Spectrophotometric investigations were carried out on soda-lime-silica glasses containing iron in high content, corresponding to 4.9 wt.% Fe(2)O(3), and melted under the various reducing conditions, as a part of series of studies
on the redox behavior of glass. Absorption measurement was made in the range from 340 mμ to 1000 mμ. Ferric ions could not be reduced to ferrous ions completely even by the amount of carbon of four equivalents. The ratio of the strength of light absorption at 1000 mμ as a typical band of Fe(2+) ions and that at 380mμ of Fe(3+) ions was expressed as E(1000)/E(380), which was examined for glass compositions as well as various carbon amount added to
glass batches. The ratio decreases as the carbon amount increases and increases with decreasing content of Na(2)O or CaO. These changes of spectrophotometric properties of glasses were discussed from the view point of state of the the iron ions. Besides, some results on oxidized glasses were also added

Kambara, Ginjiro

Okahara, Masamichi

Takahashi, Katsuaki

Okayama University Scientific Achievement Repository

MEMOIRS OF THE SCHOOL OF ENGINEERING, OKAYAMA UNIVERSITY, Vol. 3, No. I, SEPTEMBER, 1968Spectrophotometric Studies on High Iron ContentSilicate GlassesKatsuaki TAKAHASHI, Ginjiro KAMBARA*and Masamichi OKAHARADepartment of Industrial Chemistry(Received March 13, 1968)Spectrophotometric investigations were carried out on soda-lime-silica glassescontaining iron in high content, corresponding to 4.9 wt. % FezOs, andmelted under the various reducing conditions, as a part of series of studieson the redox behavior of glass. Absorption measurement was made in therange from 340 mi' to 1000 mfl. Ferric ions could not be reduced to ferrousions completely even by the amount of carbon of four equivalents. The ratioof the strength of light absorption at 1000 mfl as a typical band of FeZ+ ionsand that at 380 mi" of Fe3+ ions was expressed as ElOoo/E380, which wasexamined for glass compositions as well as various carbon amount added toglass batches. The ratio decreases as the carbon amount increases and in-creases with decreasing content of NazO or CaO. These changes of spec-trophotometric properties of glasses were discussed from the view point ofstate of the the iron ions. Besides, some results on oxidized glasses were alsoadded.§ 1. IntroductionThe state of irons contained in glass changeswith the various conditions, namely, composi-tion of base glass, content of iron, atomos-pheric condition, melting temperature, heattreatment etc. To the understanding of thesebehaviors of iron in glass the spectrophoto-metric information has been playing very im-portant role, for it is sensitively influenced byvalence and configuration of iron ions.According to the previous works sodiumsilicate glasses containing iron indicate theabsorption bands approximately at 380, 425,440 and 1200m,a. These absorption spectra innear ultraviolet and near infrared regions arerespectively assigned to Fe3 r ions and Fe2+ions both subject to an octahedral ligand fieldof oxigen ions. :Many studies on the nature ofthese absorption bands including some at-tempts to relate them to the concentrations offerric and ferrous ions have been carried outfrom the viewpoint of coloring of glass as wellas approaching structure of glass. 1-8)* Department of Inclustrial Chemistry, Faculty ofEngineering, Kyoto University63The purpose of the present investigation isalso to study the spectrophotometric propertiesof soda-lime-silica glasses containing iron inhigh content corresponding to 4.9 wt. % FeZ03and melted mainly under the various reducingconditions which were produced by addedcarbon powder. This was intended as a partof the overall series of studies on the redoxreaction and equilibrium in glass.§ 2. Preparation of glass salllplesThree groups of glasses were used in thisstudy. One was the group of the standardcomposition, that is 74.0 SiOl, 16.0 NazO,10.0 CaO,7) by wt. 96 and the other two weresoda rich group and lime rich group. Theglass batches were prepared from high puritysilica sand, reagent grade sodium carbonate,precipitated calk and ferric oxide as the ironsource together with required amount of car.bon powder. Ferric oxide used were preparedby decomposing reagent grade ferric oxalatein the oxidizing atomosphere. The amount ofcarbon added to the batch was varied withinthe range from one to four times of the follow.ing stoichiometric requirement.64 K. TAKAHASHI, G. KAMBARA and M.OKAHARA (Vol. 3,(1)The glasses were melted in sintered aluminacrucibles in the electric furnaces heated bysilicon carbide heating elements. The batchcorresponding to about 25 gram glass wasweighed precisely and divided into severalparts being about 7 gram each in weight.They were charged one after another every 20minutes into the crucible in the furnace keptat l400 oe.After all batch charged the furnace tempera-ture was elevated to l440 0 e and kept there foran hour. Then the molten glass was pouredon the stainless steel plate which was pre-viously heated up to about 550 0 e and pressedwith another plate of the same temperature sothat the glass sheet was obtained. The processmentioned above was followed by keepingthe glass at 520 0 e for 10 minutes, after this,the glass was cooled at the rate of lO o e aminute to 200 o e, and then it was allowed tobe cooled in the air.The glasses thus obtained generally showeda dark green colour, but in some cases a darkbrown colour. Even when extremely excesscarbon, that is, four equivalents, was added tothe batch, reduction of the iron ions to themetal was not observed. The samples used formeasurement of optical transmission propertieswere prepared by grinding and polishing theglasses to 0.3-0.5 mm shickness.§ 3. Results of spectrophotollletricllleasurelllent and discussionThe optical transmission properties of thesample glasses were measured as a function ofwavelength in the region between 340 mIl and1000 mIl. Of the data obtained the principalones are indicated in the figures. In Table Ithe composition of glasses corresponding tothe spectrophotometric data cited is showntogether with the amount of carbon used asthe reductant. The colour of polished glassplates as viewed with white transmitted lightis also involved in the table. At first in Fig. 1are shown the transmittance curves of the ironfree glasses, that is, base glass (001" soda richglass (002), and glass containing Alc03 (003).The glass 003 was prepared to examine the in-fluence of Al,03 came into the glass fromcrucible.Table I Glass usedGlass 1__~G_I_aSS compositionNo. Sl02 CaO Na20wt. %-~--AI20 3 I Fe20aAdded carbon(equivalent)IAdded J'\a:'\103* I(equivalent) !Colour of glass------'---------- --,----_._--_._-001002003310 I311312313314 I74.067.861.370.410.0 16.09.7 15.611.1 27.89.5 15.27.04.9oI234I I! IIIicolourlessgreenish-browngreenish-bluegreenish-bluegreenish-brown22o244.9 I4.9 II19.9' ---------4.9-1--15.224.09.69.519.9330 I\332 f' 61.6334-342167~5241968) Spectrophotometric Studies on Silicate Glasses 65\\;l\'C lcngl h (Ill!')Fig. 1 Transmittance curves of the ironfreeglasses, i. e. base glass 00 I, soda rich glass002, and Alz03 containing glass 003.!i1TI)()-,".'LlOO-DO---"'c:blOaDO·lOO( 1-------.....,11,1.111'1::],--------------=-d (, ',' I:;SOOregion and the sharp ones in near ultravioletregion as the glasses containing iron oxidegenerally do. From the figures it can be saidthat the principal wavelength of transmittedlight shifts to short wavelength side as theamount of carbon increases.§ 80'"u" 60~.;; 40"c:b 20In every case the curves illustrate flat andalmost equal percentage transmittance overall range of wavelength from 360mJ~ to 1000mp. Fig. 2 shows the influence of amount ofcarbon added to the batch of glass containing4.9 wt. % ferric oxide on transmittance curve,Fig. 3 shows the similar influence of carbonwhen NazO content was increased. Both ofFig. 2 Influences of amount of carbon addedto the batches on transmittance of glasseswhich contain iron corresponding to 4.9wt. 96 Fez03.these two groups of glasses whose principalwave length of transmitted light are approxi-mately 500~550mJI show a deep absorptionand which covers from near infrared to visible'\;l\'C length (m!,)Fig. 3 Influences of amount of carbon addedto the batches on transmittance of soda richglasses which contain iron corresponding to4.9 wt.% Fe203.Moore2l indicated, as to the soda-lime-silicaglass of low iron content, that the strength ofabsorption bands at 390mp and lOOOmJ~ canbe related quantitatively to the concentrationof FeF and Fe2+ ions respectively. Ihara3l alsousing soda-lime-silica glasses confirmed that thispropotional relation between transmittance ata characteristic wave length and ion concentration may be applied to the extent of iron con-tent O. 5~2. 5% Fe203. For the alkali silicateglass of high alkali content, however, it wasshown by Bishay4l that the relation did nothold. Although the glasses used were soda-lime-silica glasses, in the present case, havingthe composition near to those used by Ihara,they contained iron oxide as much as two timesor more of Ihara's glasses and the proportionalrelation mentioned above was not confirmedfor them. But the authors intend to advancethe argument assuming, for the present, thatthe relation can be also applied, at least as arough approximation.Considering from the vIew point, it can besaid that the change of the nature of transmis-sion curves shows the change of the concentra-tion ratio of Fe2" ions to Fe3 c in the glasses. Astrength of light absorption E generallychanges with the changes of the concentra-tion, C, of absorptive substance as well as thethickness, d, of the absorption layer. Thisrelation is given by well known Lambert-Beer's66 K. TAKAHASHI, G. KAMBARA and M.OKAHARA (Vol. 3,~ 2.0"'~0 3lJ :,11 ~0"~ 0 0311 33:0 331 10law which is given using the absorption coeffi-cient k as follows,E = -IOgi; = log ; = ked, (2)3.0Wave length Cmf')Fi:s. 5 Transmittance curves of the glasseshaving different Na20 content but samecarbon addition of 2 equivalents.Fig. 5 indicates the transmittance curves ofthe glasses having different Na20 content butsame carbon addition of 2 equivalents. Thecurves tell the fact that the change of trans-mittance in the short wavelength region seemsto be relatively large, while in the long wave-length region very small. This appears to bea very curious thing when it is recollected thatthe absorption coefficient kr of the 1000 mplight is by far larger3 )8) than that of 380 m!~light, kv • In Fig. 6 is shown the ratio of theabsorption E 1OJO/ £380 as a function of NacOcontent. The ratio also decreases linearly withincreasing Na20 content. To explain this ex-perim.ental results, following inferences can beamount of added carbon increasing to thevalue of 2 equivalents, beyond which the ratioremains almost constant. This tendency gene-rally looks to be held even when the Na20content is changed, but the value of the ratioof absorption decreases with increasing Na20content.(3)(4)~ :2.0~J1.0310330E38J = O.03kv [Fe 3+],E looo = O. 03k r [Fe2+] ,As it was pointed out in the previous work,kr has a value of 8-10 times kv for the glassesused here. Consequently, it can be ~een thatthe change of absorption at 1000 m/l is usuallylarger than at 380m//,. From the equations (3)and (4), the proportional relation,E looo _ kr [Fe2+] _ K [Fe2 "] (5)E381 - -k~[Fe~j - TFe3+Jis obtained between the ratio of absorptionand that of ion concentration. In Fig. 4 areshown the values of the ratio E lOoo / E380 cal-culated from the curves in Fig. 2 and Fig. 3.From the figure it may be seen that the ratioE1000 / E380 increases propotionally to thewhere, In and I are the strength of initial in-cident light and transmitted light, respec-tively. It is necessary, in general case, to con-sider the reflection loss at each surface of theglass. The effect of reflection, however, canbe approximately calculated from the refrac-tive index of the glass. In the present investi-gation, curves of transmittance except thosefor ironfree glasses were corrected by thismethod. Furthermore, these curves were con-verted to those of the plate glasses having thethickness of 0.3 mm.If we select the two absorption bands, 380m!~ as the typical one for the near ultravioletregion and 1000 rn!~ for the near infrared re-gion, their absorption can be shown by follow-ing equations, using the absorption coeffi-cients, kv and kr , respectively.3\l20(II-----'---_J......__--'-__-'10531oc\mOllnt o( carbon by equl\'alantFig. 4 Plols of the ratio, £10001 £38U a3ainst amwlIltof added carbon expressed by equivalent.:\j,O Can len t C\Yl 96)Fig. 6 Plots of the ratio of absorption £1000/£380a:sainst Na20 content.1968) Spectrophotomettic Studies on Silicate Glasses 67constructed.In the first place, it can be said examiningFig. 5 and Fig. 6 that the change of E lOoo / E380must be attributed not to the change of theamount of ferrous and ferric ions, that is Fe2+->-Fe3+ but to the change of the absorptioncoefficient Fe3 c- ion, kv , originated in thechange Na~O content.On the other hand, Bishay4l stated basingon his experimental results that the absorptioncoefficient of Fe2+ ion increases appreciably asthe alkali content of the glass increases. If thismatter is to be applied for the present case, itis necessary in order to keep ElOoo constant thatthe increase of Fe3+ ions and decrease of Fe2+ions takes place as Fe2+ ->- Fe3+. Such changeof concentration of ions with alkali contentwas confirmed by Douglas et alJl •If it is assumed that the former idea of in-crease of kv can be acceptable, this change ofabsorption coefficient may be attributed to thechange of the oxigen configuration arroundthe Fe3+ ions. Ferric ions in octahedral co-ordination, Fe3+ (0):, taking the sites whichthe network midifier ions favor can be ex-pected to change into the state in tetrahedralco-ordination, Fe3+ (0):, as network formerswith increasing Na20 content. The absorptionband at 380mj! is known as the one whicharises from Fe3 + (Oli groups. Consequently,the change of strength of this absorption bandmeans the change Fe J+(O)o->-Fe3+(0)1 suggest-ing the requirement that Fe3+ (0)1 group musthave the absorption band at 380mjt, too. InTable II the bands predicted by ligand fieldTable II Ligand field predictions for ferric iron8linstance 735, 570 and 427 mp, in the curves oftransmittance in Fig. 5 also seem to be favor-able to consider this configurational change.There is, however, no substantial basis todeny the latter idea, so it is inadequate atpresent situation to form the conclusion onthis problem and further investigation is de-sired. At all events, Beer's law does not holdfor the iron in these glasses."8el~8 GO"~·10 332"iF.g 20 312~0,100 GOO 800 1090Fig. 7 Transmittance curves of the glasses havingdifferent CaO content but same carbon additionof 2 equivalents.In Fig. 7 which shows the effect of CaO con-tent, it is indicated that the transmittance inthe short wave length region decreases andthat in the long wave length region increasesas CaO content increases. The principal wavelength of the transmitted light also shifts tothe long wave length side with increasingCaO content. Fig. 8 indicates the change ofthe ratio E lOoo / E380 when the CaO content in-creased. From the same stand point as thecase of changing Na"O content, similar deduc-tions can be drawn. For instance, the changeFe2 c ->- Fe3c as well as the increase of absorp-tion coefficient of Fe3 + is brought about by---- -------- - ----Calculatedposition__(~t'L_735570427380500446427380ValencestateFe3+Fe3+Fe3+Fe3+Fe3 ;Fe3+Fe3+Fe3+Co-ordinationsymmetryOctahedralOctahedralOctahedralOctahedralTetrahedralTetrahedralTetrahedralTetrahedral1.' ,----------~---------theory are listed. 8) It is shown that both Fe:l '(0)6 group and Fe37 (0)1 group have the sameabsorption band at 380 mjt and the above re-quirement is satisfied. The other hands, for(J'------'------'------'--JLaO Lontcnl (v, I ;10)Fig. 8 Plots of the ratio, £1000/£380 against CaOcontent.68 K. TAKAHASHI, G. KAMBARA and M.OKAHARAReferenceslight as to the glasses used is approximately500 ~ 550 mp and shifts to short wave lengthside with increasing amount of carbon addedto the batch.2. Ferric ions in the glasses can not be com-pletely reduced even by the amount of carbonof 1~4 times equivalents.3. The ratio of strength of absorption at1000 mp to that at 380 mp, E IOOO / E38C , increaseswith incressing carbon amount to 2 equi-valents, beyond which the ratio is held atabout constant.4. The ratio E IOOO / E380 decreases with in-creasing Na20 or CaO content. For thischange the change of absorption coefficientcaused from the configuration change of ferricion, that is Fe3+ (Ob Fe3+ (0);, seems to playthe important role.5. For the iron ions in these glasses Beer'slaw does not hold.6. Amount of oxidizer added to the glassbatches shows only a little influence.I) N. E. DENSEN and W. E. S. TURNER, J. Soc. GlassTechno!., 22, (1938) 94, 327-89T.2) H. MOORE and S. N. PRASAD, J. Soc. Glass Techno!.,33, (1949) 155, 336-70T.3) M. IHARA and T. YAMAMOTO, J. Cerarn. Assoc.Japan, 66, (1958) 144.4) A. BISHAY, J. Arner. Cerrn. Soc., 42, (1959) 9,403.5) C. R. BAMFORD, Phys. Chern. Glasses, 1, (1960) 5,159.6) C. R. BAMFORD, Phys. Chern. Glasses, 2, (1961) 5,163.7) M. KUNUGI, T. KONISHI, T. MORI and K. TAKA-HASHI, J. Soc. Materials Sci. Japan, 12, (1963) 7,62.8) F. N. STEELE and R. W. DOUGLAS, Phys. Chern.Glasses, 6, (1965) 6, 246.9) A. PAUL and R. W. DOUGLAS, Phys. Chern. Glasses,6, (1965) 6, 207."8058 60 112'";'10 214~ 213'"'" 20....f:-i,a400 600 800 1000Waye length em!')Fig. 9 Transmittance curves of the glasses meltedunder the various oxidizing conditions.412, 414 glass and 213, 214 glass contain 4.9wt. % Fe203 and 9.3 wt. 96 Fe203 respectively.the increase of CaO content.In Fig. 9 are shown, for reference, thetransmittance curves of the glasses meltedunder the various oxidizing conditions. Theglasses used were prepared from the corres-ponding batches added with 2~ 4 equivalentsof NaN°j as a oxidizer and consisted of 4.9wt. % Fe,Oj and the other part with base glasscomposition. Melting and sample formingprocesses were same as those of reduced glassesmentioned already. The glasses of 4 9 wt. %Fe203 indicated a light greenish brown colourand iron rich ones, namely 9.3 wt. % Fe"OJglasses, a dark similar colour. The iron richglasses indicate the near ultraviolet cut off atabout 380 m/I, and that their principal wavelength of transmitted light is approximately650 mjl. This peak value is 580 mjt in the caseof 4.9 wt. 96 Fe"Oj glasses. Strength of absorp-tion at 1000 mj~ is not so large for both kindsof glasses and has a tendency of decreasingwith increasing amount of oxidizer. Thechange, however, seems to be very small.§ 4. Conclusions1. The principal wave length of transmitted

Spectrophotometric Studies on High Iron Content Silicate Glasses

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