firing are uniformly thick and dense and adhere well to the reflective coating in further explanation three examples are given below for the practice of the method of the invention in connection with the preparation of test pieces sections of envelope tube provided with a protected reflective coating and with the determination of the thermal stability of these test pieces and of infrared radiators in accordance with the invention a solution of 70 6 g of zirconium octanoate dissolved in test benzine 8 5 zirconium content and 29 4 g turpentine oil is applied with a brush to the gold coating of an envelope tube section of transparent fused silica externally gilded half way around and is fired at 800 u00b0 c for 15 minutes the thickness of the protective coating thus produced is approximately 0 15 micrometers a solution consisting of 26 0 g of silicone resin 23 si and 74 g of pine oil and containing 6 silicon is sprayed onto the gold coating of an envelope tube section of transparent fused silica externally gilded half way around and is fired at 800 u00b0 c for 15 minutes the thickness of the protective coating thus produced is approximately 0 1 micrometer a solution of 14 8 g of tin octanoate containing 27 sn 12 0 g of dammar and 70 2 g of pine oil is brushed onto the gold coating of an envelope tube section of transparent fused silica externally gilded half way around and is fired at 800 u00b0 c for 15 minutes the thickness of the protective coating thus produced is approximately 0 1 micrometer to test for thermal stability the partially gilt envelope.