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Tin(IV) oxide
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<h2 id="structure">Structure</h2> <p>Tin(IV) oxide crystallises with the <a href="/facts/Rutile/qwJH5ElN">rutile</a> structure. As such the tin atoms are six coordinate and the oxygen atoms three coordinate.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> SnO2 is usually regarded as an oxygen-deficient <a href="/facts/N-type_semiconductor/d9GPL5tV">n-type semiconductor</a>.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p><p>Hydrous forms of SnO2 have been described as stannic acid. Such materials appear to be hydrated particles of SnO2 where the composition reflects the particle size.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p> <h2 id="preparation">Preparation</h2> <p>Tin(IV) oxide occurs naturally. Synthetic tin(IV) oxide is produced by burning tin metal in air.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> Annual production is in the range of 10 kilotons.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> SnO2 is reduced industrially to the metal with carbon in a <a href="/facts/Reverberatory_furnace/fuILeeWX">reverberatory furnace</a> at 1200–1300 °C.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h2 id="reactions">Reactions</h2> <p>The reaction from tin(IV) oxide with hot <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a> is practiced on a large scale as this <a href="/facts/Carbothermal_reduction/gj0GA25m">carbothermal reduction</a> is used to obtain tin metal from its ores: </p> SnO2 + 2 CO → Sn + 2 CO2 <p>Some other reactions relevant to purifying tin from its ores are:<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> </p> SnO2 + MgCl2 + CO → SnCl2 + MgO +CO2 4 SnO2 + 6 FeCl2 → 2 SnCl2 + 2 SnCl4 + 2 Fe3O4 <p>SnO2 converts to the monoxide at 1500 °C:<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> 2 SnO2 → 2 SnO + O2 <p>SnO2 is insoluble in water. It dissolves in sulfuric acid and in molten sodium hydroxide. It is not <a href="/facts/Amphoteric/2UrXEsyl">amphoteric</a>. Like rutile, it is not attacked by solutions of acid or base. </p><p>Dissolution of SnO2 in sulfuric acid gives the sulfate:<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p> SnO2 + 2 H2SO4 → Sn(SO4)2 + 2 H2O <p>The latter compound can add additional <a href="/facts/Hydrogen_sulfate/yuWVxsUf">hydrogen sulfate</a> ligands to give hexahydrogensulfatostannic acid.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>SnO2 dissolves in molten alkali to give "<a href="/facts/Stannate/Dq0PxxjC">stannates</a>," with the nominal formula <a href="/facts/Sodium_stannate/2Xx7zlr4">Na2SnO3</a>.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> Dissolving the solidified SnO2/NaOH melt in water gives Na2[Sn(OH)6], "preparing salt," which is used in the dye industry.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p> <h2 id="uses">Uses</h2> <p>In conjunction with vanadium oxide, it is used as a catalyst for the oxidation of aromatic compounds in the synthesis of <a href="/facts/Carboxylic_acid/U4xuMosy">carboxylic acids</a> and acid anhydrides.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p> <h3>Ceramic glazes</h3> <p>SnO2 is used as pigment in the manufacture of glasses, enamels and <a href="/facts/Ceramic_glaze/eWJqJTYk">ceramic glazes</a>. Thousands of tons of SnO2 are produced annually for this application. Pure SnO2 gives a milky white colour; other colours are achieved when mixed with other metallic oxides e.g. <a href="/facts/Vanadium(V)_oxide/LPtvz4pq">V2O5</a> yellow; <a href="/facts/Chromium(III)_oxide/5C6XdjrQ">Cr2O3</a> pink; and <a href="/facts/Antimony_pentoxide/0uUFFAER">Sb2O5</a> grey blue.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a><a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> This use probably led to the discovery of the pigment <a href="/facts/Lead-tin-yellow/erk0lXEB">lead-tin-yellow</a>, which was produced using tin(IV) oxide as a compound.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> The use of tin(IV) oxide has been particularly common in glazes for <a href="/facts/Earthenware/xz481YuA">earthenware</a>, sanitaryware and wall tiles; see the articles <a href="/facts/Tin-glazing/2tdPtuLg">tin-glazing</a> and <a href="/facts/Tin-glazed_pottery/ywk4JX5b">Tin-glazed pottery</a>. Tin oxide remains in suspension in vitreous matrix of the fired glazes, and, with its high refractive index being sufficiently different from the matrix, light is scattered, and hence increases the <a href="/facts/Opacity_(optics)/l90q0sAV">opacity</a> of the glaze. The degree of dissolution increases with the firing temperature, and hence the extent of opacity diminishes.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> Although dependent on the other constituents the solubility of tin oxide in glaze melts is generally low. Its solubility is increased by Na2O, K2O and B2O3, and reduced by CaO, BaO, ZnO, Al2O3, and to a limited extent PbO.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> </p> <h3>Glass coatings</h3> <p>SnO2 coatings are valued as <a href="/facts/Transparent_conducting_oxide/jBUvd5Fa">transparent conducting oxides</a> (TCOs). Like other TCOs, SnO2 has significant <a href="/facts/Electrical_conductivity/akyCKvMy">electrical conductivity</a> but is transparent, an unusual combination of properties. Windows coated with SnO2 also reflect <a href="/facts/Infrared_radiation/44XdJNcc">infrared radiation</a>, which is relevant to temperature control for <a href="/facts/Smart_window/trssnScQ">smart windows</a>.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> Coatings can be applied using <a href="/facts/Chemical_vapor_deposition/7khLxeU5">chemical vapor deposition</a>, vapour deposition techniques that employ <a href="/facts/Tin(IV)_chloride/MgGG3FJJ">SnCl4</a><a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> or organotin trihalides<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> e.g. <a href="/facts/Butyltin_trichloride/vYt7w1oB">butyltin trichloride</a> as the volatile agent. This technique is used to coat glass bottles with a thin (<0.1 μm) layer of SnO2, which helps to adhere a subsequent, protective polymer coating such as polyethylene to the glass.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> </p><p>Thicker layers doped with Sb or F ions are electrically conducting and used in electroluminescent devices and photovoltaics.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> </p> <h3>Gas sensing</h3> <p>SnO2 has been evaluated as <a href="/facts/Gas_detector/QHH5sQgp">sensors of combustible gases</a> including <a href="/facts/Carbon_monoxide_detector/YQBfbVFt">carbon monoxide detectors</a>. In these the sensor area is heated to a constant temperature (few hundred °C) and in the presence of a <a href="/facts/Combustibility/1QexTQ4T">combustible</a> gas the <a href="/facts/Electrical_resistivity/akyCKvMy">electrical resistivity</a> drops.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> </p> <h3>Historical uses</h3> <p>This oxide of tin has been utilized as a <a href="/facts/Mordant/KGkRrkba">mordant</a> in the dyeing process since ancient Egypt.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> A German by the name of Kuster first introduced its use to London in 1533 and by means of it alone, the color scarlet was produced there.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> </p><p>Tin(IV) oxide for this use is sometimes called as "putty powder"<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> or "jeweler's putty".<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> </p> <h3>Polishing</h3> <p>Tin(IV) oxide can be used as a polishing powder,<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> sometimes in mixtures also with lead oxide, for polishing glass, jewelry, marble and silver.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> </p> <h2 id="further-reading">Further reading</h2> <ul><li>Wang, Chun-Ming; Wang, Jin-Feng; Su, Wen-Bin (2006). "Microstructural Morphology and Electrical Properties of Copper- and Niobium-Doped Tin (IV) oxide Polycrystalline Varistors". <i>Journal of the American Ceramic Society</i>. 89 (8): 2502–2508. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1111%2Fj.1551-2916.2006.01076.x">10.1111/j.1551-2916.2006.01076.x</a>.<a href="http://www.scielo.br/pdf/mr/v9n3/31795.pdf">[1]</a></li> <li>Dibb A.; Cilense M; Bueno P.R; Maniette Y.; Varela J.A.; Longo E. (2006). <a href="https://doi.org/10.1590%2FS1516-14392006000300015">"Evaluation of Rare Earth Oxides doping SnO2.(Co0.25,Mn0.75)O-based Varistor System"</a>. <i>Materials Research</i>. 9 (3): 339–343. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1590%2FS1516-14392006000300015">10.1590/S1516-14392006000300015</a>. <a href="/facts/Hdl_(identifier)/rdebSxmC">hdl</a>:<a href="https://hdl.handle.net/11449%2F30580">11449/30580</a>.</li> <li>A. Punnoose; J. Hays; A. Thurber; M. H. Engelhard; R. K. Kukkadapu; C. Wang; V. Shutthanandan & S. Thevuthasan (2005). <a href="http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1170&context=usdoepub">"Development of high-temperature ferromagnetism in SnO2 and paramagnetism in SnO by Fe doping"</a>. <i>Phys. Rev. B</i>. 72 (8): 054402. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2005PhRvB..72e4402P">2005PhRvB..72e4402P</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1103%2FPhysRevB.72.054402">10.1103/PhysRevB.72.054402</a>.</li></ul> <h2 id="further-reading">Further reading</h2> <ul><li><a href="http://www.pilkington.com/resources/howpilkingtonenergyadvantageworks.pdf">"How Pilkington Energy Advantage™ Low-E Glass Works"</a> (PDF). Pilkington Group Limited. July 18, 2005. Retrieved December 2, 2012.[<i>permanent dead link</i>] Technical discussion of how SnO2:F is used in <a href="/facts/Low-emissivity/DwawYlP5">low-emissivity</a> (low-E) windows. The report includes reflectance and transmittance spectra.</li> <li><a href="https://www.cdc.gov/niosh/npg/npgd0616.html">"NIOSH Pocket Guide to Chemical Hazards - Tin(IV) oxide (as Sn)"</a>. Centers for Disease Control and Prevention. April 4, 2011. Retrieved November 5, 2013. Information on chemical safety and exposure limits</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 447–48. ISBN 978-0-08-022057-4. <a href="978-0-08-022057-4" target="_blank">978-0-08-022057-4</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 447–48. ISBN 978-0-08-022057-4. <a href="978-0-08-022057-4" target="_blank">978-0-08-022057-4</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Solid State Chemistry: An Introduction Lesley Smart, Elaine A. Moore (2005) CRC Press ISBN 0-7487-7516-1 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5 <a href="0-12-352651-5" target="_blank">0-12-352651-5</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5 <a href="0-12-352651-5" target="_blank">0-12-352651-5</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5 <a href="0-12-352651-5" target="_blank">0-12-352651-5</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Tin: Inorganic chemistry, J L Wardell, Encyclopedia of Inorganic Chemistry ed R. Bruce King, John Wiley & Son Ltd., (1995) ISBN 0-471-93620-0 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Graf, Günter G. (2000). "Tin, Tin Alloys, and Tin Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a27_049. ISBN 978-3-527-30385-4. <a href="978-3-527-30385-4" target="_blank">978-3-527-30385-4</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Graf, Günter G. (2000). "Tin, Tin Alloys, and Tin Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a27_049. ISBN 978-3-527-30385-4. <a href="978-3-527-30385-4" target="_blank">978-3-527-30385-4</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5 <a href="0-12-352651-5" target="_blank">0-12-352651-5</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. p. 844. ISBN 978-0-08-022057-4. <a href="978-0-08-022057-4" target="_blank">978-0-08-022057-4</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5 <a href="0-12-352651-5" target="_blank">0-12-352651-5</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5 <a href="0-12-352651-5" target="_blank">0-12-352651-5</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 447–48. ISBN 978-0-08-022057-4. <a href="978-0-08-022057-4" target="_blank">978-0-08-022057-4</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5 <a href="0-12-352651-5" target="_blank">0-12-352651-5</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>'The Glazer's Book' – 2nd edition. A.B.Searle.The Technical Press Limited. London. 1935. <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>Hermann Kühn, 1967, "Blei-Zinn-Gelb und seine Verwendung in der Malerei", Farbe und Lack 73: 938-949 <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>'A Treatise On Ceramic Industries.' E.Bourry. Fourth edition. Scott, Greenwood & son. London. 1926. <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>'Ceramic Glazes' Third edition. C.W.Parmelee & C.G.Harman. Cahners Books, Boston, Massachusetts. 1973. <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>Batzill, Matthias; Diebold, Ulrike (2005). "The surface and materials science of tin oxide". Progress in Surface Science. 79 (2–4): 47–154. Bibcode:2005PrSS...79...47B. doi:10.1016/j.progsurf.2005.09.002. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 447–48. ISBN 978-0-08-022057-4. <a href="978-0-08-022057-4" target="_blank">978-0-08-022057-4</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>US 4130673, Larkin, William A., "Process of applying tin oxide on glass using butyltin trichloride", published 1978-12-19, assigned to M & T Chemicals Inc. <a href="https://worldwide.espacenet.com/textdoc?DB=EPODOC&IDX=US4130673" target="_blank">https://worldwide.espacenet.com/textdoc?DB=EPODOC&IDX=US4130673</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li> <li id="fn:23"><p>Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 447–48. ISBN 978-0-08-022057-4. <a href="978-0-08-022057-4" target="_blank">978-0-08-022057-4</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> <li id="fn:24"><p>Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 447–48. ISBN 978-0-08-022057-4. <a href="978-0-08-022057-4" target="_blank">978-0-08-022057-4</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li> <li id="fn:25"><p>Joseph Watson The stannic oxide semiconductor gas sensor in The Electrical engineering Handbook 3d Edition; Sensors Nanoscience Biomedical Engineering and Instruments ed R.C Dorf CRC Press Taylor and Francis ISBN 0-8493-7346-8 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></p></li> <li id="fn:26"><p>Sir Thomas Edward Thorpe History of Chemistry (1909) Vol. 1, pp. 11-12. <a href="/wiki/Thomas_Edward_Thorpe" target="_blank">/wiki/Thomas_Edward_Thorpe</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></p></li> <li id="fn:27"><p>Thomas Mortimer, A General Dictionary of Commerce, Trade, and Manufactures (1810) "Dying or Dyeing" <a href="/wiki/Thomas_Mortimer_(writer)" target="_blank">/wiki/Thomas_Mortimer_(writer)</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></p></li> <li id="fn:28"><p>Inorganic & Theoretical chemistry, F. Sherwood Taylor, Heineman, 6th Edition (1942) <a href="#fnref:28" class="footnote-back-ref">↩</a></p></li> <li id="fn:29"><p>"Material Name: stannic oxide". Museum of Fine Arts, Boston. February 10, 2007. Archived from the original on November 4, 2012. Retrieved March 29, 2013. <a href="https://web.archive.org/web/20121104011622/http://cameo.mfa.org/browse/record.asp?subkey=8907" target="_blank">https://web.archive.org/web/20121104011622/http://cameo.mfa.org/browse/record.asp?subkey=8907</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></p></li> <li id="fn:30"><p>Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5 <a href="0-12-352651-5" target="_blank">0-12-352651-5</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></p></li> <li id="fn:31"><p>"Material Name: stannic oxide". Museum of Fine Arts, Boston. February 10, 2007. Archived from the original on November 4, 2012. Retrieved March 29, 2013. <a href="https://web.archive.org/web/20121104011622/http://cameo.mfa.org/browse/record.asp?subkey=8907" target="_blank">https://web.archive.org/web/20121104011622/http://cameo.mfa.org/browse/record.asp?subkey=8907</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></p></li> </ol>