A simple laboratory method for making partially reflecting mirrors of high efficiency is provided by fuming titanium tetrachloride onto glass. The reaction of the tetrachloride with atmospheric water vapor allows a thin coating of titanium dioxide to deposit on hot glass. TiCl4+2H2O→TiO2+4HCl. This coating is extremely durable and can be washed frequently without producing pinholes. The critical thickness for maximum reflectivity is regulated by watching the color of the reflected light. A single coating reflects 30 and transmits 70 percent of incident light.
Alternate layers of fumed titanium dioxide and evaporated magnesium fluoride afford reflectivities over the visible range of from 30 to over 60 percent, depending on the number of layers.
MARY BANNING, "Partially Reflecting Mirrors of High Efficiency and Durability," J. Opt. Soc. Am. 37, 688-688 (1947)