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B2O3 (Boric Oxide)
Notes-Boric oxide has no melting point, but a progressive softening and melting range from 300-700C. The crystals begin to break down at 300C, and a series of suboxides are produced with partial melting until full fusion is reached at 700C. Boron glazes tend to have a fluid melt and lower surface tension.-The way in which boric oxide combines with oxides like calcia and soda is not as well understood as other systems. -Its low expansion makes it valuable in preventing crazing. However, each glaze recipe tends to have an optimum amount above which the effect is can be reversed and crazing increase (typically 10-14%). This effect is due to the loss of elasticity associated with excess B2O3. Predicting the expansion of high boron glazes can thus be misleading due to this factor. -Boric oxide is a unique oxide often not fully appreciated for all its qualities. It reacts with whatever is available to behave as both the 'bones' and the 'blood' of glazes (acidic glass former and flux). In some ways, it can thus be considered a low temperature equivalent of silica (although silica still needs to be present for boron to act as a flux, see below). Because of its dual personality, technicians often are not sure where to place it in the unity formula. If placed with the amphoterics, where chemically it should go, it becomes difficult to relate the formula to others that have no boric oxide. -B2O3 can actually be a refractory, frits with very high contents are using in the refractory industry. These frits do not contain SiO2, boron needs it to react and form a borosilicate glass. -Like silica it does not crystallize on cooling unless significant calcia is present to form calcium borate. -Borax and Boracic Acid are both soluble and unsuitable sources for glazes, but fine for frits. -Boron has many advantages as a glass-forming oxide. Borosilicate glazes have been the major alternative to lead based formulations (melting as low as 750C), and thus boron is critical to the ceramic industry. 'Pyrex' ware, for example, is a low expansion high silica borosilicate glass. -Boron glazes are less fluid and this has been the major challenge in switching from lead. While many users have increased firing temperatures to compensate, this has not fully solved the 'healing' and bubble clearance problems. -In low temperature glazes, it both substitutes for fluxes of high-expansion, and for silica which cannot be present in large amounts. -Boron's reactivity helps to form good clay-glaze interfacial zones that inhibit crazing. -The action of B2O3 depends upon the ratio of bases to silica existing in the glaze before the addition. If the ratio is greater than 1:2, the glaze will tend toward opalescence and crazing; if less toward clear and transparent. -Boron can form both borosilicate and borosilite alkali glasses in the same melt. These tend to separate in what is called 'phase separation'. Glasses solidified from such a non-homogeneous melt can have thermal expansion properties, for example, that are much lower or higher than expected. Frits very high in boron demonstrate this phenomenon. -Boron can form a strong eutectic with BaO and it is possible to produce glossy and runny glazes that can solidify below 500C. -Boron is very important in glass manufacture. It is employed to obtain the low expansion and quick heat transfer necessary for hot-cold cycle endurance. It also imparts corrosion resistance and lower temperature workability (at the expense of working range). Small amounts in ordinary soda-lime glass (1-1.5%) give greater strength, brilliance, durability, thermal shock resistance and protect against the tendency to crystallize during the cool cycle. Adding boric oxide for silica reduces melting temperature, substituting it for soda improves thermal properties and durability. Mechanisms
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