Milk glass is an opaque or translucent, milky white or colored glass, blown or pressed into a wide variety of shapes. First made in Venice in the 16th century, colors include blue, pink, yellow, brown, black, and the white that led to its popular name.

Milk glass contains dispersion of particles with refractive index significantly different from the glass matrix, which scatter light by theTyndall scattering mechanism.

The Tyndall effect in opalescent glass: It appears blue from the side, but orange light shines through.
http://en.wikipedia.org/wiki/Milk_glass

The particles are produced via addition of opacifiers to the melt
The opacifiers can be e.g. bone ash, or tin dioxide and arsenic and antimony compounds. They are also added to ceramic glazes, which can be chemically considered to be a specific kind of milk glass.

First made in Venice in the 16th century, colors include blue, pink, yellow, brown, black, and white. 19th-century glass makers called milky white opaque glass "opal glass". The name milk glass is relatively recent

Made into decorative dinnerware, lamps, vases, and costume jewelry, milk glass was highly popular during the fin de siecle. Pieces made for the wealthy of the Gilded Age (1870-1900 ) are known for their delicacy and beauty in color and design, while Depression glass pieces of the 1930s and '40s are less so.

Milk glass is often used for architectural decoration when one of the underlying purposes is the display of graphic information. The original milk glass marquee of the Chicago Theatre has been donated to the Smithsonian Institution. Perhaps one of the most famous uses of opal glass (or at least the most viewed example) is for the four faces of the information booth clock at Grand Central Terminal in Manhattan.