How do you clean a bath tub from the early 1900's are they porcelain?

Borax and water to make a paste. Rub on with a damp sponge to the damp porcelain . Let set until dry then rinse. You may need to reapply if really dirty or stained.

Yes, its porcelain. Nothing works easier, nor better than Bar Keepers Friend.

Found next to Comet in groc stores. Scrub just like you would using comet. It won't hurt your tub, but will make it like NEW again! Removes even old rust stains & scratches as well

Oven cleaner

Bar keepers friend and a magic eraser

White vinegar and baking soda. It's the only way I can truly say you won't scratch the tub.

White vinegar and sea salt works well at removing stains, hard water deposits and discoloration. If it is really bad, you may have to do it more than once.