It is generally known to manufacture infrared radiation absorbing soda-lime-silica glass by the incorporation therein of iron. The iron is present in the glass as both ferrous oxide (FeO) and ferric oxide (Fe.sub.2 O.sub.3). The ferrous oxide imparts a bluish tint to the glass, and provides for the absorption of radiant energy at the infrared end of the visible spectrum. Ferric oxide, however, imparts a yellowish tint, and causes the glass to absorb ultraviolet radiation. Consequently, a proper balance must be achieved in order to produce a blue-colored glass having desirable infrared energy absorption and light transmittance characteristics.
U.S. Pat. No. 3,652,303 discloses a heat absorbing blue soda-lime-silica glass composition wherein at least 80% of the total iron in the glass is maintained in the ferrous state, during the forming and any subsequent reheating process, by the inclusion of an amount of tin metal or stannous chloride in the melt so as to result in a tin concentration (the majority of which is in the stannous state) in the finished glass article of up to 15% by weight. The purpose of the tin is to serve as an internal reducing agent in the glass, to prevent any tendency toward conversion of the ferrous ion to the ferric species arising from oxidizing influences, such as for example the diffusion of oxygen from the air into the glass during the reheating incident to conventional subsequent fabrication processes like press bending or thermal tempering.
U.S. Pat. No. 2,755,212 discloses a blue-green infrared radiation absorbing glass containing iron, wherein the ration of ferric to ferrous iron is maintained by the addition of carbon as the reducing agent.
U.S Pat. No. 3,326,703 discloses a heat-absorbing boroaluminosilicate glass composition, wherein carbon, aluminum, or tin oxide are utilized to reduce the Fe.sub.2 O.sub.3 to FeO.
Japanese Patent Publication No. 60215546 (1985) discloses an infrared radiation absorbing clear glass composition which utilizes barium oxide to shift the absorption peak of FeO toward the infrared portion of the visible spectrum. The amber coloration caused by the sulfur, which is present as a reducing agent for the iron, is suppressed by the addition of zinc oxide.
Finally. U.S. Pat. No. 3,779,733 discloses a process for manufacturing heat absorbing glass, by employing a moderate amount of iron in the batch composition, and controlling the reducing conditions to maintain a large portion of the iron in the ferrous state. Control is achieved by conducting the melting and refining operation in discrete stages, without a large volume of melt being retained in either stage, while at the same time minimizing the sulfur content of the melt by avoiding the addition of sulfur-containing refining aids into the batch and by evacuating the process vessels so as to eliminate even trace impurities of sulfur.
It must be noted that the prior art referred to hereinabove has been collected and reviewed in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention.