Journal of the American Ceramic SocietyKurkjian and Prindle Vol 81. No 4 Table 1. Glass Compositions Oxide content(wt%) Glasst SiO, BO3 Na,O ,O Cao Mgo AlO Fe2O3 (1) Egypt, 1500 BC 678 3.8 3.22 (2)Palestine, 4th Centur ()Sudan, 3rd century (4)Italy, 9th-10th centurie 77.8 8.7 0.7 ()Container glass, 1980 (6)1: 1: 6 soda-lime-silica 15 (9)Schott thermometer glass 12.0 II msil glass I 1)Schott Welsbach chimney 47000602 0.3 1.8 7.0 3.3 12)Nonex' 0.4 (14)E-glass, typical 54.0 17.5 high sodium); (3)Brill, calculate materials.Glass contains other oxides:(I (x708902:1m place of sand if high clarity was desired. The stones were light by small gold or copper crystals(-50 nm in diameter) that reduced to fine particles by pounding in a mortar, and the silica are formed by the precipitation of the metals in their atomic owder then often was fritted with the alkali salts. Neri gave state. The formation of the metal crystals is enhanced by re- onsiderable attention to alkali preparation, discussing in some heating the glass ("striking")and by the presence of reducing detail the washing of various plant ashes to prepare alkali salts agents, e.g., stannous chloride. The red glasses found in old for clear crystal glass. The purification process consisted of church windows are most likely copper reds, either coppe repeated sieving of the raw salt, dissolving it in boiling water, rubies, suspensions of cuprous oxide, or copper stains, because filtering, and evaporating. Thus the impurities causing color, gold rubies do not seem to have been made with any certainty such as iron compounds, were left behind Unfortunately, much of the alkaline-earth and alumina of the Opaque glasses colored by suspensions of relatively large shes were left behind as well; therefore, many clear glasses crystals(with diameters in the micrometer range), where the prepared from the purified raw materials had relatively poor crystals behave essentially as color pigments, have been known resistance to attack by moisture. The much-admired clear since antiquity. The pigments are generally insoluble or of cristallo"glass ed in Venice-Murano in the early limited solubility in the matrix glass. Some of the opaque 1500s suffered from low lime and magnesia content. As a ors formed in this way are white glasses containing suspensions of tin oxide, arsenic pentoxide, or calcium antimonate, and lass of that period now in museums have developed surface yellow glasses colored by lead antimonate. Opaque blue crizzling(a multitude of fine surface fractures) because of their glasses colored by copper calcium silicate or cobalt alumi- poor chemical durability; some extreme examples are sticky to nate, green glasses colored by chromic oxide, and brown or the touch and appear to sweat. 26 These cristallo glasses provide red-browns from iron or iron-manganese oxide mixtures also an example of an unintended consequence of the desire to re used optimize one glass property, colorless clarity in this case, caus- The most dramatic examples of colored glass are probably ng a deterioration in another property, durability the church windows of the middle with the greatest ( Colored Glasses created during the 10th through 14th centuries. Most of these windows also contain much stained glass, wherein a colorant is Although the preceding section described colorless glasses diffused into the glass surface at temperatures well below that purposely made free of unwanted color, other glasses were of molten glass. Copper reds and silver yellows are perhaps the colored purposely for decoration since the earliest days of best-known examples of surface stains glassmaking. Glassworkers in Egypt, the Middle East, and the Roman Empire knew that small amounts of certain salts could (4) Lead Glasses be incorporated in the melt to produce strongly colored glasses, The first major departure from alkali-lime-silica glasses probably the first example ue. This addition of colorants was came during the 17th century with the commercial introduction some transparent, some opac of the use of minor ingredients to of lead flint glasses. Lead had long been a minor constituent in change glass properties to produce a desired effect glazes, mosaics, and artificial gems. It was introduced as cal- The earliest and most widely used solution colorants were cined lead or lead oxide, primarily for its fluxing effect. Neri salts of copper(blue-green from the presence of Cu), iron discussed lead glasses at some length in L'Arte Vetraria and (blue to green from Fe2*, yellow to brown from Fe+), and emphasized that great care must be taken to thoroughly calcine manganese(amethyst or purple from Mn+).27 The use of small the lead to avoid the formation of molten lead because"the quantities of manganese as a decolorizer to compensate for iron least lead remaining breaks out the bottom of the pots and lets colors, also known in the Middle Ages, was referred to by all the metall run into the fire. "25 Agricola and Neri and was used by the Venetians in the pro- Shortly after the publication of Merrett's English translation duction of cristallo. Cobalt was first used in the 14th century of Neri's work in 1662, George Ravenscroft, an English glass BC (deep blue from Co). The use of chromium as a solution merchant, turned glassmaker to develop a clear glass based colorant probably began early in the 19th century English ingredients.29, 30 This latter requirement was motivated Copper and gold ruby glasses were prized highly for their by the difficulty English glassmakers were experiencing beauty and for their scarcity, the latter a result of the difficulty obtaining raw materials at acceptable cost, because a monopoly of producing these colloidal colors. Both glasses presented se- controlled the import of plant ashes for soda. 31 The glass mer rious challenges to the glassmaker because of their sensitivity chants also were struggling with unresponsive to composition, melting conditions, and subsequent thermal ers, much breakage in transit, and oppresduced history. The ruby color is caused by the selective absorption of series of experiments, Ravenscroft intplace of sand if high clarity was desired. The stones were reduced to fine particles by pounding in a mortar, and the silica powder then often was fritted with the alkali salts. Neri gave considerable attention to alkali preparation, discussing in some detail the washing of various plant ashes to prepare alkali salts for clear crystal glass. The purification process consisted of repeated sieving of the raw salt, dissolving it in boiling water, filtering, and evaporating. Thus the impurities causing color, such as iron compounds, were left behind. Unfortunately, much of the alkaline-earth and alumina of the ashes were left behind as well; therefore, many clear glasses prepared from the purified raw materials had relatively poor resistance to attack by moisture. The much-admired clear ‘‘cristallo’’ glass produced in Venice–Murano in the early 1500s suffered from low lime and magnesia content. As a result, many of the elegant examples of the elaborate Venetian glass of that period now in museums have developed surface crizzling (a multitude of fine surface fractures) because of their poor chemical durability; some extreme examples are sticky to the touch and appear to sweat.26 These cristallo glasses provide an example of an unintended consequence of the desire to optimize one glass property, colorless clarity in this case, caus￾ing a deterioration in another property, durability. (3) Colored Glasses Although the preceding section described colorless glasses purposely made free of unwanted color, other glasses were colored purposely for decoration since the earliest days of glassmaking. Glassworkers in Egypt, the Middle East, and the Roman Empire knew that small amounts of certain salts could be incorporated in the melt to produce strongly colored glasses, some transparent, some opaque. This addition of colorants was probably the first example of the use of minor ingredients to change glass properties to produce a desired effect. The earliest and most widely used solution colorants were salts of copper (blue-green from the presence of Cu2+), iron (blue to green from Fe2+, yellow to brown from Fe3+), and manganese (amethyst or purple from Mn3+).27 The use of small quantities of manganese as a decolorizer to compensate for iron colors, also known in the Middle Ages, was referred to by Agricola and Neri and was used by the Venetians in the pro￾duction of cristallo. Cobalt was first used in the 14th century BC (deep blue from Co2+). The use of chromium as a solution colorant probably began early in the 19th century.27 Copper and gold ruby glasses were prized highly for their beauty and for their scarcity, the latter a result of the difficulty of producing these colloidal colors. Both glasses presented se￾rious challenges to the glassmaker because of their sensitivity to composition, melting conditions, and subsequent thermal history. The ruby color is caused by the selective absorption of light by small gold or copper crystals (∼50 nm in diameter) that are formed by the precipitation of the metals in their atomic state. The formation of the metal crystals is enhanced by re￾heating the glass (‘‘striking’’) and by the presence of reducing agents, e.g., stannous chloride. The red glasses found in old church windows are most likely copper reds, either copper rubies, suspensions of cuprous oxide, or copper stains, because gold rubies do not seem to have been made with any certainty until the 17th century.25,28 Opaque glasses colored by suspensions of relatively large crystals (with diameters in the micrometer range), where the crystals behave essentially as color pigments, have been known since antiquity. The pigments are generally insoluble or of limited solubility in the matrix glass. Some of the opaque col￾ors formed in this way are white glasses containing suspensions of tin oxide, arsenic pentoxide, or calcium antimonate, and yellow glasses colored by lead antimonate. Opaque blue glasses colored by copper calcium silicate or cobalt alumi￾nate, green glasses colored by chromic oxide, and brown or red-browns from iron or iron-manganese oxide mixtures also are used. The most dramatic examples of colored glass are probably the church windows of the Middle Ages, with the greatest created during the 10th through 14th centuries. Most of these windows also contain much stained glass, wherein a colorant is diffused into the glass surface at temperatures well below that of molten glass. Copper reds and silver yellows are perhaps the best-known examples of surface stains. (4) Lead Glasses The first major departure from alkali–lime–silica glasses came during the 17th century with the commercial introduction of lead flint glasses. Lead had long been a minor constituent in glazes, mosaics, and artificial gems. It was introduced as cal￾cined lead or lead oxide, primarily for its fluxing effect. Neri discussed lead glasses at some length in L’Arte Vetraria and emphasized that great care must be taken to thoroughly calcine the lead to avoid the formation of molten lead because ‘‘the least lead remaining breaks out the bottom of the pots and lets all the metall run into the fire.’’25 Shortly after the publication of Merrett’s English translation of Neri’s work in 1662, George Ravenscroft, an English glass merchant, turned glassmaker to develop a clear glass based on English ingredients.29,30 This latter requirement was motivated by the difficulty English glassmakers were experiencing in obtaining raw materials at acceptable cost, because a monopoly controlled the import of plant ashes for soda.31 The glass mer￾chants also were struggling with unresponsive foreign suppli￾ers, much breakage in transit, and oppressive tariffs.32 After a series of experiments, Ravenscroft introduced a clear potash Table I. Glass Compositions Glass† Oxide content (wt%) SiO2 B2O3 Na2O K2O CaO MgO Al2O3 Fe2O3 (1) Egypt, 1500 BC‡ 67.8 16.08 2.08 3.8 2.89 3.22 0.92 (2) Palestine, 4th Century 70.5 15.7 0.8 8.7 0.6 2.7 0.4 (3) Sudan, 3rd century 64.2 15.9 2.65 10.2 2.73 2.06 2.3 (4) Italy, 9th–10th centuries 77.8 6.4 8.7 2.1 0.7 2.2 0.8 (5) Container glass, 1980 73.0 13.7 0.4 10.6 0.3 1.8 (6) 1:1:6 soda–lime–silica 75.3 13.0 11.7 (7) Faraday ‘‘heavy glass’’‡ 10.6 15.6 (8) ‘‘Jena Standard Glass’’‡ 67.2 2.0 14.0 7.0 2.5 (9) Schott thermometer glass 72.0 12.0 11.0 5.0 (10) Schott utensil glass 73.7 6.2 6.6 5.5 3.3 (11) Schott Welsbach chimney‡ 75.8 15.2 4.0 (12) Nonex‡ 73.0 16.5 4.25 (13) Pyrex 80.5 12.9 3.8 0.4 2.2 (14) E-glass, typical 54.0 10.0 17.5 4.5 14.0 † (1) Morey,5 Table I-1 (10); (2) Brill,20 Jalame glass, (low potassium, high sodium); (3) Brill,21a Sedeinga tomb glass, (high potassium, high magnesium); (4) Brill,21b Frattesina glass, (mixed alkali); (5) Ryder and Poole43; (6) by calculation; (7) Faraday;37 (8) Hovestadt,40 Jena glass 16III, 1884; (9) Hovestadt,40 p. 246, Jena glass 59III, 1889, ‘‘ideal thermometer glass’’; (10) Steiner,42 p. 172, Jena glass 202III, 1893, recalculated from batch; (11) Steiner,42 p. 172, Auer von Welsbach gas light chimney, Jena glass 276III, 1895, recalculated from batch; (12) Corning code 7720; (13) Corning code 7740, Morey;5 (14) Aubourg and Wolf,46 typical composition, can vary, depending upon manufacturer and materials. ‡ Glass contains other oxides: (1) 0.54% Mn2O3, 1.51% CuO, and 1.0% SO3; (7) 70% PbO; (8) 7.0% ZnO; (11) 4.0% Sb2O3 and 0.9% As2O3; (12) 6.25% PbO. 798 Journal of the American Ceramic Society—Kurkjian and Prindle Vol. 81, No. 4