<p>The preservation of cultural heritage, particularly in fire-prone settings like churches and museums, is critical, yet traditional flame-retardant treatments often involve harmful chemicals, emphasizing the need for eco-friendly alternatives that do not compromise fabric integrity. This study develops and evaluates a sustainable, eco-friendly flame-retardant finish for polyester fabrics that enhances fire resistance while preserving the necessary mechanical properties for use in these surroundings. A layer-by-layer application method was employed, utilizing Diammonium hydrogen phosphate (DAP), gelatin, and silica gel to treat selected polyester and polypropylene fabrics. The treated fabrics were assessed for flame retardancy, thermal stability, and mechanical properties using TGA and FTIR analysis. Results showed substantial improvements in flame retardancy, with average burning times reduced to 3–5 s and char residue between 4.28% and 6.51%. TGA analysis confirmed enhanced thermal stability through increased residual char formation, while FTIR results indicated successful chemical modifications. Although there were minor reductions in tensile strength (up to 7.64%) and elongation (up to 16.36%), the fabrics retained adequate mechanical properties. These findings present a viable approach to achieving eco-friendly flame retardancy in textiles, balancing fire protection with the preservation of fabric integrity, and offering a promising solution for protecting cultural artifacts in fire-prone environments, thus contributing to both safety and sustainability in heritage conservation.</p> Graphical Abstract <p></p>