Dorsey Metrology uses satin ground glass of superior quality for all our optical comparator glass charts. The charts are very durable and will last for many years with proper care. Glass charts should be cleaned only with mild soap and water solution using soft cotton cloth. Window cleaners, thinners, and alcohol should never be used to clean your glass chart. All optical comparator glass charts are traceable to the N.I.S.T. and are available with certification at no extra cost.