Tuckpointing is when new fresh cement is applied to the chimney. It is the process of grinding out the old mortar joint with a diamond blade saw, preparing the surface with muriatic acid, then putting new mortar back. Acid washing is done again after setup (usually 2-3 days) to remove smudges and smears. 

The firebox of your masonry fireplace has an inside lining of ASTM C27 fire clay brick laid in ASTM C199 refractory mortar or equivalent. Total thickness of the back and side walls including the lining should not be less than 8” inches (203mm) when built in strict adherence to NFPA 211 life safety code.

 

Problem #1: Water Damage

ASTM C199 refractory mortar has a sodium silicate base, which makes it, water-soluble. A deteriorated chimney crown and/or lack of a rain cover allow water to run down the chimney inside the tile. This water may mix with chimney soot (this makes it slightly acidic) before it settles on the smoke shelf. Here it begins to dissolve the water-soluble mortar in the firebox.

Problem #2: The Firebox Deteriorates

NFPA 211 requires a minimum of 8” inches total fire wall thickness to effectively block heat transfer to combustible materials next to the chimney, for example, 2 x 4 wall studs, and siding. When the firebox deteriorates and the refractory mortar joints are loose or missing, the ability of the firebox to block heat transfer can be reduced to just one half the original requirement.

Problem #3: Pyrolysis

Defined as a chemical decomposition caused by heat, pyrolysis is the process by which a burnable material exposed to temperatures of approximately 220° degrees Fahrenheit or more for a prolonged period of time (and that is not really very hot) will dry out, break down and burn. It doesn’t need the presence of a direct flame to ignite either; it simply needs enough heat and oxygen.

Time Is the Enemy

Because your fireplace has worked just fine for a number of years is of little comfort, because time is working against you. Pyrolysis works its destruction in a matter of years, months, weeks, days, or even hours. In a test conducted by the NFPA, a 22″ inch high stack of wood fiberboards (1/8″ inch thickness) was exposed to a heat source of only 228° degree Fahrenheit. The pile of wood, self ignited in only 96 hours.