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Technical Update: Soot (Part 2)

soot classification

Welcome to part two of our three part article on soot. In this blog we talk about different types of soot, fuel types and cleaning methods.

Chimney sweeps generally classify soot by the fuel type that it was formed by when it was combusted.

There are a few general rules;

  1. Clean burning of a dry, quality fuel produces small amounts of very fine soot which contains low levels of remaining volatile residue.
  2. Poor burning technique of low quality or damp fuel produces lots of dense heavy soot with high levels of volatile residue.
  3. Burning wood which is damp, in a limited oxygen environment, with insufficient heat or inadequate turbulence produces flammable creosote. It is flammable as the volatile compounds that were previously contained within the wood have not combusted inside of the fire box where they were supposed to. The volatile gasses that leave the burning area can condense within the chimney as creosote. This creosote is the main risk of chimney fire as it still has great energy potential.
  4. The harder the deposit the more aggressive the cleaning method required to clear them
  5. The more abrasive the cleaning method the more the risk of damaging the flue.
  6. Soot is not black dirt. It is harmful, contains cancer causing compounds and must not be tackled without the correct PPE, specialist equipment and experience.
  7. Soot contains compounds which are highly corrosive especially when wet and must be removed from the flue system to prevent premature failure of components.
  8. Creosote is the main cause of chimney fire
  9. Blocked chimneys and flues are often responsible for carbon monoxide fatalities around the globe

Fuel types


Wood when burned properly produces black soot with a relatively fine powder. In most cases It can be cleaned with a moderate up and down scrubbing action with a medium or firm brush. A chimney can hold a large volume of wood soot so beware. When cleaned with a rotary flail a standard firmness plastic flail on a low speed is usually sufficient.

burning wood
Wood logs. An excellent renewable fuel when correctly dried, stored and used.
Wet wood or wood burned with poor burning practices

This type of situation leads to creosote soot deposits being formed within the chimney system. Creosote can come in many forms. It can range from liquid tar, rock hard black toffee apple like deposits (referred to as glazed), hard flakey deposits and expanded creosote which has a structure like a sweet honey comb.
The cleaning methods for creosote vary massively and some forms cannot be removed by abrasion alone. These tend to be the very hard glass like creosote deposits which must be modified with chemical treatment and heat prior to removal.
Most other types of expanded creosote respond well to being cleaned with a firm plastic rotary flail, with a large surface area on a moderate drill setting.

Some creosote which is beginning to expand. Creosote can expand up to six times its thickness and can easily block flues.
Some creosote that has fully expanded
Some creosote that has fully expanded. In this form it becomes easier to remove.
Wet glazed creosote
Wet glazed creosote. Almost impossible to remove without first treating with chemicals that modify it into an expanded form.

House Coal

House coal produces lots of smoke and therefore lots of soot. Appliances which still burn house coal must be cleaned very regularly to prevent blockage or soot falls. House coal soot is often a dense medium fine powder, jet black and there can be lots of it. A medium firm brush or flail can be used to clean it.

Good old house coal
Good old house coal. this coal type is quickly being replaced with smokeless variants that are better for the environment.

Manufactured smokeless coals.

Some smokeless coal is broken down to a dust and then reformed. During the pressing process it is normal to add binding agents such as molasses or similar and sulphur. Occasionally when these fuels are used some very hard rock like deposits form within the chimney system sweeps often refer to these as clinker. Clinker can be difficult to remove using traditional methods and is hazardous as it causes blockages. The cleaning and removing of this type of deposit is best tackled with use of a stiff rotary flail. In some extreme situations a rotary chain or even a coal chisel may be necessary to break up these rock hard deposits. Warning chains and chisels will damage some chimney types!

Some clinker beginning to form
Some clinker beginning to form

Smokeless coal

Regular smokeless coal has often been heated or cracked to drive if much of the volatile petro-chemicals. This means it produces much less smoke and therefore less deposits when compared to none smokeless fuels. The soot produced is often a yellows grey powder which can be cleaned from the flue in most cases with a medium flail or brush.

Some smokeless fuel soot
Some smokeless fuel soot. This can have a fairly high sulphur content and be corrosive when wet
Smokeless soot from a automatically fed boiler.
Smokeless soot from a automatically fed boiler.

Oil soot.

Oil soot is very, very fine it can be compared to talcum powder only it’s sticky, black and toxic.

Being such a fine powder a very fine brush or light plastic flail on very low speed should be used. If sweeping traditionally then a swabbing or stroking action should be used as the fine dust is difficult to control. Additional sealing methods must be utilised to prevent contamination of the area. Vacuums will struggle with oil soot as it is fine enough to block the pores within the filters lowering their effectiveness. This same issue with dust masks exists they too block up quickly with oil soot.
Finally oil soot is the most toxic of all of the standard exhaust products we deal with. It contains very high levels of heavy metals along with VOCS.

wood pellets

Wood pellets are an excellent fuel type. They are carbon neutral, renewable, produce lots of heat per m3 and most importantly produce very little soot. The soot that is produced tends to be very fine and dark grey in colour. In most cases it is easy to clean from the flue system using a soft brush or light flail.

Gas soot

Gas by nature does not produce much particulate soot. That does not mean gas chimneys do not require sweeping. Dust from the house gets sucked up the chimneys day to day, spiders build cobwebs which block updraft. leaves, twigs and all manner of wildlife infiltrate from the outside. Gas chimneys can usually be swept with very fine bristles such as Perlon. In older masonry flues that were previously used for solid fuel it is recommended to sweep these with a firmer brush or flail in order to the clear degrading flue material or previously built up soot.

In conclusion. There are many types of sootsnd we cannot cover every type in this short article. You will require various tools and techniques to deal with them all. Some chimneys will contain various mixtures of soot and as a chimney sweep you must be able to adapt job by job. I cannot over emphasise enough how important practicing what you learn in a real life sooty environment is.

I hope you have learned a little from todays article. There will be a third part to this series where we talk about disposal, transport and storage of soot.
Thanks for reading.