here is a article from a online resource that can be found at:

http://www.chimneysweeponline.com/hocslog.htm

Q: Lately I've seen two different brands of logs you can buy called "Chimney Sweeping Log" and "Supersweep" that supposedly clean your chimney. How can the logs I'm burning CAUSE creosote buildup in the flue, while these other logs are supposed to REMOVE it? What is so magical about these logs? Or do they even work? If so, is one better than the other?
A: It isn't really the logs that are supposed to do anything, it is the chemicals they're impregnated with, and chemical products that claim to clean or assist in cleaning chimneys are nothing new. In fact, there are actually some chemical products that are used by professional Chimney Sweeps in conjunction with mechanical cleanings. Specifically, in some extreme situations, a chimney can develop third-stage, glaze creosote that is so hard that it cannot be removed by mechanical brushing alone. In these cases, certain liquid chemical catalysts may be sprayed directly onto the glaze to alter its chemical composition, turning it into a brittle or powdery condition so it can be swept out.
From what we've read and observed, the "Chimney Sweeping Log" and "Supersweep" products currently being marketed likely contain some sort of similar chemical catalyst. If they do, here's how they would work: the chemical would be carried up the flue by the rising exhaust gases, where it would deposit on the glaze in dry form and, over the course of several subsequent fires, break it down so it could be swept out.
We have a couple of problems with the marketing of these logs: first, their names are misleading. You might expect a product called The Chimney Sweeping Log or Supersweep to sweep your chimney, or at least perform an equivalent function. The actual claim in the fine print is that the chemical contained might reduce creosote by as much as 60%. Second, we have not found chemical catalysts to be of any use whatsoever on first- or second- stage creosote deposits, which comprise about 90% of the deposits we find in chimneys (glaze deposits are an extremely rare occurance). Third, even if your flue was coated with glaze creosote and the chemical in the chimney sweeping log broke it down as much as 60%, it would still represent a considerable safety hazard until it was physically removed by sweeping.
The Washington Public Fire Educators Association has come to a similar conclusion, and published the following position paper:
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Chimney Sweeping Logs:
The use of chimney sweeping logs (and similar products) alone is not an adequate substitute for mechanical chimney cleaning and inspection because it does not provide for the same level of protection to the chimney system.
Vital Points:
Each time you burn wood in your fireplace or woodstove, tar and creosote are formed and over time, will build up on the inside of your chimney. This build-up is highly flammable and can ignite causing a chimney fire. To prevent chimney fires, the fire service has long recommended having your chimney cleaned and inspected annually by a licensed professional. But now, a new product called the "Chimney Sweeping Log" has many citizens wondering whether an annual mechanical cleaning remains necessary.
The manufacturer of the Chimney Sweeping Log claims that the product contains "specially developed minerals" that act to reduce deposits of tar and creosote thus reducing the risk of chimney fires. To use the product, you simply place the log in your fireplace or woodstove and allow it burn for roughly an hour and a half. The products website boasts that "the burning of a single Chimney Sweeping Log can reduce build-up by up to 60%".
Washington Public Fire Educators (WPFE) is concerned about these claims. While we wonÂt dispute what these fire logs will do, we feel that itÂs vital to address what they wonÂt do. If these logs manage to loosen creosote so it flakes off the flue walls as the advertisements claim, where does that creosote go? It either catches fire as it flakes off and increases the potential for a chimney fire through the intense burning, or it falls to the bottom and collects on the smoke shelf, thus causing a future hazard.
WPFE agrees with the Chimney Safety Institute of AmericaÂs statement: "The use of these products alone is not an adequate substitute for mechanical chimney cleaning and inspection because it does not provide for the same level of protection to the chimney system." (The Chimney Safety Institute of America is a non-profit, educational institution focused on the prevention of chimney and venting hazards.)
WPFE believes that the safest and most effective chimney maintenance is achieved through annual inspections and mechanical sweeping.
The basic task of a chimney sweep is to clean chimneys. The cleaning process includes 1) removing the hazard of accumulated and highly combustible creosote produced by burning wood and wood products, 2) eliminating the build-up of soot in coal- and oil-fired systems and 3) removing bird and animal nests, leaves and other debris that may create a hazard by blocking the flow of emissions from a home heating appliance. In addition to the cleaning, chimney inspections often reveal hidden problems within the structure that could be potentially dangerous such as breaks or breaches in the flue.
Mechanical sweeping of chimneys not only removes layers of creosote from surfaces, it also eliminates the resulting debris from the chimney, fireplace, or woodstove. Many chimneys are not constructed in a straight path from the firebox to the outside. If chimney-cleaning products perform as claimed and cause debris in the chimney to fall, that debris must still be removed from the smoke shelf, baffle, catalytic combustor, or offset in order to ensure a safe and properly functioning chimney.

In a word, snake oil.

OK, that's two words.

There's no substitute for a thoroughly cleaning and visual inspection.

Everything you have ever wanted to know about the creosote sweeping log here .

http://creosotelog.com