Fireplace window: Can one influence how fast soot is building up?

Question

I'm sorry for such a stupid question, but after googling for half an hour I know two things

1. There are at least a zillion pages, blog-posts and articles about how to keep your fireplace window clean or how to clean it.
2. There is not a single page which explains what I'm looking for and which doesn't make claims without supporting them with scientific facts.

Therefore, here the short form of my question: Assuming I have a fireplace with a window, which looks like this after a while:

Further assume I'm able to keep all other factors constant (like type of wood, temperature of the fire etc.), is there something I can apply/do to the cleaned window to prevent the building of soot for a longer time?

---

The reason I'm asking is this article I stumbled over, which claims that normal corn-starch can be used to fill the pores of the glass. This left me with more questions

• does each glass (especially my one) have pores with a size that starch particles can fit in? I read somewhere that starch has a diameter between 2-170 µm depending on the type.
• I'm making a fire where I burn wood! Why wouldn't the starch simply burn and create itself the soot?
• As we know for a while now making a surface really smooth doesn't mean it keeps clean. With a size of 10-20 µm of the lotus papillae, it seems to fall in the same range as starch.

And for the diligent, here two more sub-questions:

• How does soot building work? I mean, why does the soot stick so hard at the window? After all it's just little particles flowing around and landing on the window. (I know it's called burning in, but how does the heat work on a already burned particle?)
• Is there some connection to the pores (if any) in the glass in this process?

Update

After using my fireplace one winter, I'd like to make recommendation based on my experience so far. Important is that I can easily remove the window from my fireplace for cleaning. In case the window is fixed, cleaning might become a bit messy. Let me bring it down to 3 tips:

• My current method is to use a cleaner for ceramic glass cooktops. This works like a charm. In case someone wants to know the specific product: it's a cleaning stone for class ceramic which is probably only available in Germany but I guess something similar can be found all around the world. This thing comes with a sponge and you just have to rub the wet sponge a few times over the cleaner to apply some. Cleaning the window with this is done in less than a minute, even if there is a lot of soot.

• If you don't want to invest any money, the most simple and very effective cleaning method is to use the ash itself to scrub the window. Simply take water and a newspaper, if possible make the window itself wet or better rinse it with water. Take a sheet of newspaper, make it wet and apply ash directly from the fireplace itself. It's really surprising how easily the soot can be scrubbed away with this method.

• Last tip: Heat! As already said in the accepted answer, soot is basically the deposition of incomplete combustion products from a flame. There is no doubt and I have tested this very often: a dirty window can partially be cleaned by make a hot fire with very dry wood and a lot of air. Therefore, try to fill your fire-room with the maximum possible amount of wood, open all air-valves and let it burn for a while.

Answer 1

I don't have a fireplace or wood stove, so I don't have a practical technique that I know works to prevent soot buildup on glass, but I can answer some of your questions about glass and soot. To start, I should add a caveat here: while many fireplaces use tempered glass windows, wood stoves generally have glass-ceramic windows to withstand the increased heat. As far as surface properties go for this, glass-ceramic isn't dramatically different from normal glass.

does each glass (especially my one) have pores with a size that starch particles can fit in? I read somewhere that starch has a diameter between 2-170 µm depending on the type.

While special techniques exist for making porous glasses, normal glass is not porous and float glass (the standard technique for fabricating flat glass panes) produces quite smooth pieces even without polishing (hundreds of nm peak-to-peak), so probably whole starch particles are not getting stuck in the surface of the glass and the surface roughness probably has little impact on soot adhesion in this case.

I'm making a fire where I burn wood! Why wouldn't the starch simply burn and create itself the soot?

Unless the flame is right up to the glass, it probably doesn't get hot enough to really burn starch (if any is even there). That site didn't compare it directly with not applying starch so it's not clear that it makes any difference.

How does soot building work? I mean, why does the soot stick so hard at the window? After all it's just little particles flowing around and landing on the window. (I know it's called burning in, but how does the heat work on a already burned particle?)

Soot is basically the deposition of incomplete combustion products from a flame. A wood flame appears yellow or orange because tiny particles of mostly carbon are incandescing. There is insufficient oxygen to fully combust the carbon in the fuel to $\ce{CO2}$, so the combustion results in carbon molecules in various states of oxidation from small polymers to elemental carbon. These species are really only stable as gases within the heat of the flame itself and will thus readily deposit onto cold surfaces, like the window glass. Many modern wood stoves seem to have "air-wash" systems that flow air past the inside of the window to reduce how much soot can actually reach the surface.

What is actually deposited as soot is a complex mixture, but what is emitted from the flame is related to how much air is available and the temperature of the flame (the two are also connected with each other). If the oxygen supply is increased, more complete combustion is favoured so less carbon species that can deposit as soot are formed. The temperature that is attained influences the composition of soot particles: higher temperatures produce more aliphatic and less aromatic compounds. The practical significance is that reducing the flame intensity by limiting oxygen can greatly increase the production of soot. (and may have some effect on how well it adheres to the glass, though I wasn't able to find anything concrete to that effect)

From the non-scientific sources I glanced through, it seems to be well known that excessive soot buildup can be due to wet fuel or low airflow and that burning a very hot fire can help make soot easier to remove. As for the corn starch, it may indeed be leaving some deposit that hinders soot adhesion (there are commercial cleaners that claim to leave a film behind that makes soot easier to remove), the act of applying it may be removing existing soot that's too small to see but new soot adheres readily too, or it may do nothing. Perhaps you might want to try applying corn starch to half of a freshly-cleaned window to see if there is a noticeable difference.

Answer 2

Excellent question. One I've been pondering myself. I moved to Norway 3 years ago. Before then, I'd never had a wood burning stove in any of the places I had lived in. Now, our main sources of heat are wood burning stoves with electrical unit for the coldest times. So, it has been a bit of a learning experience for me and one that I've found fascinating, having had a Biology & Chemistry tertiary education (as well as having slight pyromaniac tendencies!).

To answer your question... I've found that soot deposition on the glass doors is dependant on wood composition. More resin means more soot on the door. It also makes the soot difficult to remove. Pine is the worst, closely followed by fir. Hard wood is the best. My personal favourites are beech and elm but both can be difficult to come by. Birch and willow are what I use mostly. Although I've had a bit of a windfall - one of our old elms was blown down in this year's autumnal storms. It is a bit of a shame as I really like the tree but I'll be glad to make use of it. I tend to use a mixture of coniferous and hard woods. We have an excess of the former. They burn very easily and quickly so are good at the start. Hard wood comes later once I've got a good collection of embers and the oven has heated up properly. but I digress... Starch is combustible so, after the coating applied and fires have been burnt the chemical structure will be have changed. The process is called The Maillard Reaction. You'll find plenty of articles about that online. The products of the Maillard reaction tend to be brown in colour so the starch film would need to be very thin or you would see it. They also tend to have a hydrophylic nature. If there are indeed pores then these will end up as pockets of hydrophylic Maillard reaction products. Coniferous resin is mainly composed of hydrophobic compounds. (I found this fascinating paper that gives details of them and their structure. Also discusses high temperature treatment products. See text around Fig. 2.1 in "Chemical Investigations of the Organic Embalming Agents Employed in Ancient Egyptian Mummification" https://research-information.bristol.ac.uk/files/34498395/396674.pdf) Hydrophobic and hydrophylic repel each other, think oil in water. So, if this theory of pores and starch holds true then it will the mutual repulsion that is most likely to be the factor that influences the soot deposition rate. I have to say though - I do doubt it because starch is combustible. There will be plenty of oxygen and heat to facilitate that.

As stated elsewhere, high airflow / oxygen availability prevents soot build up. That is because the resin combustion reaction is allowed to go to full completion. That is why, as you've noticed, a roaring fire at a later time clears the glass. It basically finishes the job.

Delivering the appropriate airflow is a fine art, one I've yet to master. It dependant on air pressure and wind as well as how wide the stove's vents are. My step father just burns with his vents full open. He has done this all his life so perhaps he is right. However, imho his method wastes a lot fuel and heat as the wood burns very quickly and draught sucks a lot of the air out of the room which is replaced by cold air from outside. So, I try to find the balance.

For cleaning, I use a hoover with plastic scraper to start (reduces mess), then a steam lance (gets most of it off) and finally oven cleaner (removes the most hard stuck deposits). That works a treat. I like the idea of using the ash. Thank you!

By the way - I got to this forum post via google. It was highly ranked. So thanks for asking and thanks for your update.