Learn Portuguese (Portugal): A porta velha é de ferro.

We use ser because being made of iron is a permanent, unchanging characteristic of the door. In Portuguese, materials are always described using ser.

Why is it de ferro instead of just an adjective for iron?

In Portuguese, you rarely use adjectives to describe what something is made of. Instead, you use the preposition de followed by the noun for the material, like de ferro (made of iron), de madeira (made of wood), or de vidro (made of glass).

Could I use antiga instead of velha to say old door?

Yes, but it changes the meaning slightly. Velha usually means the door is physically aged, worn out, or dilapidated. If you said porta antiga, it would sound more like an antique, historic, or previous door.

What happens if I put the adjective before the noun, like velha porta?

Placing velha before the noun makes it more subjective or emotional rather than a literal physical description. A velha porta might imply that same old familiar door we always use, while a porta velha just states the factual, worn condition of the door.