Could you be damaging the glass of your oven door?

Borosilicate glass will indeed withstand thermal shock – for example putting a cold casserole on a hot oven door. Having had a considerable amount of specialist glassware made from borosilicate glass at work, I believe that it is the best choice for oven doors.

I presume that the reason that toughened glass sometimes remains in place and sometimes explodes results from differences in the tempering (heat treatment) used in different applications.

It would be interesting to know which type of glass different manufacturers use for their oven doors.

Sorry; correction – the outside surfaces are put into compressive stress.

Patrick, thank you! This is surely the kind of information Which? should be including in its reports and Convos so we can see things ino perspective? Do they not have the resources or the expertise to dig out such information or do they choose not to as it might spoil the headlines? They could ask their Members to help of course.

I think it is misleading to withhold useful information like this, and would like the way topics are presented to be much more balanced.

The whole report makes interesting reading: https://www.ovenu.co.uk/shattered-oven-door-glass-survey/

Toughened glass will ensure that there are no large shards to cause injury if breakage occurs, but the shock of an explosion could result in a user dropping food at extremely high temperature.

I wonder whether some manufacturers opt for borosilicate (e.g. Pyrex) glass or if there is a standard that demands use of toughened glass.

The safety standard is BS EN 60335-2-6. It specifies a test for the glass, whether curved or flat, and whether loose or held in a frame. The test requires the use of a specified punch and hammer that is used to break a test panel; it requires that if the panel is loose, the number of crack free particles within a 50mmx50mm square number not less than 60. If the glass is held within a frame, it requires that none of the glass shall become released or dropped from their normal position. A thermal shock test is also specified.

This seems to include borosilicate or similar glass as well as toughened glass, providing they pass the tests.

Thanks Malcolm. This makes it clear that on breakage, the glass is intended to remain in place, so any door that exploded sending fragments of glass round the room is clearly non-compliant with the standard.

I do not know if borosilicate glass can be toughened by heat or chemical treatment. In scientific glassblowing using borosilicate glass it is normal to anneal glass (heating it to between the glass transition temperature and the deformation temperature followed by slow cooling) to remove stresses to reduce risk of cracking in use.

The glass that is held in a frame is intended to remain in place when broken. So borosilicate say, that breaks into large shards must comply.

Boroslicate can be toughened but as it is a low expansion glass I don’t believe it will have the same characteristics as the higher-expansion soda lime.

I have never seen a toughened glass panel “explode” across a space. We used them widely, they are used a lot in larger window and door glazing, in car windows and used to be in windscreens. Whenever they broke or were broken they stayed in place, rather like a jigsaw. So “exploding” would not, in my experience, be normal. It would be interesting if Which? asked Sheffield University to comment on this.

I have seen broken toughened glass windows in cars and doors and the glass has either remained in position or mostly so. I have not seen a broken oven or washing machine door.

It would certainly be interesting to have more input from Sheffield, and Which? has already made contact here.

My experience as a schoolboy made it very clear that toughened glass can occasionally explode and I doubt that all the photos that can be turned up with a simple web search have been contrived.

I see no problem at all in providing comprehensive reports to satisfy members of all abilities. A summary can provide the basic information, and the details can then be dealt with in the body of the report. We should not dumb down reports nor construct them to suit only one group of people. If extensive information is available but too long for publication the website can be used. However I don’t see this as necessary in most cases.

The inner door glass of an oven may be subject to scraping, abrasive materials and chemical cleaners that are typically highly alkaline. In contrast, the outer glass is unlikely to receive this harsh treatment and will not be subject to the same extreme temperatures. It would be interesting to see statistics for breakage of inner and outer door glass.

Apart from abrasion and impact, one reason given for spontaneous breakage of toughened glass is nickel sulphide inclusions: https://www.pilkington.com/resources/ats165spontaneousbreakage20130114.pdf We have discussed these before in the context of breakage of mobile phone screens.

Can you advise us on availability of ovens without glass doors?

I’m not too concerned but anyone who has suffered a breakage – particularly a spectacular one – might be interested in an oven without a glass door.

Very few do it now, mainly on very high end stuff into commercial and large range cookers only sadly as, customers demand glass doors.

I really don’t understand why but, they do.

K.

Range cookers were the only ones I could think of.

In a previous Convo I and others have said that inclusions in glass (sometimes called “stones”) can be the source of spontaneous shattering – the stress built into the glass is suddenly released.

If Which? used the information given by commenters in past Convos when new versions or similar topics were presented, I would feel they were more than just a talking shop.

I’ve recently completed a survey for Which? and been contacted by them. My main gripe was that Which? seems not to want to engage with its members – a valuable source of help and information.

Patrick – I doubt that many would be interested in technicalities, even though we are. What would be useful would be to know how frequently breakage occurs. My reading suggests that it is infrequent and cuts and burns (because the pieces of broken glass can be hot) are rarely serious. It would be wrong of Which? to ignore the subject and I do hope the claims made in the current magazine are fair and can be substantiated.

For years I used an oven with an inner glass door and a metal outer door. For anyone with children, that might be a good practical compromise between safety and convenience.

Malcolm – In research we made use of fused quartz for various applications where conventional glasses were unsuitable. From your experience in the lighting industry, would fused quartz be economically viable for use in oven door windows? It would certainly be resistant to thermal shock and being made from purer ingredients should avoid the problem of nickel sulphide inclusions.

I doubt that fused quartz would be viable. It is generally used for optical applications. We need to look at the practicalities of this. Toughened glass is successfully used in a huge range of applications and very, very rarely spontaneously breaks. (think car windows and older windscreens for example). Quality of glass and the toughening process are factors of course, and these are covered by both quality and process standards You cannot prevent occasional process faults. I wonder, though, whether some(cheap) brands have less-than-reliable suppliers.

“Technicalities” – or factual information – are important to keep discussions on a sound basis.

It would be good if we could ask manufacturers which supplier(s) they use but I would not expect them to tell us. 🙁

Some of us are very keen on having additional technical information available on the Which? website and I’m keen to push for this to happen. I don’t envisage that this is likely to appear in the magazine, though I’m personally keen that the magazine should improve our education.