If a smoke test shows no leakage and the flue is straight, a hand mirror
held at the proper angle at the base affords a means of examination for
obstructions. Usual causes of stoppage are broken tile leaning inward,
mortar accumulations, loose bricks, bird's nests, partly burned paper,
soot from soft coal, tarry deposits from burning wood, etc. A weighted
bag of hay or straw attached to the end of a rope may be passed up and
down the flue to clean it if there is not too great an offset in it.