Etching, or applying thin surface coating to constructional glass for windows, can increase the reflectivity of UV and infrared light and reduce heat intake. This can reduce air conditioning costs.
Low-E glass is produced either as soft-coated or hard-coated. Both processes deposit a thin, transparent coating of a metal oxide (indium, tin or zinc) or silver on the glass surface. Some processes use multiple coating layers. This coating allows short-wavelength daylight to pass through and blocks long-wavelength infrared heat.
Soft-coated glass is the most common. In this application, the layer of silver is deposited onto the glass through a physical vapour deposition (PVD) sputtering process using argon after the glass has been manufactured. Although it provides the lowest possible solar heat gain coefficient (SHGC), the resultant coating is delicate and must be protected within a double- or triple-insulated glass unit (IGU) to prevent scratching and wear.
During the manufacturing process for hard-coated low-E, a thin layer of a metal oxide (generally tin oxide) is applied to the glass while it is still hot in a pyrolytic chemical vapour deposition (VCD) process. Hard-coated glass is more durable than soft-coated glass and may therefore be used in single-glazed windows or storm doors.
Pyrolytic glass coating is performed on the glass production line, typically after the tin bath and before the annealing lehr. The PVD sputtering coating process can be performed off-line at the glass factory or at a separate specialist coating facility. PVD operates under vacuum conditions and at low temperatures.
Within the vacuum chamber, gas ions - typically Ar+, occasionally Kr+ - are accelerated by a high voltage-producing plasma discharge. A target silver cathode is bombarded by these ions and atoms of the silver target material are ejected by the momentum transfer and land on the glass to form the nano coating. The PVD process is therefore a significant consumer of argon or krypton gas.
Glass can be etched by hydrofluoric acid, or anhydrous hydrofluoric acid gas (HF). The gaseous HF produces a surface which resembles ground glass. The aqueous acid produces clear etching.
Corrosive chemical gases such as HF, HBr and HCl are used in the semiconductor industry for etching silicon chips, which have similar chemistry to constructional glass.