Do not use iron in fashioning an alter

Rashi:

For iron is created to shorten man’s days, and the altar is created to lengthen man’s days: what shortens may not rightly be lifted up against what lengthens. (Mishna, Middot 3:4)

Rashbam,  Rashi’s grandson, offers a different interpretation. He quotes Isaiah 44:12-13:  

The craftsman in iron, with his tools, works it over charcoal, and fashions it by hammering…He forms it with scraping tools, marking it out with a compass. He gives it a human form….