Abstract Some historical glasses (lead‐wood ash glasses, lead‐crystal glasses…) are silicate glasses with high content of lead and potassium. This work presents the evaluation of the chemical stability of high‐lead glasses in a high relative humidity atmosphere and as result of aqueous immersion. In both situations, the alteration mechanism begins with the lixiviation of alkali metal and lead ions, followed by the hydrolytic attack of the silica glass network. According to the results, the glasses with a higher content of lead show the fastest degradation due to their higher hygroscopicity. Environmental CO 2 can be dissolved in the adsorbed water and favor the formation of intermediate degradation compounds.