The effect of heating on the refractive index of glass was investigated in order to decide whether the screening of glass fragments by RI measurement could be employed before elemental analysis when a burglar was suspected to enter a house after breaking the windowpane using a burner. Glass fragments (1-2 mm in a maximum diameter) were taken from the bulk of sheet glass samples produced by 3 different manufacturers for the windowpane of buildings. The RI values of these fragments were measured before and after heating by a pyrolyzer. It was found that heating over 670°C caused the decrease in RI values of all glass fragments by more than 0.0002, which corresponds to the defined limit of RI value variation for discriminating glass samples. The reduction of RI was observed after 30 seconds of heating at 764°C and was kept for 7 weeks. Heated and non-heated glass fragments from a same source provided identical RI values after both samples were heated at 900°C for 2 minutes and cooled to room temperature under the same condition. It should be emphasized that the direct comparison of heated glass fragments with non-heated reference glass by RI measurement could result in a wrong conclusion. The screening of glass fragments by the comparison of RI must be performed after the re-heating of heated and non-heated glass under the same condition when it was suspected that the examined glass had been subjected to high temperature.