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Cloudy, 'milky' or crazed glass is caused by unburned acidic condensates etching the ceramic glass and unfortunately this cannot be easily removed. It is definitely not faulty glass, but has to do with the quality of the fuel that you burn or the way that you operate your stove.
Elsewhere on the internet, you might see the odd Youtube video showing someone (obviously with a lot of time on their hands) polishing and cleaning their cloudy glass to something like its original state – but not quite. However, this will mean removing the glass and will take a lot of time and hard work, as well as involve you in the purchase of a proprietary grinding paste which is unlikely to be available from your local stove dealer or DIY merchant, to which you can therefore also add the cost of post and packing. In our view this is really more trouble than it's worth.
Cloudy glass is only cosmetic and the performance of the glass is not affected, but if it really bothers you, then you should buy yourself a new piece of heat resistant glass and fit it yourself. It's actually very easy and not that expensive – check out the 'how to' video on our sister Stove Spare Glass website here, where you can also buy your replacement stove glass (even if you don't know the make or model of your stove).
How to prevent cloudy glass...
To start, always use the correct fuels as recommended by the stove manufacturer. In our experience this usually means avoiding any fuel with a high sulphur content such as bituminous coal (aka smoky coal) and manufactured coal products which contain petroleum coke (aka petcoke – one of the dirtiest fuels in the world) as these are loaded with sulphur.
A by-product of burning these fuels is that, given an incorrect combustion environment, the sulphur which is released, when mixed with water condensation, can produce sulphuric acid which in turn will damage the glass if it settles on it. Since most fossil fuels contain some sulphur, slumber burning will only increase this risk, because turning down the secondary air for long periods reduces the effectiveness of the airwash system which would normally 'wash' it away. This encourages incomplete combustion and allows the resultant matter, including acidic condensates, to settle on the glass.
Although when compared with coal, wood has a much lower sulphur content, the incomplete combustion of wood can also produce similar conditions which can encourage the glass to cloud. If the wood is wet or unseasoned then this will also add to the problem – as well as create a host of other problems for you.
The good news is that from the 1st of May 2021, as part of steps to improve air quality, England and Wales will make it illegal to sell any bagged bituminous house coal, as well as unseasoned wood that has a moisture content greater than 20%. In addition the sulphur content in manufactured smokeless coal products will be significantly reduced. In the Republic of Ireland smoky coal has already been banned. All of this should help reduce the instances of cloudy glass.
In The Stove Yard's experience cloudy glass occurs more often when a large stove has been installed so that its heat output is clearly well above that recommended for the size of the room. This means that such stoves are often operated with a small fuel load so that the fire chamber and airwash system do not reach the correct operating temperature for efficient combustion. Alternatively, when burning with the designed fuel load, the stove then produces so much heat that it has to be turned down for very long periods, again encouraging the incomplete combustion which promotes cloudy glass and all other manner of problems (eg nuisance smoke, blocked flueways, furred flue and corroded liner).
Initially, the cloudiness will usually start with a series of white deposits which can be easily removed if you regularly clean your glass prior to operating the stove. Leaving such deposits for prolonged periods and then continuing to operate your stove inefficiently should therefore be avoided. In short, the cleaner you keep your glass and the less you slumber burn, and the longer you maintain an effective airwash, then the better the chance you have of preventing cloudy glass.
Top images: The initial tell-tale deposits of Sulphur Dioxide on the inside of the stove glass that indicate the use of sulphurous fuel or the prolonged inefficient combustion which promotes the production of acidic condensates.
To sum up...
• If possible, clean and dry the glass every day prior to using the stove
• Only burn the fuel types recommended by the manufacturer
• Avoid burning petcoke or products with petcoke in them or those with a high sulphur content
• Avoid slumber burning (after low burning always burn on maximum heat for 20 to 30 minutes)
• Always maintain some airwash flow (do not fully close the secondary air)
• Never burn wet or unseasoned wood
• Ensure your smokeless mineral fuel is dry and is stored in a dry place
• Avoid any ash build up on the lower part of the glass
