Complex sentence: It should not be moved when about to burn through, lest the log break at the place most consumed by the fire, which is usually strongest at the center of the fireplace; the separation of the log should be a result of the fire only.
Split into simpler sentences: It should not be moved when about to burn through, lest the log break at the place most consumed by the fire, which is usually strongest at the center of the fireplace. The separation of the log should be a result of the fire only.