Joints should be as narrow as possible, preferably under 1/8". Because this is a high-temperature, ready-mixed product, proper attention to bond widths and heat cure is essential for successful installation. Fireplace Mortar is water soluble until fire-cured, therefore a flue cap is strongly recommended to preclude joint washout.

Use for joining or sealing masonry to masonry.
A high temperture resistant (2000° F) silicate mortar used for the joining and repairing of firebrick. It's the best choice for joining masonry to masonry. Use to tuckpoint between firebricks and repair broken firebricks. This is an air-set mortar requiring heat (1200°F) for final cure. When properly dried and firecured, becomes brick-hard and brick-strong.