As part of an ongoing project at the World Heritage Site of Bukhara, we investigated the glue used in the construction of the Khoja Zaynuddin mosque. Analysis by a range of techniques confirmed that it consisted of a collagen-based glue. However, the glue contained many non-protein constituents. The presence of lipid material suggests that the glue was produced by a relatively unsophisticated process. More surprisingly, various marker compounds of urine were found. Study of the viscosity of a mixture of a modern collagen-based glue and urea showed that the presence of urea at a concentration typical of human urine reduced the viscosity of the glue, thus improving its spreading characteristics and enabling its use at a lower temperature. Whilst the advantages of adding urea to glue were known to craftsmen in Europe and America in the 19th century, it would appear that the builders of Bukhara were aware of the technique, using less pure components, at a significantly earlier date.