(Sources: Borax Frits, Gerstley Borate/Colemanite, Boric Acid, Borax, Ulexite)

-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.

Some other borate minerals are:

NaBO2: Sodium monoborate or sodium metaborate
BO2H: Metaboric acid
Na2B4O7.4H2O (Na2O.2B2O3.4H2O): Kernite, rasorite
Ca4B10O19.7H2O (4CaO.5B2O3.7H2O): Pandermite
Mg6Cl2B14O26 (5MgO.7B2O3.MgCl2): Boracite

-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. 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.

-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 (although Gerstley Borate, an important source, does have some consistency problems and can flocculate a suspension). 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.

Toxicity of Boron:

The EPA Health Advisory Level for boron in drinking water is 0.6 ppm (set primarily for water taste). The level for chronic harm from regular exposure to boron is not well established.

Properties

• Fusion - 1923F

• Glaze Color - Blue haze, Boron blue

  Low fire transparent glazes employing boron frits, which have CaO and lack alumina, will have opalescent blue cloudy effects from the formation of calcium borate crystals. These 'boron blue' glazes work well visually on terra cotta bodies. These crystals do not form well if there is adequate alumina to stiffen the melt.

Linked Articles

• Boron Compounds and Their Toxicity by Edouard Bastarche

  Toxicity of Boron by Edouard Bastarche

• G1916M Cone 06-04 Base Glaze

  This is a frit based boron base glaze that is easily adjustable in thermal expansion, a good base for color and a starting point to go on to more specialized glazes.

URLs

• Using borates in brick and roofing tile - http://www.ceramicindustry.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2710,123649,00.html

Authors

• Tony Hansen (Owner)

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