Q: I look down my chimney periodically so I'll know when it's time to sweep it out. About a month into this past burning season, I climbed up for my
regular inspection, and found something I'd never seen before. Instead of a thin layer of dry, crunchy creosote, the inside of my chimney was coated
with a thick, gooey substance that looked and felt like tar! I chalked it up to the rainy weather we'd been having, figuring the rainwater had soaked
into the creosote or something. A couple months later I climbed up for another look, and the tar had hardened into a shiny mass that looked nothing
like the creosote I'm used to seeing. When I tried to chip a little off with my penknife, I discovered the stuff was as hard as a rock! What IS this
It is a form of creosote. Creosote starts out as a liquid that condenses onto the inner walls of connector pipes and chimneys as wood smoke
cools. When this liquid dries, it gradually hardens, taking one of three forms: Stage 1 (velvet soot), Stage 2 (the porous and crunchy form you're
used to) or Stage 3, the shiny stuff you found in your chimney this time, which is also known as glaze. Glaze is denser and harder than brick, and
sticks to the chimney like glue. Chimney brushes won't cut it, and any blow strong enough to break the glaze could damage the chimney structure as
Q: What caused this glaze to form?
A: Glaze is formed when fresh layers of creosote accumulate so rapidly that the previous layers don't dry completely. The freshly formed layers
then insulate the partially-hardened previous deposits from the heat of the wood exhaust that dries them, resulting in a heavy buildup of sticky goo,
which eventually solidifies, creating the rocklike substance known as glaze. The excessive creosote accumulation that leads to glaze formation is
usually caused by the improper burning or venting of airtight wood stoves. Specifically, glaze will often form when the chimney is larger than the
vent opening on the appliance (causing sluggish draft), when unseasoned or wet fuelwood is burned, or when the draft control on an airtight
appliance is habitually set too low, causing the fire to smolder. In your case, since you've never found glaze in your flue before, I suspect the rain
you mentioned somehow got onto your woodpile, turning each piece it saturated into a little creosote manufacturing machine. You can read about
this phenomenon in our Sweep's Library by clicking
Q: So how do you remove this glaze?
A: Prior to removal, the glaze must be broken down chemically. There are two ways to accomplish this:
The slow way: do it yourself.
There are products available to the general public that contain a chemical catalyst that breaks down glaze creosote gradually, over time. Some, like
TSR (Third Stage Remover) and ACS (Anti Creo Soot) brands, are in liquid form and are sprayed on each load of wood as it is added to the fire.
Liquid catalysts burn up in the fire, and emit exhaust chemicals which deposit on the glaze as they travel up the flue. Some, like Cre-Away, are in
powder form, and are puffed into the firebox above the flames to be carried aloft by the chimney updraft. Activated by the heat from hot, dry-wood
fires, these catalysts gradually "etch" the creosote, turning it into a dry, powdery form that can be swept out with a standard brush. Depending upon
the thickness of the glaze, this technique can sometimes drag out over several months.
The fast way: hire a professional Chimney Sweep.
This process involves spraying a super-concentrated chemical directly onto the glaze via a special pressure applicator that is lowered down from the
chimney top. This chemical is not available to the general public, so you're going to have to call in a professional Chimney Sweep. Most
professional glaze removal treatments require three visits over the course of two weeks. On the first visit, we score the surface of the glaze with a
chimney brush, then saturate it with the chemical catalyst. The chemical needs to be above 50 degrees to work, so glaze removal is best
accomplished during the Spring and Summer. In colder weather you must have at least 4 hot fires during the following week, opening the draft
control and damper or baffle-bypass on your stove enough to allow as much heat as possible up the chimney (we recommend you monitor these fires
in case the glaze ignites). One week later, on the second visit, we sweep out the broken-down glaze and re-apply the chemical to any remaining
glaze. On the third visit, we remove the last of the residue and spray the flue with a chemical that neutralizes the catalyst. Occasionally, when
deposits are extra dense or extra thick, additional treatments may be needed to break down and remove all the layers.
Q: How can I prevent the formation of glaze creosote in the future?
A: Burn dry, seasoned wood, avoid smoldering your fire and vent into a properly sized chimney liner.
The Chimney Sweep, Inc.
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Bellingham, WA 98225-7032
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