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Building codes require refractory products that meet ASTM C-199 (medium duty) for fireplace and chimney construction. All Heat Stop products (including the castable products Insul Stick, Insul Cast and Heat Cast 40) meet or exceed this ASTM C-199 (medium duty) requirement. Firebrick must meet the ASTM C-27 requirement, and clay flue lining must meet the ASTM C-315 and ASTM C-1283 requirements. ASTM C-105 is a discontinued specification for fireclay and should NOT be used in fireplace and chimney construction. Fireclay mixes with regular mortar do not meet current building codes.