A non-destructive technique giving an early diagnosis of glass degradation is presented here. It allows to implement a new sol-gel method that stops further deterioration of the glass and avoid the classical removing of crusts. Reflectance spectroscopy in the visible range is used with a portable fiber-optics spectrophotometer in a back-scattered configuration and underlines very fine levels of glass alteration. Quantitative validations on reference samples are first presented. The reference glasses have varying amounts of lead and have been damaged using a defined and controlled degradation process. The deterioration begins by an increase in the roughness of the upper surface, quantified by an upward vertical translation of the reflectance spectra. For glasses containing an important amount of lead, it is followed by a decrease in this roughness and the apparition of periodic oscillations of spectrum, due to interferences inside a transparent layer created by the loss of lead. The analysis of these oscillations leads to the quantification of the thickness of the altered layer and to its evolution with the degradation. These results have been validated by RBS analysis. The same measurements have been implemented on archeological samples that have been damaged by naturally occurring reactions while being buried in soil. Comparisons of features and oscillations of the reflectance spectra discriminate non-, weakly and strongly altered areas of the glasses, no matter the amount of lead in the samples.