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Abstract

A sealing composition is provided for joining crystalline ceramic or cermet parts, e.g. in an arc tube for a discharge lamp. The sealing composition comprises, as starting materials, a mixture of alumina (Al.sub.2,O.sub.3), silica (SiO.sub.2) and magnesia (MgO), the mixture comprising in the ranges of from 22% to 24.5% by weight of Al.sub.2 O.sub.3, from 45.5% to 49.5% by weight of SiO.sub.2, and from 26.0% to 32.5% by weight of MgO. The sealing composition contains less than 40% by weight, of the mixture, of the crystalline phase .alpha.-Cordierite when subjected to the following test procedure. (i) sintering the mixture at 1200.degree. C. for 10 hours to produce a friable lump which is crunched and heated at 1550.degree. C. for 2 hours while the glass is in a molten condition; (ii) pouring the resulting material into water to form a brittle frit and, after drying at 160.degree. C., grinding the glass to a powder particle less than 180 .mu.m; (iii) mixing the glass with 4% by weight of a solution in toluene of methyl/butyl methacrylate, and pressing the mixture in a 16 mm diameter die at 100 MPa to produce 1 g samples; (iv) sintering a sample continuously for 6 hours at a temperature equal to or greater than the temperature a seal made from the sealing composition is intended to withstand; and (v) evaluating the proportion by weight of the crystalline phase .alpha.-Cordierite in the samples sintered at the sintering temperature.

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