Cor-Ten: Why Does It Look Like Rusty Metal?

Ever seen a gorgeous surface that looked like rusty metal?  Well, if it’s weathering steel, often called by a brand name, Cor-Ten, it looks like rusty metal because it IS rusty metal.

This material is really striking, has a great texture, has an interesting color, and is loved by architects.

However, it has some extremely problematic negative aspects… but we might be able to get around some of those.

Check out my latest technical article, published on the website of the Denver Chapter of the Construction Specifications Institute (CSI), “COR-TEN and Other Weathering Steel Alloys in Architectural Applications” .

8 thoughts on “Cor-Ten: Why Does It Look Like Rusty Metal?

  1. Liz:

    Another consideration for weathering steel in addition to those you mentioned in your article, is that the air must be sufficiently polluted in order to trigger the rusting/weathering process.

    About 1970 or so (I don’t know the exact date) a building was constructed at Cornell University in Ithaca NY, with the exterior envelope consisting of weathering steel Vierendeel truss framing and glazed openings. Because of the clean air in Ithace, the steel patina took forever to develop, and in the interim the building has been a combination of rust and white streaks; according to people I have spoken with recently, the building still looks that way after 40 years. I can send you photos I took in 1972, if you like.

    If anyone has more current information or a recent photo, I’d be glad to hear of it.

    • Dave, that’s a good point to bring up. People around here (Colorado and the southwest) often pre-weather the weathering steel, and then install it already rusty, so that it looks right.

      Methods of pre-weathering vary, and I don’t know much about them, but I have heard that in desert climates, people sometimes use acid to “pre-weather” the steel, since the dry atmosphere would never get it done.

      I think that connections, which are always an issue with weathering steel, might be even more of an issue with pre-weathered steel.

  2. That’s interesting, Liz. I’ve specified pre-patinated copper roofing on several projects, but had not heard of pre-weathered Cor-ten before. Actually it’s been years since Cor-ten was used on one of our projects; it seems to me a very 1970’s material.

    I agree with you that the connections for pre-weathered steel could be an issue, especially with rust on the faying surfaces. I’d be interested to hear from a structural engineer about this; Mark Gilligan, are you out there?

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