Traditionally, as the chimney was being built, the internals flues were parged with the same bedding material to protect the brick and create a smooth surface so creosote and other organic compounds were less likely to build up. Over time and from lack of maintenance, the original liner slowly degrades and falls off, often times pulling some of the mortar joint with it. When the mortar joint is compromised, the result can be leakage of heat, flames, smoke, sparks, and carbon monoxide into adjoining flues or other rooms and the attic space. When dealing with older chimneys, it's necessary to have it inspected prior to any use.
Rust Jacking, which is also called oxide jacking or rust burst, can cause damage to structures made of stone, masonry, concrete or ceramics. The expansive force of rusting from metal reinforcing components causes the masonry material to become displaced and many times leads to further damage when not addressed.