IMPROVING THE COLOR OF LEAD AND SODIUM CALCIUM SILICATE GLASSES V. A. Fedorova UDC 666.24 One of the characteristics which determine the aesthetic properties and consumer appeal of articles is the pure color of the glass or the absence of any very obvious, colored tints. Colored tints in either lead-containing or in lead-free glasses depend on the concentra- tion of coloring impurities which enter the glass with the raw materials and glass cullet used in the melting process; on the composition of the physical and chemical decoloring agents; and on the character of the redox processes which affect the color of elements of variable valency; and soon. In the production of high-quality hollow ware the most contaminating coloring impurities are the sands, chalk, dolomite, Dotash, and glass cullet. In accordance with the require- ments of the Standards Technical Documentation (STD), the concentration of iron oxides in the sand used for the production of crystal should not exceed 0.01% and for sodium calcium sili- cate glass, 0.015-0.025%. However, sands with a higher concentration of coloring impurities are used at the glass plants although this is against regulations. The current methods of enriching sands makes it possible to obtain a good quality raw material and therefore the question here is one of management. The concentration of iron oxides in carbonate raw material according to the operative STD, even in the best grades, is 0.15% for chalk and 0.05% for dolomite. In fact this raw material introduces into the glass compositions virtually as much contaminating impurity as the sands. The problem of using a purer carbonate raw material for the production of high- grade hollow ware is very urgent and must be borne in mind when reviewing the STD. According to the data from the chemical analysis it is well known that, for example, Belgorod chalk grade MMI is the purest with a concentration of 0.04-0.08% of iron impurities. It is clear that it would be reasonable to separate into a separate category the organization of the production of high-grade hollow ware using carbonate raw materials. The Gusev branch of the SIG has tested and recommended calcium borate as a composite raw material in accordance with the Tech. Spec. 6-08-305-74. The use of this material is widely recommended for melting crystal glass but the addiiton of calcium borate to lead-free composiitons is limited by reason of cost. In particular we must spend some time to consider the quality of the potash currently in use. According to State Standard 106-90-73, not only the concentration of iron in potash must be limited but also (in the case of the glass industry) the concentration of Cr203 (not more than 0.0005%). Quite often potash is contaminated by chrome impurities above the estab- lished norm. On the basis of many analyses undertaken at the Gusev Branch of the State In- stitute of Glass it was found that in various plants the concentration of Cr203 was 0.0005- 0.002%. If we assume that the intensity of the coloring by chromium compounds is tenfold greater than that by iron oxides, the effect of this impurity on the color of the glass becomes clear. The amount of iron oxides in the glass increases also as a result of the use of glass cullet. In recent years the addition of cullet, particularly in mechanized production, has increased. More serious attention must be paid to the quality of the cullet. During granula- tion, the cullet is contaminated by a film of iron oxide as a consequence of the use of un- purified water, from the apparatus during transportation, and by machine oil which acts as reducing agent. It is not by chance that the color of the glass has deteriorated at plants where the addition of return cullet without strict production control is increasing. The cullet used must be graded as far as possible and thoroughly washed. In other countries purified water is used during the granulation process and acidified HCI solutions are used in the washing. Gusev Branch, State Scientific-Research Institute of Glass. Translated from Steklo i Keramika, No. 9, pp. 8-10, September, 1984. 380 0361-7610/84/0910-0380508.50 (cid:14)9 1985 Plenum Publishing Corporation
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`O-I Glass, Inc.
`Exhibit 1018
`Page 001
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`BOO 700 ~, nm T.% 80 / 70506074_U ~ ~:"~'-" "~" 30#00 .... 50Q Fig. i. Spectral transmission T of glasses with weakly expressed tints: i) neutral; 2) sky-blue-violet (im- purities C a+ and Ni3+); 3) greenish (impurity Cra+); 4) purple (impurity Nia~); 5) yellowish brown (sulfides and selenides of metals). FeB/FezO J 0'$14~k~ I 0,424[ ~ 0'571- I k (cid:12)9 0,Z98[' ' " I 0,1951 0,188 [ Mass concentration of As203 and CeO2, % Fig. 2. Dependence of the Fe0/Fe20a ratio on the addition of Ce02 and As203 to sodium calcium silicate glass An analysis of the reasons for the deterioration in the color of glass at plants shows that cases occur where the glass is contaminated by iron and chrome imputities as a result of damage to the charging units. Particular attention must be paid to the possible contamina- tion of glass by chromium compounds from the aluminochronium phosphate binder used to insulate the furnace crown. The intense evaporation of akali, lead, and boron oxides during the melt- ing of high-grade glass creates favorable conditions for the development of low-melting eutectics of these compounds with the chromium phosphates and thus for the contamination of the glass. Cases were observed where the glass had become colored as a result of colored cullet falling into the furnace, the use of mixing machines contaminated by a colored-glass batch, the transporation of batch in contaminated containers, and inaccuracy in the dosing of physical decolorates (oxides of Ni, Cr, etc.) in particular, which intensely color the glass. All these factors can be elimianted and, with a stric production control, the amount of coloring impurities in glasses can be reduced to a minimum. With the aim of improving the color of the glass it is reasonable to set up at each plant, on-line control of the impurities in the raw materials, cullet, and glass based on up-to-date analytical methods for impurity iron, chromium, sulfur, titanium, etc. It is very important to establish as soon as possible the reason for the appearance of tints in the glass. One of the on-line monitoring methods is spectrophotometric analysis [i]. Figure 1 gives the curves of spectral transmission of glasses with weakly expressed tints most frequently encountered in practice. In this case the characteristic absorption bands indicate the presence of particular elements. The characteristic absorption bands for several elements in the composition of high- grade glasses are given below: 381
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`O-I Glass, Inc.
`Exhibit 1018
`Page 002
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`TABLE 1 Composites / CeO, L eaa and low- .I 0,15...0,3 read crystal and 0,15...0.3 crystal glass 0,15...0,3 0,15...0,3 0,0003L.0,0~8 0,00~25~..,0,0(103 Mass concentration of decolored, %* Er~O~ Nd2Os I r 0,01 .~.0,06 0,003...0,015 0,02...0.016 -- 0,02...0,03 NiO Co2Q~ 0,000@6~..0,0001 0,0000~... 0.00005 0,00008_ .0.00'0,2 0,00008...0,0001 I Sodium calcium 0,05...0,121 -- silicate glass 0,05...0,121 --~ 0,05...0,191 0,0001...0,0002 Se -- -- 0 052. 0 008 0,010...0,015 / 0:002 ... o:oos - ,0.002 ... 0,004 *Added to 100% of the host glass Coloring ion Absorption band, nm Fe=+ i000 ... ii00 Fe 3+ 370 ... 380 Ni 3+ 450, 600 ... 650* Co 3+ 545, 600, 650 Cr 3+ 450, 650 Nd 3+ 430, 575 ... 590 Er 3+ 520 *Intense in lead-containing and potassium glasses. Establishing the character of the contaminating impurity enables us to take steps to neutralize the coloring tint. Various methods are known to ensure that the glasses are decolored and numerous compositions for decoloring mixtures are also well known [2]. The use of arsenic or potassium nitrate compositions as chemical decolorants is well known but the toxicity of arsenic and the scarcity of saltpeter poses the problem of replacing them with other effective compounds. Systematic investigations were carried out and the oxidizing effect of As203 and Ce02 in sodium calcium silicate glass was studied. It is quite clear from Fig. 2 that CeOa has the stronger oxidizing effect. Decolorizing mixtures were prepared using Ce02. Table 1 shows the most commonly used composition. It must be pointed out that the combination of decolorants may differ but there are a few rules for the use of new decolorants. Thus the replacement of As20s by Ce02 is accompani- ed by an increase in the thermal transparency of the glass and the transmission in the IR region of the spectrum increases with the concentration of Ce02 (Fig. 3). This leads to heat- ing of the bottom layers of glass contaminated by compounds of iron and to the degrading of the color in the initial stage of decoloration when Ce02 is used. However, after two or three days the decoloration process is stabilized and the color improves. In this case the use of glass cullet decolored by As203 is not recommended. When chronium impurities are found in the glass composition the obligatory physical decolorants are neodymium and erbium oxides (2-30 g per 100kg of glass depending on the amount of impurity) which smooth out the spectral curve by compensating the absorption at 450 and 650 nm. At the present time CeO2 is always used for decoloring glass at the Neman (24% of PbO), the Volodarsk (18.12% PbO), Krasnodar Glass-Container (18% PbO) and the Pervomai and Krasnyi Fakel Plant (sodium calcium silicate glass). Laboratory studies established the possibility of lowering the concentration of salt- peter by using Ce02 as the chemical decolorant in lead crystal. It is clear from Fig. 4 that to achieve identical transmission with the decoloration by Ce02, it is possible to have 1.8-2 times less potassium nitrate than when decoloration is effected by As=03. This clearly is because of the fact that with the decomposition of potassium nitrate the oxygen mainly goes to oxidize the iron (II) oxide. 382
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`O-I Glass, Inc.
`Exhibit 1018
`Page 003
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`L% 80 ,, Z , I $ (cid:12)9 / 5O 0,02 O, Ofi 0.06 ~,08 O,f (cid:12)9 Mass concentration of Ce02, % Fig. 3. The change in the transmission T of a glass in the Ig region of the spec- trum as a function of the addition of Ce02: i) in the 1600-1700 nm region; 2) 1500 nm; 3) ii00 nm. L% 8~ 87, ,~i "/ 86~ 2 # 6 y 8 Mass concentration of K20, % /ef-~- ,,,. A s 2 03 Fig. 4. Dependence of the light trans- mission T of lead crystal on the addi- tion of potassium nitrate witha constant concentration of Ce02 or As203 of 0.2%. r,% 8O 80 ~0~00 . 509 ~00 700 3, nm Fig. 5. Spectral transmission T of glass of the same composition contain- ing 0.03% impurity Fe203 and 0.08% impurity S03 : i, 2) oxidizing and reducing conditions of melting, respec- tively. 383
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`Exhibit 1018
`Page 004
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`The change in the color of the glass is also associated with various redox processes as a result of the action of the gas medium. Such changes can occur at the stage of silicate and glass formation in the glass during molding and annealing. This is particularly charac- teristic of sodium calcium silicate glasses in which selenium is used as the decolorant. The particular stability of the FeO/Fe203 ratio in the melting process has been discus- sed in the literature [3]. The equilibrium Fe=+~-Fe 3+ is established with the silicate and and glass formation and depends mainly on the increase in the oxygen potential of the batch (special addition to the batch of oxidizing additives, the use of raw material to introduce oxygen). The gas medium affects to a lesser extent the change in the equilibrium which is established. However, the equilibrium between the different valence states of selenium Se 4+'6+ Se~ 2- depends largely on the redox potential of the gas medium. It is only when neutral and weakly oxidizing conditions are maintained that the pink color associated with selenium (Se ~ appears and the compensating greenish-yelow color of the iron compounds are achieved. In a strongly oxidizing medium, selenium changes to the noncoloring form (Se 4+,6+) and in a reducing medium to the selenide form (Se 2-) which produces yellow and brown tints. Similar changes also occur with impurity sulfur found in sodium calcium silicate glasses in amounts of 0.4-0.5% in the formof SO~. Under reducing conditions the $6+~-S 2- equilibrium is shifted towards sulfide sulfur. The sulfide and selenides of metals (FeS, FeSe, PbS, PbSe, CoS, CoSe, etc.) which color glass intensely are capable of lowering the transmission of the glass by 10-20% and give the glass irremovable ti~ts. Figure 5 shows the spectral transmission curves of glasses of the same composition melted in controlled reducing and oxidzing media. The iron and sulfur compounds are present in the form of impurities. It is clear from Fig. 5 that there is a significant change in the absorp- tion in the shortwave region of the spectrum. The glass acquires a clearly expressed yellow color. Consequently, the decisive factors in the decoloration of high-grade glasses are the monitoring and maintenance of the Constant composition of the gas medium. Automated control systems for glass furnaces using the Kaskad equipment installed in the plants of this branch of industry make it possible to provide the maintenance of the required fuel--air ratio in the furnaces and constantly to ~eep the gas medium constant. Thus the solution of the l, roblem of improving the color of a glass demands a complex approach. It is necessary to improve the production discipline at the stages at which the raw material is prepared and inspected; to provide for the maintenance and monitoring of the necessary furnace atmosphere; to make scientifically based selection of the chemical and physical decolorants allowing for possible redox processes and the character of the spectral curves. The use of rare-earth oxides makes it possible to stabilize the color of the glass, to eliminate the use of toxic components, and to reduce the consumption of scarce raw materi- als. 1. 2. 3. LITERATURE CITED V. A. Borgman, Steklo Keram., No. 2, 30 (1970). V. A. Fedorova and Yu. A. Guloyan, Production of High-Grade Hollow Ware [in Russian], Legkaya i Pyshchevaya Promyshlennost', Moscow (1983); USSR Patents Nos. 368195, 614038, 496242. V. A. Fedorova et al., in: Production and Study of Glass and Silicate Materials [in Russian], No. 6 (1978), p. 66; L. G. Baiburt et al., Steklo Keram. No. i0, 6 (1973). 384
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`O-I Glass, Inc.
`Exhibit 1018
`Page 005
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