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A board of sandwich construction which comprises two outer preformed support membranes consisting of or comprising fibrous material, the said membranes having interposed therebetween an aerated intermediate layer comprising at least 70% by weight based on the dry board of an unfired clay which conta

A board of sandwich construction which comprises two outer preformed support membranes consisting of or comprising fibrous material, the said membranes having interposed therebetween an aerated intermediate layer comprising at least 70% by weight based on the dry board of an unfired clay which contains less than 20% by weight of expandable minerals and the intermediate layer also including fibres therein. The boards of the present invention are intended mainly for use as plasterboard substitute.

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1. In a board of sandwich construction which comprises two outer preformed support membranes consisting of or comprising fibrous material, the said membranes having interposed therebetween an intermediate layer, the improvement which comprises the said intermediate layer being aerated, the said inte

1. In a board of sandwich construction which comprises two outer preformed support membranes consisting of or comprising fibrous material, the said membranes having interposed therebetween an intermediate layer, the improvement which comprises the said intermediate layer being aerated, the said intermediate layer consisting of at least 70% by weight based on the dry board of an unfired clay which contains less than 20% by weight of expandable minerals and the said intermediate layer including fibres therein. 2. A board according to claim 1 wherein the fibres are cellulosic fibres. 3. A board according to claim 2 wherein the cellulosic fibres are paper fibres. 4. A board according to claim 1 wherein the intermediate layer additionally contains a sizing agent to adhere the intermediate layer to the support membranes. 5. A board according to claim 4 wherein the sizing agent is starch. 6. A board according to claim 5 wherein the starch is selected from the group consisting of dextrin and corn starch. 7. A board according to claim 1 wherein the intermediate layer additionally contains a deflocculant. 8. A board according to claim 1 wherein the intermediate layer has no more than 20% porosity of diameter greater than 0.2 mm. 9. A board according to claim 1 wherein the unfired clay contains less than 5% by weight of expandable minerals. 10. A board according to claim 1 wherein the intermediate layer contains the following constituents, the percentages given being by weight based on the dry board: (a) 70-95% by wt clay (as hereinbefore defined) (b) 0-15% by wt paper fibres (c) 0-5% by wt starch (d) 0-2.0% by wt deflocculant (e) 0-1.0% by wt detergent (f) 0-7% by wt water 11. A board according to claim 1 wherein the preformed support membranes are layers of a cellulosic material. 12. A board according to claim 11 wherein the cellulosic material is reconstituted paper board. 13. A method of manufacturing a board of sandwich construction as claimed in claim 1, which method consists essentially of the steps of: (a) providing first and second preformed support membranes consisting of or comprising fibrous material; (b) providing the said aerated intermediate layer as defined in claim 1 between the preformed support membranes to form a composite board; (c) subjecting the composite board to an applied pressure of up to 30N/mm 2 ; (d) cutting the resulting composite into discrete lengths; and (e) drying the boards so produced; or (f) drying and rolling the boards during drying to control the dimensions of the boards so produced. 14. Method according to claim 13 wherein in step (c) the composite board is subjected to an applied pressure of less than IN/mm 2 . 15. Method according to claim 13 wherein the intermediate layer is prepared by adding a pulp of paper fibres to a mixture of an unfired clay, starch, deflocculant and water, subjecting this resulting mixture to centrifugal high shear mixing and aeration. 16. Method according to claim 13 wherein the intermediate layer is aerated by the addition thereto of a separately prepared foam detergent. 17. Method according to claim 13 wherein the intermediate layer is aerated by incorporating therein an unfoamed detergent and subjecting the mixture to air injection.