Building Materials

It is difficult to exaggerate the radical change that the new industrial world of the nineteenth century brought to architectural materials. Since the beginning of architectural history, the same basic substances had been employed. They were provided directly by nature and used in their natural or near-natural state, only cut, shaped, and dried into the functional forms of timbers, stone blocks, and clay bricks. The exceptions were lime mortar and Roman concrete. Metals, which had the tensile strength that masonry materials lacked, were employed in minor and supplementary ways. Bronze was expensive as well as brittle. Iron, the structurally more important metal, was available in limited quantities and uneven quality, and was too easily converted to rust by the elements. It was, therefore, restricted in use to things such as tie-rods and chains, and, along with bronze, to masonry clamps and decoration. Prior to the nineteenth century, the structural presence of iron in architecture was scarcely noticeable.

The Industrial Revolution changed all that. Iron materials became available in such large quantities that they could play far more than a minor architectural role. In 1800, the world production of iron stood at 825,000 tons; by 1830 it was 1,825 000 tons, and nearly 40,000,000 in 1900 — an increase of almost 50-fold over the century. The growth was not only in quantity, but quality as well. Iron, found bountifully in Earth's crust as an oxide, is a material of almost protean variability. It is not simply pure or impure, but can be made hard or soft, brittle or ductile, strong or weak. These qualities depend on carbon content, freedom from impurities (slag), and heating and cooling treatments of the refined metal, Traditionally, three versions existed: cast iron, wrought iron, and steel. Cast iron is the crudest form, containing the most impurities and thus being extremely brittle. Wrought iron, because it includes almost no carbon, is highly malleable (hence its name), but also comparatively soft. The optimum material is steel, which incorporates a restricted amount of carbon for hardness but is otherwise free of impurities, giving it great strength, and which, as a result of tempering treatments, is also malleable.

Cast and wrought iron came into prodigious manufacture in the early and mid-nineteenth century as a result of rapid growth in demand, new means to transport materials, and more efficient iron-founding techniques. But the mass production of steel required further technological innovations to rid the metal of weakening impurities and to control more perfectly the amount of carbon added. Such advances were made with the Bessemer process (put into use in 1860) and the open-hearth process of 1864; scientific iron metallurgy in the last third of the century perfected these techniques.

A significant but little-known fact is that the main form of increased steel production was technically not steel but a kind of wrought iron. It lacked a crucial property of true steel—its hardening power—yet it differed from the older forms of wrought iron because it was free from the weakening presence of slag, at the same time being malleable (unlike cast iron). It was called steel only because the name carried the status of a high-quality and high-priced product.

Another alternative new building substance was concrete, composed of an aggregate of broken stone, gravel, or other small chunks of hard matter embedded in a matrix of lime, sand, and water. First used in Roman times, its modern revival depended on the invention of portland cement in 1824, a substance of many times greater strength, durability, and fire resistance than ancient lime cement. Mass-produced concrete began to come into widespread use in the 1850s and 1860s, in the construction of the sewers of Paris, for example. However, even with portland cement, the use of concrete was still severely restricted by its low tensile strength, but the remedy was at hand in the newly available iron and steel; their properties complemented those of concrete. The latter material was cheap, easily molded into large structural forms with great compressive but little tensile strength. Iron and steel, on the other hand, were expensive, difficult to shape, yet endowed with extreme tensile strength and easily procurable in the simple form of long, thin bars.