Scratches/marks/lines on ceramic basin

Hi all, I bought a ceramic bathroom basin last year (Sottini Oracle 50cm) which seemed okay at first. However, during the next few months I started to notice what looked like scratches on the surface of the basin. These appeared faint at first but seemed to become darker and more distinct as time went on. I used Cif and other cleaners, in the hope that they were marks that would come off, but these didn't work. Then I was told they could be metal marks, so I bought a Cramer bath rubber, but that didn't make any difference either. Looking closely at the marks, and touching them, they didn't seem to be scratches or gouges in the surface (rubbing across a mark with a fingernail, you can just feel it but can barely tell it's there), although they do look like scratches.

Anyway, to cut a long story short, I got a replacement basin from Sottini, so that was all right. But I'm still curious about the marks. If they were metal marks (e.g. if someone had dropped a pair of nail scissors into the basin), shouldn't the bath rubber have shifted them? And also why did they start off faint and become slowly darker and more obvious as time went on? Also I don't think anything has been thrown or dropped into the basin hard enough to cause scratches in the surface of the ceramic.

So it's a bit of a mystery to me. Has anyone experienced something similar?

Could it have been a crack in the glaze caused by uneven expansion/contraction in the firing of the basin that had allowed water to discolour the ceramic?

Hi TicklyT, I think that's possible, these do seem to be marks that existed from the start but became more and more distinct, so that could well be it. Will keep an eye on the replacement basin, just in case it's a defect with this particular make. Many thanks!
All the best,
Alex

A cleaning powder called "Barkeepers friend" is good for marks. I recommended it to someone recently, he told me on Monday that it has worked, but he got more marks on the sink next time he did the washing up. He discovered it happened when his titanium watch touched the surface.

Hi JohnD and many thanks for the tip; Bar Keeper's Friend looks like an excellent product (I just had a peek at the website and read messages from some very happy customers.) Will definitely look into this.
Cheers,
Alex