ARC TUBE FOR A DISCHARGE LAMP

A sealing composition is provided for joining ceramic or cermet parts, eg in an arc tube for a discharge lamp. The sealing composition comprises, as starting materials, a mixture of alumina (Al2O3), silica (SiO2) and magnesia (MgO), the mixture comprising in the ranges of from 22% to 24.5% by weight of Al2O3, from 45.5% to 49.5% by weight of SiO2, and from 26.0% to to 32.5% by weight of MgO. The sealing composition contains less than 40% by weight (expressed as the percentage of the total weight of the mixture) of the crystalline phase alpha -Cordierite when subjected to a test procedure comprising the following steps: (i) thoroughly mixing the constituents of the sealing composition and sintering the mixture at 1200C for 10 hours to produce a friable lump which is crushed, transferred to a platinum (10% Rhodium) crucible, and heated in a muffle furnace at 1550C for 2 hours while the glass is in a molten condition; (ii) pouring the contents of the crucible into distilled water to form a brittle frit or cullet and, after drying at 160C, grinding the glass in an Agate Cone Grinder to form a powder of particle size less than 180 mu m; (iii) mixing the powdered glass with 4% by weight of a binder comprising a solution in toluene of methyl/butyl methacrylate, and pressing the mixture in a 16 mm diameter die at 100 MPa to produce 1g samples; (iv) sintering a sample continuously for 6 hours at a sintering tempterature equal to or greater than the temperature a seal made from the sealing composition is intended to withstand; and (v) evaluating, by X-ray Diffractometry, the proportion by weight (expressed as a percentage of the total weight of alumina, silica and magnesia) of the crystalline phase alpha -Cordierite in the samples sintered at the sintering temperature. o