A thin nonporous disk of YSZ (solid electrolyte) is sandwiched between two porous electrodes. Wax pencils are primarily used to write onto nonporous surfaces such as porcelain or glass. Additionally, some nonporous surfaces may be polished. This method involves applying a nonporous coating of titania onto the membrane surface. Its polished nonporous surface allows sterilization in boiling water or an autoclave. The membrane is a nonporous polymer filter (nylon, cellulose, or polycarbonate). This would include softwoods that are nonporous. Nonporous caprock such as halite (rock salt) or anhydrite is more effective in trapping helium.