Staining/etching/deposit on enamel casserole dish

[NY]

My wife has a cast iron casserole dish, coated with (I presume) enamel - certainly a glazed vitreous coating of some sort.

Recently we've noticed that it has an irregular dull whitish staining on the outside surface -

It feels rough, like very fine sandpaper - the sort of roughness you get with limescale on a washbasin - unlike the smooth surface of the enamel. I'm trying to work out what's caused it. The fact that it's an irregular shape, all round the circumference, rather than a steady "tidemark", suggests it's not caused by the dish standing in a caustic liquid, more as if something has smeared the sides. It's only on the outside.

I've tried scraping it off with a fingernail and with a pan scourer, and I've tried limescale remover. Nothing makes any difference. I can't work out whether it's something deposited on the surface of the enamel or whether it's something that has attacked/etched the glaze.

The dish has just been used for normal cooking, and has been cleaned in hot soapy water, so I can't think where the deposit/etching could have come from.

Any thoughts?

[Muddymike]

My system gives me a virus alert when clicking that link.

Mike

[NY]

I wonder why. It's just a link to a normal photo-sharing site like Flickr - it's a photo of the casserole dish. I wonder what other photo-sharing site I could use - I don't have a Flickr account.

[Roger Hayter]

Not seen this with our cast iron cookware. Your description does make it sound like damage to the enamel rather than an external coating. When overheated ours cracks the enamel rather than melting the surface. Has it been used on a gas ring, the irregular pattern seems like the shape of flames playing on the surface? Our stove's electric. I don't think the prospects for cure are very good. Sorry I have not really added much info.

[Chris Hogg]

The fact that the enamel in that area feels rough, coupled with the fact that the marks are pale, suggests the enamel is re-crystallising in those areas. I'd go with Roger Hayter's suggestion that it's the effect of localised heating by flames from a gas ring, which in turn suggests it's always put on the ring in the same orientation.

You'll now tell me that it's only ever been used on an electric hob or in an oven!

As to getting rid of it, a lamb's wool polishing pad loaded with very fine alumina polishing powder, in an electric drill, will probably restore the smoothness and shine, but probably won't restore the darker colour; it will remain whitish.

[The Natural Philosopher]

I disagree, proper T-cutting of any solid colored surface will restioe shine and get rid of 'whiteness'.

Things that have mild abrasives in are: T-cut and frieninds Toothpaste or better, tooth powder Most metal polishes (Brasso) Some scouring powders and liquids (can you still get VIM?) Valve grinding paste.

Even ultra fine carborundum paper (600-1200 grit wet and dry) will prove the point.

I suggest a trip to Halfords for fine grit and T-cut or equivalent.

[Martin Brown]

My guess would be someone used oven cleaner sponge stick on the outside and there was active residue still present next time the thing was heated up. I am a bit surprised that it has etched the surface though.

It looks like a sponge tip applicator from the edge patterns.

I'm with TNP on the judicious application of Tcut/metal polish and elbow grease to see if it will buff out.

[The Natural Philosopher]

Either something really aggressive has attacked te glass - and I know of only one acid that can - or something nasty has been baked on at high temps. I guess the latter and its something like a dish cleaner that hasn't been rinsed off

Either way mechanical removal and polishing should work

[NY]

Maybe "rough" is the wrong word. The smooth glaze has come off leaving a surface like *very* fine sandpaper or like limescale on a porcelain washbasin - abrasive enough to start to wear down a fingernail if rubbed across the surface.

We don't have gas. It may have been heated sometimes on the ceramic hob (the sort that has a glass top and glows cherry red in a disc where the pan is placed). I wonder if the more localised heating than you'd get in an oven might have caused it. Now I think about it, I saw something similar (a dulling of the shiny surface, leaving it slightly abrasive) on a aluminium pan that had some sort of enamel coating on the outside - and that *was* used all the time on a gas hob.

I'll see if we've got any T-Cut. Out of curiosity, I'll try a bit of toothpaste first in a localised area since I can certainly lay my hands on that.

[Chris Hogg]

Depends...If it's the enamel re-crystallising, as I suggested, then it may well be right through the depth of the enamel, and polishing the surface won't remove the whiteness. If it's just a surface effect, then yes, I'll agree with you.

[NY]

One of the frustrations of testing anything like T-cut is the fact that as soon as the enamel gets wet, the blooming disappears completely and only reappears once the surface has completely dried again.

[harry]

If you can't polish it off the enamel must be shelling away from the article itself. Faulty manufacture/not fired correctly or maybe local overheating?

ISTR reading that there are two layers, the colour and a transparent glaze over. Separation of the two?

[Clive Arthur]

Differential expansion - has the hot pot been plunged into cold water?

Well, you did say *any* thoughts.

Cheers

[Jeff Layman]

A strong solution of sodium hydroxide will attack glass. The ref here:

states "And glass is much more prone to etching by caustic soda than any acid." I don't know whether that's true or not, particularly for hydrofluoric acid.

Molten NaOH is very corrosive against glass, but its MP is 318C, which will never be reached in a domestic oven. However, hot NaOH solution will "frost" glass.

[The Natural Philosopher]

Its false. Caustic soda needs to be hot and in touch with the glass for long periods before any reaction occurs. I.e. its technically true that hot caustic will slowly dissolve glass, but in practice its pretty unlikely to cause serious damage

Over a very very long period