When you use wood as fuel most of what you see burning is volatile hydrocarbons, these are tars, creosote and resins etc. They make up 70 – 80% of the available heat energy.
it’s the volatile tars etc. that can cause a problem. These must first be turned in to a gas (vapourised) before they can burn.  If the fire is not burning hot enough these volatile vapours are still given off but are not completely burned in the appliance and they escape to the chimney.
If the chimney is cool enough, some of these vapours will condense and solidify on to the inside of the chimney, In a cool chimney they can immediately solidify to form the tarry deposits or creosote glaze, this possess a high risk of chimney fires also restriction of the flue can cause deadly carbon monoxide gas to enter your home.



It will first need to be assessed by a professional chimney sweep, it will then be swept usually with a flail to remove as much of the tar deposits as physically possible, the flue will then need heating either with a electric/gas burner or a small fire, after that the powder treatment can be applied, if safe to do so it is then left with the customer to use the fire daily for a minimum of 4 hours a day for 2 weeks, once the reaction period is over the flue will be swept again to remove any remaining powder and modified creosote, most flues only require one treatment, very large or heavily tarred flues may need additional treatment.
Cre-Away Pro is a four-component powder that safely and effectively modifies glazed or third-degree creosote so that it can be more effectively removed by chimney sweeping. First a reactive agent neutralizes the slightly acidic creosote, making it less corrosive to metal components and less flammable. Second, a dehydrator absorbs the oils and moisture contained in some forms of creosote. Third, a combustion inhibitor helps reduce the chance of chimney fires. Finally, a magnesium catalyst helps break down third-degree creosote when the flue is heated by normal use.