what is an iron clamp?

Answer 1

This may have a specific technical meaning somewhere, but it appears to be simply a noun qualified by an adjective. Clamps of all sorts, such as C-clamps and carpenter clamps are often made of cast iron due to its formability, strength, and stiffness. You may mean an angle iron clamp, though. That would be a specialized clamp for joining angle irons. Angle irons are a kind of structure steel beam made from bent or extruded steel.