Boric acid is used as an adjuvant for the liquid catalyst used to cure furan-based binder systems for forming a foundry mold in a warm box process. The binder system comprises furfuryl alcohol, a furan resin, and a furan monomer or oligomer containing at least two terminal hydroxymethyl groups. The binder system also comprises a latent acid curing catalyst and the boric acid. The boric acid improves tensile strength of the foundry mix as well as through cure of the binder.