 These low-cost ethers are excellent low viscosity solvents for leuco dyes and developers, and this facilitates color formation at a well-defined temperature and with minimum energy input.Dyes in thermally sensitive paper are often unstable and return to their original colorless, crystalline forms when stored in hot or humid conditions.To stabilize the metastable glass formed by the leuco dye, developer and sensitizer, a fourth type of material called a stabilizer is often added to thermal papers.