Arched door and window openings required the use of wood centering during erection, as did corridor arches and any type of vault or domed construction. Windows were kept small and to a minimum, and placed high on walls as a protective measure in case of Indian attack. A few of the missions had imported glass window panes, but most made do with oiled skins stretched tightly across the openings. Windows were the only source of interior illumination at the missions, other than the tallow candles made in the outposts' workshops. Doors were made of wood cut into planks at the "carpintería", and most often bore the Spanish "River of Life" pattern or other carved or painted designs. Carpenters used a ripsaw (or "pitsaw") to saw logs into thin boards, which were held together by ornate nails forged in the mission's blacksmith shop. Nails, especially long ones, were scarce throughout California, so large members (such as rafters or beams) which had to be fastened together were tied with rawhide strips. Connections of this type were common in post and lintel construction, such as that found over corridors. Aside from nails, blacksmiths fashioned iron gates, crosses, tools, kitchen utensils, cannons for mission defense, and other objects needed by the mission community. Settlements had to rely on cargo ships and trade for their iron supplies as they did not have the capability to mine and process iron ore.