an assembly for high - pressure high - temperature processing comprising a can , a cap , a meltable sealant and sealant barrier , and a superhard mixture comprising superhard particles . the superhard particles may be positioned adjacent a substrate of cemented metal carbide . the can and cap contain the superhard mixture with the sealant barrier positioned within the assembly so as to be intermediate the sealant and at least a portion of the mixture , thereby preventing the sealant from coming in contact with the mixture during processing . the assembly is placed within a vacuum chamber and heated to a temperature sufficient to cleanse the assembly and then melt the sealant providing a hermetic seal for the assembly in preparation for further hpht processing .