[pjaj]

Some years ago I annoyed my wife  by making marmalade on our ceramic hob.
Towards the end of the boiling process, as the jam thickens, it tends to splash out onto the hot glass.
I failed to wipe this up straight away, and when it came to cleaning up we found the surface of the glass notably pitted  where the splashes had been allowed to caramelise.
Can anyone explain the reaction that took place?
Was it the sugar or the fruit juice or the combination?
This was the type of ceramic hob with simple heating elements under the glass, so the glass gets very hot.

[Borek]

Sugar, but I have no idea about the process taking place.

[Arkcon]

I can't see you ceramic hob from here, but I'd expect the roughness of the surface isn't a pitting, or other loss of material.  Instaed, I think some burnt sugar has crusted there, and you're just assuming you've gotten it all off and you're looking at pitted ceramic.  The same thing happens at mom's house, sometimes the ceramic cooktop gets crusted with burnt milk, or marmalade sugar, or even ordinary dirt.  We scrub it with the proprietary scrubbing compound and it's like new again.  But that burnt crust can look pretty thin, and still be there.

Or maybe the hot sugar cracked the glass.  But that shouldn't happen, if it was to be used for that purpose.

[Borek]

Manual to my electric stove with ceramic top very directly addressed the problem - if you allow for the sugar caramelization you will end with a pitted surface, period. So far I have not checked it experimentally, we do try to not cook anything sweet.

[pjaj]

No it was definitely pitted. We always used the razor blade type scraper supplied with the hob to remove any burnt on food. This would have taken off any caramelised sugar. You could feel the pits with your finger tips.

On one hob manufacturer's current site they warn about this, but don't give any technical reasons.

On page 10 of http://www.vitralux.co.uk/downloads/VitraluxHobsBrochure.pdf

it says "Exception: In the case of sugar deposits, remove all residues before the cook-top cools down. (Be careful not to burn yourself!).
Sugar will cause pitting on the glass surface if not removed instantly."