"Professional potter here. The dark marks are caused by your spoon rubbing against the glaze. It’s traces of metal from the spoon. It’s not harmful to you, but you can remove it with scouring powder products. Fun fact, the material that makes glazes white is Zirconium, same thing used to make fake diamonds. It is an incredibly hard material, which is why it will scrape the metal off of your cutlery."

"Try denture cleaning tablets. They’re cheap and amazing at removing tea tanning stains without scrubbing, so may work on your metal deposits."