Bitumen brown oil paint reconstructions based on 19th century production records from the British colourman Winsor & Newton were analysed using thermally assisted methylation with tetramethylammonium hydroxide (Py-TMAH-GC/MS), and pyrolysis comprehensive two-dimensional gas chromatography (Py-GCxGC/MS). Reconstructions were compared with the starting material, Trinidad Lake asphalt (TLA), to determine how detectable this asphalt is after heat processing in lead treated linseed oil. The use of brown paint containing asphalt/bitumen (the names are used interchangeably) was blamed for severe film-forming defects in 18th and 19th century oil paintings, yet this material has rarely been identified in earlier studies of historical paintings. This research offers a possible explanation for the paucity of evidence as it reveals that asphalt markers identified in the TLA disappear in the first stage of reconstructing bitumen brown oil paint.