Sentence:  If you burn wood on its own, the flue gases are cooler than from a coal fire. This can result in tars condensing out within the <WSD>parging</WSD> and brickwork of the flue, sometimes causing brown or yellow staining on upstairs chimney breasts. 
Meaning of the word marked with <WSD> tags:  A coat of cement mortar on the face of rough masonry, the earth side of foundation and basement walls; a parge.