Vertical black lines appeared in toilet bowl

Recently we noticed several black lines appear in the bowl of one of our toilets. The thickest line runs down the centre rear of the bowl from under the rim down to the waterline, there is another less pronounced line to one side from rim to waterline where there is a slight channel in the surface of the bowl. Not sure if the lines were there before or after the last clean about 5 days ago but the central line is quite pronounced so I think we would have noticed it.

There are 2 other toilets in the house which have some similar faint lines but nothing like this one. They are all the same age more or less, about 8 years, in fact the most stained one is slightly more recent than the others. So far no sign of any staining on basins.

The lines have resisted all attempts at cleaning, scrubbing, bleach and anything else we could think of. It is quite possible the surface may have been damaged/worn/scratched in the past by over zealous cleaning with scourers but that would apply all over all 3 toilets.

No sign of any discolouration in the tap water. We have a water softener if that might be relevant, which appears to be working OK.

What could have caused these lines to appear (suddenly?) and how can we get rid of them?

Many thanks.

 

Dry the bowl with tissue paper then lay one square of paper in the bowl where the lines are. If paper becomes wet then the cistern flush valves could be leaking. At a guess, if braided flexi hoses are used on the cistern fill valve`s then these have black rubber internals which might be breaking down. Are there any signs of blackness in the cistern either in the water or the water level line around the cistern?

 

Have you tried T-cut?

 

Or a Cramer china and bath rubber

 

Have you tried a scale remover like viacal often dirt gets lodged in hard to see scale and cleaners like bleach will not touch it.
https://www.tesco.com/groceries/en-GB/search?query=viakal liquid

 

Toothpaste may work too... but not on your toothbrush!

 

I've used spirit of salts on similar marks. Carefully in small amounts using an old toothbrush.

 

I haven't been able to get at the cistern of the worst toilet because its in a cabinet but the other 2 have no sign of black in them.

On closer inspection there are vertical black streaks all around the toilet bowl and a black mark around the waterline. Not sure if they were there before and I didn't notice or if it is getting worse.

Apart from trying to clean the marks off, any ideas what could have caused the marks to appear so suddenly in the first place?

 

Might be something in the cisten rusting like the tops of the pan fixing bolts.

 

Did you try post #2?

 

I haven't tried the test with paper yet, and I can't get at the cistern to see inside it. However in the other 2 toilets, which are not as bad but still showing some black marks in the toilet bowl, there is no sign of any black discoloration in the cistern.

If the black colour is caused by something rusting or breaking down in the cistern it seems strange it would affect all 3 toilets at the same time, and have such a sudden effect. My best guess is a reaction between something in the toilet bowl and the bleach used to clean it (Harpic white & shine, contains sodium hypochlorite & sodium hydroxide) since the only place the black marks have appeared is in the toilet bowl, but we have used that cleaner for some time and not had any issues before.

Is there anyone with a degree in chemistry?

 

Could also be a bad glaze on the surface

 

Try a limescale remover.
Killrock comes in a gel form which can be painted on.
Use a fingernail to detect if the surface is smooth or rough... Rough suggests a deposit. But don't bite your nails after.

 

The surface feels smooth so I don't think there is much of a deposit.

If it was limescale, or a bad glaze, or something in the system rusting or breaking down, then I would expect it to have a gradual effect. But the black marks appeared quite suddenly, in 3 different toilets to a greater or lesser degree, but not in any basins. I would have to say the area of the toilet bowl below the water line always seems to look a bit discoloured in all the toilets even after cleaning and that I think is getting worse over time. I could believe that might be limescale buildup (although with a water softener we should not have limescale) or maybe rust coming from somewhere. But these new marks that have appeared are a different colour, and have made a permanent stain around the bowl just from the water passing over it and not even staying in contact.

 

Are the bowls all the same manufacturer?