Several of the special aspects of optical glass properties can only be understood with some knowledge about the production processes and their restrictions. The traditional way, introduced more than 100 years ago, is to melt optical glass in a clay pot (Fig. 2.1). A raw material batch consisting of precisely weighed and carefully mixed compounds is put in the clay pot. When the batch is molten, the next batch follows, until the pot is filled with molten glass.
Although compositions of optical glasses are usually given in the form of the oxides used, in reality, many needed elements are introduced as different compounds. The reason for this is that many oxides have high melting temperatures that would require high energy. These temperatures might even be higher than that of the refractory material used. The compounds actually used, such as carbonates and hydro-carbonates, release gas when being incorporated into the glass melt. In order to eliminate resulting gas bubbles, the temperature is raised considerably above the melting temperature. This so-called glass refining leads to higher gas pressure, enlarging existing bubbles and lowering the melt viscosity, thus, allowing these bubbles to leave the melt via buoyancy forces.
Bubbles that are too small to leave the melt will be dissolved by integrating their gas atoms into the glass matrix with the help of refining agents. These agents are elements that change their valence from three to five while cooling down. After refining, the melt will be stirred for improved homogeneity. The contents of the clay pot are cast in a very short timeframe into a large mold. The mold is moved into a preheated furnace. Then starts the process of cooling down the glass to room temperature. The cooling must happen slowly in order to prevent breakage caused by internal temporal stresses.
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