Halogen Hob Glass Surface Melted?

[mikeparnell1959]

Whilst cooking the Christmas turkey my wife placed the oven tray, containing the turkey and root vegetables in a foil-sealed parcel, upon our Halogen hob. This was a new oven tray.

Previously, she.has done this for several years, but with the tray from an old oven.

The turkey is boiled gently for a while, whilst it cooks gently in it's own steam and juices, before being placed into the oven - to finish off.

When she came to remove it from the glass hob, it had stuck!

Upon close investigation, the oven tray had warped by the localised heat, and when eventually removed, we were left with two indentations where the glass had actually melted.

I presume that the warped tray had applied the weight of the turkey to two small area of the glass.

Glass was also left on the underneath of the oven tray, and as it cooled it started to crack and fall off.

I'm assuming that it will now be dangerous to continue using the hob - especially on the two rings where the glass has melted and left craters behind?

Any advice would be most welcome, but I fear that our Christmas meal will have ended up costing us more than the price of a turkey....

[Brian-Gaff]

Erm, did the send key melt as well? :)

I think it depends on how deep these craters are and if they can be pollished out, ie not very deep you might be able to carry on, but I'd guess if its changed the stresses inside then there might be a bit of a pop and goodbye element. Never had one apart myself.

Brian

[Tim Watts]

It does not sound good - I would not be happy using that myself for anything except lightly loaded pans :(

[Fredxxx]

Was the oven tray of the enamel type? It is possible that the glaze had a low melting point.

I'm not sure if the hob glass is heat treated. My instinct is it not and is a low expansion borosilicate glass that can't be heat treated.

If the marks are not too deep then they can be ground / polished out. Something I have never been successful with. I think a new hob is in order though would keep using the old one until I found one I liked. Second hand hobs aren't expensive on eBay.

[Kellerman]

Hum.... Hob manufacturers normally say "Dont Use" if the hob glass is damaged. If it is really glass. If the turkey was boiling in oil it would be around 250C to 350C, 100C if there was water there. (see

)

Ordinary glass starts to soften at 500C and borosilicate withstands

1000C (ish!! figures do vary on glass types). Assuming electric elements ~ whats the surface temperature of the element 750C ish?

Conclusion? The hob "glass" was either not borosilicate or was a fancy plastic based thing made from polyaimide or some such.

Try an insurance claim.........

[Chris Hogg]

Most glasses used for cooking hobs are technically described as 'glass-ceramics'. They are glasses in which controlled crystallisation has been induced, producing a network of microcrystals that significantly improve the physical properties of the glass, most notably strength, thermal expansion and thermal shock resistance. The commonest glass-ceramic used on hobs is a lithium aluminium silicate glass (LAS). Melting points of such glasses are high, ~1500C, white heat. (Similar glass-ceramics are used for the mirrors of modern very large reflecting telescopes, so that they are not affected by changes in ambient temperature and don't go out of focus).

I quite like your suggestion of the enamel on the oven tray melting. Enamels on steel do have relatively low melting points, and if it did melt, it might well have reacted with the glass-ceramic hob top to produce a low-melting phase. But if this was going to be a regular problem, I would have thought it would be well known and there would be warnings against using enamelled utensils directly on top of the hob in contact with the glass-ceramic. Perhaps the temperature of normal enamelled cooking pots is limited by the fact that they usually contain water, which would limit their temperature to 100C. By interposing an oven tray between the (I assume) roasting pan containing the turkey, such temperature limiting may not have happened.

[Fredxxx]

Thanks, I need to look this up when I have time.

Most ceramic hob elements I have come across have a thermal cut-out. Of course this could have failed.

My thought is that many glasses alloy, so any miscibility would effective glue the two together and pulling apart when cold would cause damage. I read in one place that the enamel is only good to 300C.

[Fredxxx]

The best known polyimide is perhaps Kapton which can only cope with 400C.

This range of hobs are generically called "ceramic" hobs which perhaps can include glass.

[Robin]

Should that be "some enamels"? Vitreous enamel pans have long been used on the hob - eg for Yorkshire pudding (to get the fat smoking hot before adding the batter) or for gravy (using the juices from the roast). I've an enamelled pan which has survived that being done many times - albeit not the many hundreds of times previous generations' pans survived.

[Fredxxx]

Whilst you may be correct, the term "vitreous enamel" has as much meaning as "Pyrex" in the culinary trade, where a Pyrex measuring container is more than likely made of soda-glass. In short there are no guarantees.

Either way "vitreous" is really another name for glass. "Enamel" is the descriptive name of an object with a glossy finish!

[The Natural Philosopher]

Well its more likely descriptive of a THICK finish.

[Chris Hogg]

I guess it depends on how you interpret my phrase 'Enamels on steel do have relatively low melting points'. Enamel holloware (pots & pans, mugs, plates etc) is usually made from steel, by applying (often two) coats of enamel powder to the steel item and firing to around 800°C, when the enamel powder melts to form a smooth and durable but brittle surface. See

By comparison with most glass-making processes, 800°C is a low temperature. But it's still much higher than I'd expect you'd get in a cooking pot on a hob, unless there was some particular circumstance that caused or allowed the heat to concentrate and the temperature to rise locally, such as the pot boiling dry, or possibly as with the OP's situation, having an enamelled oven shelf between pot and hob.

800°C is a good bright red heat, similar to what you might get on a traditional electric cooker ring:

[Fredxxx]

Most ceramics rely upon sintering for their strength/structure. Heat usually accelerates the process.

I'm wondering if the flat surface of a ceramic hob, and the flat bottom of a enameled baking tray could somehow become welded, while around their glass transition temperature.

I'm also aware that the glass transition temperature for the ubiquitous soda lime glass is

[Robin]

I'll take your word for it if you mean that "Pyrex" is used in the culinary trade as a generic (akin to hoover or biro) but FWIW the Pyrex branded stuff I have (and occasionally still buy to replace breakages) was and is sold as borosilicate.

AIUI things are different in the USA.

Gran's enamel baking tray weren't glossy - not after decades of use on the gas hob until the dripping smoked - followed later by scouring with Brillo pads :)