Around 1903, one of two Pack Houses at the California Powder Works stood where 267 Keystone is today, where 25- and 50-pound cans of black powder were packed and prepared for shipment. Before the mid 1870s, powder was shipped in wooden kegs made by Chinese workers, but these were relatively fragile and could crack if handled improperly, letting in moisture that made the powder less effective. After can-making machinery became available, the Powder Works replaced the wooden kegs with tin ones, increasing the safety and reliability of the gunpowder. The men standing in the doorway wearing shiny aprons are covered in graphite, a dry form of carbon added to finished powder to keep it from clumping. Because powder dust often sifted onto the floor during packing, floors were swept regularly and brushes were placed at each door so workers could clean their shoes before entering. Grit on boots could ignite the dust on the floor much like striking a match, and shoes made with tacks or nails were forbidden for the same reason. Avoiding accidental sparks around gunpowder was a constant concern during manufacturing, and no recorded flash fires or explosions ever occurred in the Pack House.