A method for joining clad metal plates having a protective layer (27), e.g. titanium, and a substrate layer (28), e.g. carbon steel, includes firstly removing margins (29) of protective layer along edges of the clad metal plates to be joined. The substrate layers are then welded together to form an exposed substrate weld (31). Covering material (38) of the same type as the protective layer (27) is then located along the exposed substrate weld (31) to a level substantially flush with an outer surface of the protective layer (27). The substrate layer (28) is heated prior to welding the covering material so that the covering material is pre-stressed when cooled. The method may be used to fabricate reaction vessels having seams that do not stand proud of the remainder of the interior surface of the vessel. The low profile seams are less susceptible to erosion than has been the case in the past so that longer-life vessels can be produced. The invention encompasses vessels such as autoclaves, crucibles and reaction vessels formed of clad plate joined by the method.