Hard money shortages prevailed because US exports exceeded imports and Peruvian and Mexican gold and silver sources failed to replenish specie reserves. Due to this scarcity, the terms of the bank's incorporation provided for private subscribers to invest with a combination of metallic currency and government stock. Further, they were granted an indulgence by Bank directors that effectively waived the specie requirement: ultimately, investors were allowed to purchase Bank shares on the security of the stock itself. Under its charter guidelines, the SBUS was expected to acquire specie totaling $28 million by the time it opened for business; but with only $2 million secured when it commenced operations, the bank was compelled to purchase specie at usurious rates from the London financial markets in 1817 and 1818, overburdening SBUS credit.