PICT:0955979/C1/1 A spinel-forsterite ceramic comprising SiO,2 MgO and Al2O3 has a composition lying within the volume of the quaternary SiO2-MgO-AL2O3-BaO system bounded by lines connecting, as shown in the drawing, compositions consisting of by weight per cent:- (A) 32% MgO, 45% Al2O3, 23% SiO2; (B) 35.5% MgO, 54% Al2O3, 10.5% SiO2; (C) 49.7% MgO, 19% Al2O3, 31.3% SiO2; (D) 43.5% MgO, 20% Al2O3, 36.5% SiO2; (E) 1% BaO, 53.5% MgO, 7.9% Al2O3, 37.6% SiO2; (F) 1% BaO, 46.5% MgO, 11.9% Al2O3, 40.6% SiO2; (G) 1% BaO, 49.2% MgO, 18.8% Al2O3, 31% SiO2; (H) 1% BaO, 43.1% MgO, 19.8% Al2O3, 36.1% SiO2; (I) 3% BaO, 34.9% MgO, 52.4% Al2O3, 9.7% SiO2; (J) 3% BaO, 52.4% MgO, 7.7% Al2O3, 36.9% SiO2; (K) 3% BaO, 45.6% MgO, 11.6% Al2O3, 39.8% SiO2; (L) 3% BaO, 31% MgO, 43.7% Al2O3, 22.3% SiO2. The composition is vitrefied so that it contains crystalline spinel and forsterite, with glass as a minor constituent. The raw materials used for producing the ceramic are preferably the high purity oxides or compounds which decompose to the oxides on firing, or the naturally occurring materials such as clay or talc. The minus 200 mesh mixture is mixed with a polyvinyl alcohol solution and fired in an oxidizing atmosphere. The thermal expansion of the ceramic matches that of titanium between 25 DEG C. and 800 DEG C. and is used to form sealed titanium ceramic envelopes. Specification 798,450 is referred to.
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