AbstractMelamine orthophosphate crystals vary in shape and formula according to their method of production. The types vary widely in their behaviour when incorporated into an epoxy resin intumescent coating for trials in a small scale fire test. Heat treatment of melamine phosphate crystals at 210 °C, a process designed to reduce their solubility, profoundly affects the performance of any crystal type in a fire test. In such cases crystal shape alone appears to determine performance, whereas the shape, formula, and state of agglomeration affect performance in coatings produced with untreated melamine phosphate. Small plates of side 20 μm heat treated at 210 °C, are the preferred type for optimum performance and weathering characteristi