Metal has been used in construction for centuries, but the use of structural steel and iron is a relatively recent invention (late 19th century). From the 17th to late 19th century, metals were primarily for fasteners (nails and screws), hardware (hinges), strapping, plumbing and flashing (primarily lead sheet and copper sheathing). More recently tin and aluminum have been added to the architectural toolkit, along with a dizzying array of new alloys developed each decade of the 20th century. Of course metal has also been used in leafs and foils as architectural embellishment for millennium. Metals come from the earth's surface in the form of ores that must be heat extracted. By manipulation of even minute amounts of added elements, along with tempering and other treatment methods, steel, for example, can be formed into infinite permutations that range from soft, ductile steel to heat-treated spring steel and steel so hard and tough it is used to cut other steels. A common problem in historic buildings is the later addition of structural steel or iron to correct perceived problems or design flaws. With changes in construction and the development of engineering values that rely on the strength of steel as the basis of our modern structural systems, many historic buildings have been retrofit without regard for the different way that these building materials behave and the problems steel can cause in older buildings. As a result, much of modern preservation work must focus on subtracting these steel repairs. The greater thermal expansion and contraction of steel tends to concentrate stresses in historic building systems that were otherwise designed to spread loads and accommodate a degree of movement. Once the localized expansion forces of steel push historic masonry apart, water infiltration compounds the problem. Corrosion of steel and iron creates as much as a seven-fold increase in the mass of the metal, fracturing adjacent areas.