IN DARLINGTON there is a famous new sculpture of a steam engine - built with a million bricks. I suppose if it was made out of steel then it would just look like ... well, a big steam engine. But because it's made of bricks, that makes it a sculpture.

Now, don't get me wrong; it's well worth a look if you can find it, on a bit of waste ground between the supermarket and the ring road. But it's funny that someone should use bricks to make a steam engine; just as funny as the fact that lots of architects think that steel is a good material for the outsides of buildings.

The first serious steel-clad buildings were put up by a French architect called Jean Prouve in the Thirties. One of the main differences between brickwork and steel is that one absorbs water and the other doesn't. So rainwater runs down the face of a steel-clad building, growing in volume the further down it gets, just like a river fed by its tributaries.

This constant flow creates all sorts of problems with staining and erosion, and Prouve soon realised that the only way to cope with it was to adopt the technology of the motor car, with little steel gutters above the windows, and drainage grooves between the panels; the glass was sealed in with rubber gaskets. So his early designs look more like trucks than buildings, and, despite his best efforts, there are lots of grimy stains where the water has run down - just like my car when I haven't washed it for six months.

The other difficulty of making buildings out of steel is corrosion. If you buy a new car, you'll be pleased if the manufacturer offers you a six-year warranty against body corrosion; obviously six years of rust- free motoring is quite an achievement in the British climate. But for a new building, surely you'd expect something a bit longer? House deals do not generally include a life-expectancy clause, but you'd be entitled to think that your new home would still be standing for a few years after you'd paid off the mortgage. And the same should go for factories and offices. Forty years would be a not unreasonable time to expect before any major repairs were needed, and who knows, with a bit of tender loving care and attention, the place should still be serviceable at the turn of the next century. After all, we are supposed to be living in a time of environmental awareness, when sustainability is the name of the game. It should no longer be thought clever to spend a fortune on something that is only going to last five minutes.

But now we hear that the looming and majestic Lloyd's Building in London, our most famous example of steel-cladding, completed only in 1986, is costing millions of pounds annually to maintain. The architects and their insurers have just reached a deal with the owners for compensation for the corrosion and staining that are disfiguring this landmark building.

So perhaps it is time to accept that steel is the best material for making cars, and masonry is the best material for buildings, and if we stick to that then we won't go too far wrong. As for sculptures - now that's a different matter.