My neighborhood sweep stopped by yesterday for a routine cleaning, and informed me that I had some creosote glaze in the flue. (Like me, many of us up here burn firewood in our fireplaces daily, doing our part to fend off the coming Ice Age and to provide homey comfort and warmth to our humble abodes. The farm still has a good-sized mountain of hard coal in the basement to use when needed. You can just throw a few hunks on the fire if you want to.)
It's not not a good thing to have creosote glaze, because a sweep's brushes cannot remove it and it presents a chimney fire risk. He blamed it on my habit of burning green and damp wood, but said that any long-used flue will accumulate glaze over time.
He proposed a treatment plan: Cre-Away powder and Anti Creo-Soot Spray. These things claim to change the glaze into something brushable. The active ingredient is a catalyst which degrades creosote.
There are these things too, at Amazon.
I ordered some. I wonder whether it would work on my coronary arteries.
Lastly, anybody with fire in the house needs one of these: Chimney Fire Suppressent
It's a career for people who do not wish to work in a cubicle. Our guy charges $125 per visit, and $50 for each additional chimney. (We have 3, so it's like a doctor's house call.)
He says he tries to do 8-10 calls per day, so this retired fireman makes $1200+ per work day, and takes the entire summer fishing on Cape Cod with his grandkids.