Which will clean 18 karat gold best, vinegar, baking soda, or isopropyl alcohol?

Answer 1

18 carat gold should not be tarnished at all!
It all depends on what the dirt is, whether it's organic in nature (old skin) or not. It will be safe to try all three, I assure you!

Answer 2

Toothpaste and a toothbrush will remove dirt and grime that collects on gold. Vinegar with table salt added works better than plain vinegar. The salt acts as a mild abrasive.Baking soda works like toothpaste but may be more abrasive.