JPS6231441B

PURPOSE:To give an efficient protection to reflective plates by keeping residual potassium quantity within a specific range against film thickness of potassium silicate film. CONSTITUTION:Potassium silicate solution constisting of, for example, potassium oxide, silica and water is coated on a reflctive plate machined and molded of aluminium plate. Too little quantity of residual potassium in the potassium silicate film causes ccracks and too much residual potassium causes clouding on the film. However, since there exists such interrelation as the shaded area of the figure between residual potassium quantity and film thickness, the residual potassium quantity should be selected in accordance with the film thickness. To reduce the potassium quantity, dipping in dilution nitric acid or warm water for a certain period is ecommended. The residual potassium quantity can be controlled freely by the time of dipping.