In U.S. Pat. No. 3,926,602, the present inventors disclosed a process for producing glass-ceramic articles having highly-durable integral surfaces exhibiting distinctly metallic lusters. When developed upon previouslypolished surfaces, mirror-like finishes with good color and spatial reproduction of reflected images can be produced.
Such articles were made by melting batches for glasses consisting essentially, by weight on the oxide basis, of about 0.5-3.5% FeO, 3-6% Li.sub.2 O, 16-21% Al.sub.2 O.sub.3, 65-75% SiO.sub.2, and 1.5-7% RO.sub.2, wherein RO.sub.2 consisted of 1.5-6% TiO.sub.2 and 0-3% ZrO.sub.2. The molten batches were cooled to at least within the transformation range (optionally to room temperature) and glass articles shaped therefrom. The glass articles were then exposed to temperatures between about 850.degree.-1025.degree. C. to cause the growth of beta-quartz solid solution crystals in the body interior and an integral surface layer of hematite (alpha-Fe.sub.2 O.sub.3). This surface film was extremely thin, i.e., less than several thousand angstroms thick.
The body portion of the article was highly crystalline and contained beta-quartz solid solution as the sole or predominant crystal phase. This body portion displayed a degree of transparency depending upon the amount of iron in the base composition. The crystals of beta-quartz solid solution were very fine-grained, viz., less than about five microns in diameter.
Where desired, the glass ceramic article could be heated to about 1050.degree. C. and higher, at which temperatures the article became opaque as a result of the conversion of the beta-quartz solid solution crystals to beta-spodumene solid solution crystals. And, where the heat treatment at 1050.degree. C. and higher was not extended for too long a period, the reflective surface remained intact.
U.S. application Ser. No. 757,585, filed concurrently herewith by the present applicants, discloses the production of highly crystalline glass-ceramic articles having an integral surface layer containing metallic iron crystals, and wherein either beta-spodumene solid solution or nepheline constitutes the predominant crystal phase in the interior of the articles.
The method of the invention comprehends four general steps:
First, a batch of the proper composition is melted;
Second, the melt is simultaneously cooled and a glass article of a desired configuration shaped therefrom;
Third, the glass article is heated treated in an oxidizing atmosphere at about 1000.degree.-1300.degree. C. to cause the crystallization in situ of beta-spodumene solid solution or nepheline in the interior portion of the article while, at the same time, effecting crystallization in situ of alpha-hematite (Fe.sub.2 O.sub.3) crystals in an integral thin surface layer; and
Fourth, the crystallized article is exposed to an essentially dry reducing atmosphere at a temperature about 500.degree.-800.degree. C. to reduce the hematite in the surface layer to metallic iron.
U.S. application Ser. No. 757,584, filed concurrently herewith by the present applicants, discloses the manufacture of glass articles having very thin, integral surface layers containing hematite crystals (alpha-Fe.sub.2 O.sub.3) or magnetite crystals (Fe.sub.3 O.sub.4) or metallic iron crystals dispersed within a glassy matrix. The crystal content of the surface layer is very high, i.e., greater than about 50% by volume, such that the crystals provide an essentially continuous film.
The method of that invention for producing a glass having an integral surface layer containing hematite crystals involves two general steps:
First, a batch of the proper composition is melted and formed into a glass article of a desired configuration; and
Second, the glass article is subjected in an oxidizing atmosphere to a temperature of at least 675.degree. C., but less than 975.degree. C., to cause the crystallization in situ of hematite crystals in the surface of the glass.
When the glass article having the integral surface layer containing hematite crystals is exposed to a H.sub.2 O-containing reducing environment at a temperature between about 450.degree.-650.degree. C., the hematite crystals are reduced to magnetite.
In contrast, when the glass article having the integral surface layer containing hematite crystals (or magnetite crystals) is exposed to an essentially dry reducing atmosphere at a temperature between about 450.degree.-650.degree. C., the hematite crystals (or the magnetic crystals) are reduced to metallic iron.
The instant invention is founded in, and is an improvement upon, the invention set forth by the present applicants in U.S. Pat. No. 3,926,602. In essence, the glass-ceramic articles of that patent, having a metallic luster due to an integral thin surface film of hematite, provide the starting point of the present invention. Therefore, reference is specifically made to that patent for further details regarding the composition and process parameters of that invention.