Prelog's method was refined in 1956. The decarboxylation yield was increased by the addition of the Heinsdecker pathway (11%) and the Hoffman reaction (24%) that raised the total yield to 6.5%. The process was still too complex, and a more convenient method was found in 1957 by Paul von Ragué Schleyer: dicyclopentadiene was first hydrogenated in the presence of a catalyst (e.g. platinum dioxide) and then transformed into adamantane using a Lewis acid (e.g. aluminium chloride) as another catalyst. This method increased the yield to 30–40% and provided an affordable source of adamantane; it therefore stimulated characterization of adamantane and is still used in laboratory practice. The adamantane synthesis yield was later increased to 60% and 98% by ultrasound and super acid catalysts. Today, adamantane is an affordable chemical compound with a cost of about $1 a gram.