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Applications of Borate Compounds for the Preparation of Ceramic Glazes

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Applications of Borate Compounds for the Preparation of Ceramic Glazes Glass Tech, 2000, 41 (6), 193–6 Applications of borate compounds for the preparation of ceramic glazes S. Stefanov 7, A.Ronkali Str., 1504 Sofia , Bulgaria The influence of B2O3 on ceramic glazes is discussed. hardness of glazes, intensifies the effects of colouring The main properties of borate compounds and minerals oxides and is a very useful glaze constituent.(1) used for introducing B2O3 into ceramic glazes: boric acid, borax (sodium tetraborate decahydrate), sodium tetra- Borate sources for glazes borate pentahydrate, ulexite, pandermite, colemanite, For introducing boric oxide into ceramic glazes the fol- datholite, hydroboracite, etc. are discussed. The borate lowing compounds and minerals are used. glazes are classified into four groups: lead borate, alkaliborate, alkaline earth borate and zinc borate glazes. Boric acid and sodium tetraborates Information is presented concerning the main proper- Boric acid H3BO3 is volatile and water soluble. Heated ties of the glazes and Seger formulae and compositions to 105°C it loses a water molecule and becomes of the frit and glaze blends for some typical glazes are metaboric acid; upon further heating to above 160°C given.Possibilities for the application of borate glazes in the metaboric acid changes into tetraboric acid the manufacture of different ceramic articles (heavy clay, (H2B4O7) which when calcinated yields a glassy, mol- majolica, earthenware, vitreous china, bone china, etc.) ten mass of boric oxide. Free boric acid occurs in na- are discussed. Borate glazes for the production of pot- ture as mineral sassolite. Boric acid is used in glazes tery, wall and floor tiles, stove tiles, bricks, roofing tiles, when alkali oxides are unwanted. Boric acid contains tableware, sanitary ceramics, decorative articles, etc. are 56·3%B2O3. proposed. Sodium tetraborate pentahydrate is recom- Borate compounds and minerals are used for simul- mended as a most effective and cheaper raw material taneously introducing boric oxide and alkali or alka- for introducing B2O3 into ceramic glazes. The advan- line earth oxides into ceramic glazes. Usually the tages of the pentahydrate are discussed. minerals are purified to give pure borate products. Boric oxide is a highly hygroscopic glassy solid that Sodium tetraborate decahydrate (borax) Na2B4O7. does not exist in nature and is never used in free state. 10H2O appears in nature as various minerals. The low When heated above 300°C boric oxide forms a viscous hydration phases of borax Na2O.2B2O3 are known as melt, capable of dissolving metallic oxides and form- minerals rasorite and kernite and the borax of Asian ing eutectic melts with a lower melting point. Boric origin as tincal. When heated at 400°C borax gives off oxide plays an important structural function in glazes, all its constitutional water and at 741°C forms a glassy similar to that of silica. It decreases the viscosity of melt. Borax is hygroscopic, water soluble and used only glazes and gives them better gloss, lowers the soften- in fritted glazes. Substituting boric oxide for a portion ing point, increases the resistance to thermal shock and of the silica content is a sure means of reducing the reduces the surface tension. Boric oxide exerts a strong melting temperature of the glaze without danger of solvent action on the body bringing a better interac- devitrivication. Borax contains 36·52%B2O3 and tion between body and glaze which reduces crazing. 16·25%Na2O. Adding 12–16% boric oxide to a glaze composition reduces its thermal expansion coefficient but further Sodium tetraborate pentahydrate Na2B4O7.5H2O is addition causes it to increase. Small amounts of boric also used for introducing B2O3 into glazes. It contains oxide improve the transparency of glazes and their 47·8% B2O3 and 21·28% Na2O. Theoretically 76·4 parts mechanical properties. Larger quantities have a nega- of pentahydrate are equivalent to 100 parts of borax. tive effect on crazing resistance. Such behaviour is at- The commercial form contains approximately 4·75 tributable to changes in the boron atom coordination moles of constitutional water. number Anhydrous borax Na2 B4O7 is used as a raw material BO3ÆBO4ÆBO3 ( 1 ) for glazes. It contains 69·27%B2O3 and 30·73% Na2O. Boric oxide enhances the acid resistance and the Theoretically 53 parts anhydrous borax are equivalent Glass Technology Vol. 41 No. 6 December 2000 193 152 Proc. Third Int. Conf. on Borate Glasses, Crystals and Melts to 100 parts borax. Anhydrous borax is more economi- Table 1. Lead borate glazes cal in space, transport, handling and fuel but has a Glaze no. 1 2 3 4 5 6 7 much higher price, the main obstacle for its use in ce- Melting point(°C) 750 840 940 980 1050 1080 1160 ramic glazes. Seger formula (mol) PbO 0·5 0·5 0·5 0·39 0·40 0·35 0·25 Calcium borates Na2O 0·5 0·5 0·5 0·09 – – – K2O – – – 0·04 0·15 0·05 0·25 Ulexite Na2O.2CaO.5B2O3.16H2O. In addition to the CaO – – – 0·48 0·40 0·28 0·30 mineral, a synthetic product exists consisting of so- MgO – – – – – 0·145 0·10 (2) SrO – – – – – 0·175 – dium calcium borate. Ulexite contains 38% B2O3. ZnO – – – – 0·05 – 0·10 Al2O3 0·1 0·5 0·7 0·21 0·25 0·49 0·28 SiO2 2·0 3·0 3·4 1·92 2·50 3·20 2·50 Pandermite 4CaO.5B2O3.7H2O. The hydrated calcium B2O3 1·0 1·0 1·0 0·44 0·20 0·30 0·30 borate–borocalcite derives its name from Panderma, a Frit blend (%) town in Turkey, is insoluble in water and can be used Borax 66 66 66 – – – – for introducing boric oxide directly into glazes with- Boric acid – – – 38·5 – – – Quartz 34 34 34 10 – – – out fritting. It contains 46%B2O3 and 31%CaO. Marble – – – 40·9 – – – Kaolin – – – 10·6 – – – Colemanite 2CaO.3B2O3.5H2O. This is the main min- Glaze blend (%) Frit 53 41·6 37·5 20 – – – eral source of borate compounds, it is insoluble in wa- White lead 34·4 27·0 24·4 – 29·8 – – ter and is frequently used as a natural frit in ceramic Quartz 5·7 4·5 4·1 5·4 18·5 20·98 30·2 glazes. Great deposits exist in the USA and Turkey. It Kaolin 6·9 26·9 34·0 4·4 6·5 19·27 7·9 Feldspar – – – 23·8 30·4 4·58 8·6 contains 50·81%B2O3 and 27·28%CaO. Gerstley borate, Marble – – – 6·2 7·8 2·29 13·5 a variant used in the USA is of similar composition. PbO· 1·5 SiO2 – – – 40·2 – – – Colemanite is a strong fluxing agent. It yields glazes Colemanite – – – – 6 – – Zinc oxide – – – – 1 – – with melting temperatures of 1000–1100°C. A small Dolomite – – – – – 4·51 – addition of colemanite improves the gloss of the glazes. Minium – – – – – 40·74 – Boric acid – – – – – 3·25 27 Strontium carbonate – – – – – 4·38 – Datolite 2CaO.B2O3.2 SiO2 .H2O. Sources of datolite Borax – – – – – – 12·8 are employed in the former Soviet Union. The datolite is used for obtaining low melting glazes from natural transparency and can be used over wider temperature raw materials. ranges. Lead borate glazes are preferred to other low melting glazes.(3,4) Some suitable fritted and raw Magnesium borates unfritted glaze blends are shown in Table 1. Various types of magnesium borate are used as raw Lead borate glazes are used for pottery, facing bricks, materials for introducing boric oxide into glazes. roofing tiles, earthenware floor and wall tiles, decora- Inderite 2MgO.3B2O3.13H2O tive art articles, stove tiles, majolica articles, as ‘reac- Asharite 2MgO.B2O3.H2O tive glazes’ for special decorative effects, etc. Hydroboracite MgO.CaO.3B2O3.6H2O Alkali borate glazes Boracite 6MgO.MgCl2.CaO.8B2O3 Alkali borate glazes have alkali oxides as the main flux- Classification of borate glazes ing agents and contain boric oxide. They have low melt- Borate compounds are mainly used for the prepara- ing temperatures and develop colours from metallic tion of low melting ceramic glazes. According to their oxides very well but their application is limited mainly composition the glazes can be classified as follows. to art ceramics due to a lack of crazing resistance.(5) Alkali borate glazes are prepared from alkali borate frits, Lead borate glazes feldspar, whiting and clay. Alkali borate frits are non- Lead borate glazes have lead oxide as the main fluxing toxic and insensitive to a reducing firing atmosphere. agent and contain boric oxide. Combinations of lead The difference between boric oxide frits and alkali bo- and boric oxides are used to produce a substantial rate frits is only a question of their respective alkali ox- number of low melting glazes of practical importance. ide content since boric oxide is nearly always introduced The eutectic mixture (wt%) 88PbO and 12B2O3 melts via borax. Some typical alkali borate glazes are shown at 493°C. The ternary system eutectic of (wt%) 84PbO, in Table 2. The blends of some alkali borate frits used 12B2O3 and 4SiO2 melts at 484°C. Substituting boric in the ceramic industry are listed in Table 3. oxide for aluminium oxide yields very low melting tem- Alkali borate glazes are used in the manufacture of peratures. Lead borates are readily soluble in acids. majolica articles, wall and floor tiles, decorative earth- Boric oxide additives in lead containing glazes increases enware articles and tableware. the lead release. For the preparation of lead borate glazes it is recommended that the fritting of lead and Alkaline earth borate glazes boric oxide together is avoided. Lead borate glazes are Alkaline earth borate glazes with alkaline earth oxides usually composed of two or more frits or of one acid as the main fluxing agents containing boric oxide have resistant lead frit to which insoluble boron minerals been used since the 19th century as substitutes for earth- are added as mill additives.
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