Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides (see also matte) and metal atoms in the elemental form. While slags are generally used as a waste removal mechanism in metal smelting, they can also serve other purposes, such as assisting in the temperature control of the smelting; and also minimizing any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal.

Ore smelting
In nature, the ores of metals such as iron, copper, lead, aluminium, and other metals are found in impure states, often oxidized and mixed in with silicates of other metals. During smelting, when the ore is exposed to high temperatures, these impurities are separated from the molten metal and can be removed. The collection of compounds that is removed is the slag. In many smelting processes, oxides are introduced to control the slag chemistry, assisting in the removal of impurities and protecting the furnace refractory lining from excessive wear. In this case, the slag is termed synthetic. A good example is steelmaking slag: quicklime and magnesite are introduced for refractory protection, neutralising the alumina and silica separated from the metal, and assist in the removal of sulfur and phosphorus from the steel.

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The term "opaline" in current times refers to many forms of opaque and colored glass. In France the term opaline is used to refer to multiple types of glass and not specifically antique colored crystal or semi-crystal. The idea that the term opaline is strictly antique French crystal is a misnomer. For instance when shopping in France you may see a piece of American slag glass for sale labeled opaline in reference to the color of glass and not the age, origin or content of the glass.
 
Opaline glass is also a decorative style of glass made in France from 1800 to the 1890s, though it reached its peak of popularity during the reign of Napoleon III in the 1850s and 1860s. The glass is opaque or slightly translucent, and can appear either white or brightly colored in shades of green, blue, pink, black, lavender and yellow. The glass has a high lead content which defined it as "demi-crystal" or semi-crystal. The primary influences on this style of glass were 16th century Venetian milk glass, and English white glass produced in 18th century Bristol.
 
Many different pieces were produced in opaline glass, including vases, bowls, cups, coupes, decanters, perfume bottles, boxes, clocks and other implements. The popularity of Opaline glass began during the reign of Napoleon. Cities involved in the production included Le Creusot, Baccarat, and Saint-Louis, Reunion, as well as various locations in England.
 
All opaline glass is hand-blown and has a rough or polished pontil on the bottom. There are no seams and no machine engraving. Many pieces of opaline glass are decorated with gilding. Some with handpainted flowers or birds. Several have bronze ormolu mounts, rims, hinges or holders. Real opaline glass was produced only in France. In the 20th century Italy produced a similar glass and called it opaline veritable.
 
Most all opaline glass is not branded or signed.
 
Portieux Vallerysthal made lots of robin's egg blue glass they call Opaline. This was produced later and in much greater quantity and is not nearly as rare or valuable.
 
The two best references for French Opaline Glass are Les Opalines by Christine Vincendeau and L’Opaline francaise au XIXe siecle by Yolande Amic. Both are in French.