Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 57

Even though industrial glasses may now be transported across the planet, glassmaking tends to remain a local industry. When selecting their raw materials, glassmakers target specific oxides or elements to fit quantitatively a calculated recipe [1, 2] by paying special attention not only to their composition and quality but, of course, also to their price. Raw materials in effect may represent up to one third of the total cost of glassmaking, strongly depending on local geographical and logistic conditions such as quarry‐to‐plant distance or transportation mode, which can be by waterways, rail, or truck. This concern holds especially true for commodity products which, like hollow ware, are facing fierce competition from metal or plastic materials.

Natural materials are in principle favored because, like for the mineral resources used in metallurgy or cement industry, their extraction, milling, and transportation are relatively inexpensive operations compared to the fabrication cost of synthetic materials. Some of the raw materials used in glassmaking must nonetheless be man‐made when their natural counterparts are rare, geographically restrained, or of an insufficient purity. As such, their price can dramatically increase the overall batch cost. For instance, synthetic sodium carbonate may constitute up to 50 % of the total batch cost for window or bottle glass, whereas it constitutes less than 14 wt % of the recipe.

After batch preparation, melting and subsequent chemical homogenization are complex processes whose kinetics depend on a number of factors. The grain size distribution of raw materials is particularly important in this respect as the main starting product, quartz, dissolves into the forming melt at a rate of only a few hundred μm per hour, so that pull rates tremendously decrease with increasing grain size. The impurity content is another fundamental issue. As natural products, most raw materials are far from being pure chemicals. The infusible minerals they may bring in have a very high likelihood of surviving the glassmaking process and, therefore, of causing inacceptable production losses even when present at the ppb levels. Other impurities such as iron metal originate in the initial processing of the raw materials. In any case, impurities may cause raw materials to depart from the specified physical and chemical properties. Production yield and quality can then be strongly impacted.

Since many parameters may have either positive or negative consequences if not properly mastered, an in‐depth knowledge of the chemical and physical properties of raw materials is necessary. In this chapter, the main chemical, physical, engineering, and economic criteria pertaining to raw materials specifications will thus be reviewed. Because high production yields cannot be obtained without high‐quality and permanently controlled raw materials, attention will be paid to production problems most commonly met if the specifications and management of raw materials are not respected. The focus will be here on the raw materials used for silicate glasses manufactured in very large quantities. For specialty glasses, the reader is referred to the chapters that deal specifically with optical fibers (Chapter 6.4), chalcogenide (Chapter 6.5) or metallic glasses (Chapter 7.10), sol–gel products (Chapter 8.2), or bioglass (Chapter 8.4).