It is a fact that burning wood with a high moisture content and long periods of slumbering produce high levels of un-burnt hydrocarbons that deposit within the chimney as tar or creosote.
This product is quite likely to cause a chimney fire if it is not removed however it is impossible to remove glazed creosote in most cases with abrasion based cleaning methods without also damaging the chimney.
The solution is to use a chemical cleaner or modifier to either liquefy or change the creosote tar into a product that can be cleaned.
It is my experience that the liquid method of tar removal is extremely messy using sprayed on sodium hydroxide solution which in turn runs down the flue to the fireplace and hearth.
The solution we use at clean sweep chimney services is the Cre-Away pro method dry powder application creosote modifier. This product uses a magnesium catalyst to transform wet or glazed tar into something that resembles expanded black honey comb which can be swept away using a rotary flail or stiff brushes.
It also contains a fire suppressant which helps reduce the risk of a chimney fire while it does its work.