 The surface of the paper is coated with a solid-state mixture of dye and a suitable matrix; a combination of a fluoran leuco dye for example.When the matrix is heated above its melting point, the dye reacts with the acid, shifts to its colored form, and the changed form is then conserved in a metastable state when the matrix solidifies back quickly enough.The reactant acid in thermal paper is often bisphenol A (BPA).