Abstract: A precursor, in gel form, of an oxide having the formula (I), BaO.n(Al.sub.2x Cr.sub.2y O.sub.3), where 1.ltoreq.n.ltoreq.6.6, (x+y)=1, and 0.ltoreq.y.ltoreq.0.5, said oxide being derivable from the precursor gel by the application of heat, is prepared by mixing a solution of a barium salt with a solution of an aluminium salt or a solution of an aluminium salt and a solution of a chromium salt, and polymerising the mixture to produce said precursor gel. A mixture suitable for firing to an oxide of the formula (II), BaO.m(Al.sub.2x Cr.sub.2y O.sub.3), where 4.6.ltoreq.m.ltoreq.6.6; (x+y)=1; and 0.ltoreq.y.ltoreq.0.5, comprises at least one of:(a) barium oxide;(b) a clean thermal precursor of barium oxide; and(c) barium mono-aluminate, BaO.Al.sub.2 O.sub.3 ; with at least one of:(A) alumina, Al.sub.2 O.sub.3 ;(B) hydrated forms of alumina, such as boehmite, Al.sub.2 O.sub.3.H.sub.2 O; and(C) a clean thermal precursor of aluminium oxide; and, where y is not zero, with at least one of(D) chromium(III) oxide, Cr.

Abstract: The present invention provides a method of producing a shaped object by coating a pattern with a slurry of a refractory powder in a binder. The binder comprises an acidic zirconium salt and also a gellation delaying agent. The slurry is caused to gell by contacting it with ammonia vapor.

Abstract: A refractory shape is molded on a pattern by pouring over the pattern a mixture of two slurries each of which is stable but which together react to gell. The mixture is formed by merging two slurry streams immediately above the pattern.

Abstract: The invention concerns the binding of refractory powders into self-supporting `green` shapes which are fired to provide refractory objects. In accordance with the invention the binders are based on zirconium salts and include a separate gell time delaying agent to ensure satisfactory gelling and to achieve control and to allow complex shapes to be made.

Abstract: The invention concerns the binding of refractory powders into self-supporting "green" shapes which are fired to provide refractory objects. In accordance with the invention the binders are based on zirconium salts and include a dispersion of calcined magnesia in a mixture of water and a polyhydric alcohol. The magnesia reacts with the zirconium salt to form zirconium oxide/hydroxide which gells hard. In order to control the speed of gelling a separate gell time delaying agent is also optionally included.

Abstract: A gel is prepared through the intermediary of an acid hydrolysate. The hydrolysate is produced by the hydrolysis of an organic silicate with the metal salt whose oxide it is desired to gell.

Abstract: A rigid gel is prepared by treating a solution of an aluminium hydroxyhalide solution of a specified formula and composition with an acetate or lactate accelerator. The invention has particular application to the preparation of refractories.

Abstract: A mould for casting metals by the "lost wax" process is formed by coating an expendable pattern with refractory powder in a binder dispersed in an aluminum hydroxyhalide solution. The coating is hardened by an acetate or lactate.BACKGROUND OF THE INVENTIONThis invention relates to the preparation of moulds for casting metals and alloys, particularly to the preparation of moulds which are used in the investment casting, or "lost wax" process. These moulds are prepared by applying to an expendable pattern made of wax, plastic or a low melting alloy at least one coating of a refractory powder suspended in a suitable binding agent.SUMMARY OF THE INVENTIONAccording to the present invention there is provided a method of preparing a mould suitable for use in the casting of metals or alloys includes the step of coating an expendable pattern with a composition which is a finely divided refractory powder dispersed in an aqueous alcoholic solution of an aluminium hydroxyhalide of the general formula Al.sub.2 (OH).sub.