Why does my woodburner glass turn black and how can I stop it happening?

Glass Black Woodburner

Being able to see the flames dancing around the firebox is one of the many joys of being a wood-burning stove owner, so the last thing you want is a layer of soot or dirt blocking that view.

The first thing to say is that a little bit of soot or cloudiness is normal. Given the environment in which the glass operates, it is inevitable that some of the by-products of burning wood or smokeless fuels will make their mark. You can remove this cloudiness by following these tips on cleaning wood-burning stove glass.

Secondly, we have it much better than our ancestors. Nearly all modern wood-burning stoves come with an in-built airwash system. This uses the stove’s top vent to create a layer of air that washes over the glass and prevents grime from settling.

Given the presence of the airwash system – and bearing in mind what we’ve established about a bit of blackening over time being normal – if your stove glass is regularly going back after use then you might need to chance something about the way you’re operating the stove.

What could be causing your stove glass to go black?

Let’s look at some of the main reasons for stove glass to turn black:

Burning unseasoned wood
Perhaps the main cause of blackening of stove glass is the burning of unseasoned wood. All logs should be seasoned – that is cut, chopped and left to air – for at least 12 months to allow the moisture level to drop to around 20-25%. You can check this with a moisture meter.

Using unseasoned wood means energy is used on evaporation rather than burning. This causes an incomplete burn and results in excessive smoke being produced. This settles in the form of soot, tar and creosote inside the flue system and, you guessed it, on the glass.

Incorrect use of airwash
As mentioned above, most wood-burning stoves now come with an airwash system to discourage soot from settling on the glass. If your stove is not functioning like that at the moment, you might be using airwash incorrectly.

You should try to avoid closing the airwash vent completely when the stove is in use or the glass will blacken. Read more on using airwash here.

Fuel touching the glass
If the glass is blackening in just one or two areas rather than across its entire surface, it might be that you are overloading the stove or loading fuel too close to the glass, causing the fuel to burn against the glass.

Burning coal
Most stove manufacturers advise against burning regular household coal (bituminous coal) in multi-fuel stoves. Instead, it is advisable to burn smokeless fuels.

Not burning hot enough
If the stove is not operating at optimum temperature (for instance, if you’re trying to have a small fire burning in a big stove), the appliance might not get up to a hot enough temperature for the airwash to function correctly.

Poor draw
If you’ve followed our instructions, double-checked your stove manual for correct usage of the airwash system and ruled out other causes, the problem might be the result of poor draw. In other words, the movement of air from your room, into the stove and up the chimney is not powerful enough.

The poor draw might lead to smoke lingering in the firebox, because it is not being ‘pulled’ up the flue, or prevent the airwash from functioning correctly. Possible solutions are increasing the amount of ventilation in the room or fitting an anti-downdraught chimney cowl.

10 thoughts on “Why does my woodburner glass turn black and how can I stop it happening?

  1. Just got a new wood stove insert. Had my first “real” fire although a small one. I followed the break in procedures. My glass has become blackened. Will it burn off if I have a really hot fire or is there a cleaning method? Thanks for any advice

    • OMG I’m so glad I read this, it works a treat , we just got our wood burner in December, so still learning tricks, but this is the best one yet and passing it on to all my friends with wood burners, thank you .

  2. Thanks for the great information! I received a new batch of cut oak wood and the man stated that the wood is seasoned and he just felled the “dead” tree a few days ago. After I burned the wood mixed in with my old wood, The door is extremely black. We brought the wood in yesterday to dry and I used a moisture meter and it went from 30% to 20 or 25%. I have a feeling that the wood had been in the rain for several days but that is why we brought the wood indoors to get the moisture level back to an acceptable level.

    Thanks,

    Jim G.

  3. As stated above, the best (and cheapest method) dampen an old T-Towel or a screwed up piece of newspaper then dip it into the cold ash at the bottom of your burner and use it to then clean the glass – it works a real treat 🙂

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