so I thought about it a bit more; this particular student is always very interested in the "why", in the reasons behind my observations and opinions, so it was worth me sorting out my ideas.

Another obvious thought is that metal is a better conductor than wood, so the metal compost pens would be colder in winter and hotter in summer. (I feel this is sub-obtimal).

But you can buy galvanised metal pens: round, square, solid, or slatted like this one: 

...which suggests that they are a well-accepted thing: the comments seem to suggest that most people buy a metal one in preference to a plastic one, on the (somewhat dubious) grounds that metal is more eco than plastic.

Hideous, though, isn't it? At least wood, no matter how old, has an intrinsic softness to it. And no matter how tatty a wooden pen is, you can always slap a coat of paint on it.

And personally, I'd say I'd be extremely likely to scrape myself on all those metal edges... but they'd certainly be quick to assemble, which is a big plus.
 
Rat proof, though? Hardly, with all those slats. But anyway, in my experience, rats dig their way in from underneath, and a compost pen must sit on the soil ("should" sit on the soil), so you are never going to keep rats out, if they are determined to get in.

So what would I advise? I would say "go with wood" if for no other reason that this one: corrugated metal flexes, so you'd probably have to build a wooden frame anyway, to keep it in shape.

In which case - and you just know what I'm about to say, don't you? - why not build it all out of wood!

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