Soot and creosote buildup generally comes from burning the wrong type of wood in the fireplace or not cleaning the fireplace for long period of time.
Fire place flue gases (including tares, vapors and other organic compounds) in the form condensate, condenses onto the cold surfaces of the fireplace interior and chimney. This forms a soot and creosote layer. This residue of unburned carbon particles, often looks black or brown and shiny at times.
In the early stages, it may be wiped or brushed off, but over time it accumulates in layers and becomes more difficult to remove.
Creosote residue from wood or coal particles, will stick to the surfaces of a fireplace. Each time a surface cools, another layer of creosote builds up. When dry it may be a little crackling to the touch or flake off in layers. When hardened it may be shiny or have a sheen. Creosote is flammable and if it catches fire, it can be difficult to extinguish.
Retail stores and fireplace stores often carry specialized cleaners that may used to help remove soot and creosote build-ups. If the build-up is heavy or in locations that are not easily reached, then a chimney sweep may be the best solution for removing the build-up.
Whenever there is a build up of creosote the fireplace should be cleaned. Chimney sweeps often do this kind of work and many are certified for such work. Failure to keep the fireplace relatively clean of creosote and soot buildup may result in a fire from time to time.
Creosote and soot buildups are one of the leading cause of chimney fires.
If proper maintenance and cleaning is not done from time to time these products are acidic, and they can attack the mortar or damaged the masonry portions of the chimney; so keep it clean.
Who to consult? Chimney sweeps.
Home owners can do limited cleaning and maintenance on their chimneys and flues. Chimney sweeps and fireplace contractors generally handle cleaning of fireplace chimneys and flues.