Honestly, the whole "narrow colonial chimneys" thing doesn't match my experience at all. The old bricks and mortar are usually way softer than modern ones, so a super stiff brush can actually do damage. I've found a lighter touch with a different technique works better to keep the historic liner intact.
Knight has a point about soft old mortar. Seen it myself on a 1920s house, the stuff just turns to dust if you're too rough. Hannahcraig is right that it's crumbly, but that's WHY you go gentle. A wire brush on that old lime mortar is like using sandpaper on chalk. You end up needing a full repoint, which is a huge mess. A softer brush and some patience gets the soot without wrecking the history.