i have a very old sort of chain link bracelet about one inch in width i think it's 18ct or20 but i haven't worn it for years it was a keepsake from my gran 'we think it's about 100 years old but it was before hallmarks it's a beautiful thing but i can't get it clean i've tried all the norm....
Have you tried oxyclean or Astonish oxy-plus? I found that when I used it to soak tea stains out of a cloth my gold rings came up really clean. I now clean all our gold jewellery with it--- and it is non abrasive
Hallmarks have been around since the 1300s although that does not mean that every piece of gold was hallamarked. I would take it to a reputable jewellers and ask their advice. Also I would get it valued and make sure that it is covered by your home insurance.