12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 FEATURES AND DECAY OF CAST STONE ELEMENTS IN NEW YORK CITY BUILDINGS IN COMPARISON WITH CAST STONE IN MILAN Roberto Bugini1, Mariachiara Faliva2 and Luisa Folli3 1 CNR – ICVBC, Istituto per la Conservazione e la Valorizzazione dei Beni Culturali, via Roberto Cozzi 53, 20145 Milan (Italy) 2 Thornton Tomasetti, Inc., 51 Madison Avenue, New York, NY 10010 (USA) 3 Independent Researcher, viale Calabria 18/B, Lodi (Italy) Abstract Cast stone was a new construction material introduced in the second half of the 19th century for the production of architectural elements such as sills, window frames, cornices, ornamentations, statues; technological advances allowed use of this material for complicated shapes with considerable savings in costs compared to carved natural stone. Cast stone use followed similar patterns in North America and Europe. This paper compares and contrasts production and performance of this innovative material between two countries by analyzing material samples from historic structures in New York and Milan. Samples dating to the first decades of the 20th century were analyzed using petrographic methods. Samples were described in terms of aggregate composition, aggregate grain size and binder colour. The main decay observed was surface erosion of the cementing matrix. This decay can be attributed to exposure to the elements and pollution, but can be accelerated by the type of tooling used to finish the cast stone surface. Keywords: cast stone, artificial stone, New York City, Milan 1. Introduction Since the first half of the 19th century improvements in material science studies and technologies led to many attempts at finding new and better methods of construction. One of the outcomes of these experiments was the introduction of the first modern hydraulic cements, above all Portland cement 1. A natural consequence was the production of “artificial stone,” a mix of cement and lime in various forms and recipes with a varietyDRAFT of aggregates, formed in blocks or other decorative elements. Throughout the second half of the 19th century various attempts were made in different European countries and in USA to produce artificial stone. François Coignet in France was one of the pioneers in this field and started producing concrete or béton aggloméré by mixing Portland cement, lime, hydraulic lime, and aggregate (Gilmore 1871: 1-73). Cast stone, especially in block form, started to introduce a new way of building based on some desirable and useful characteristics: it permitted faster construction and it provided fire protection, reducing production and construction costs. Initially used mainly for these reasons, starting from the 20th century cast stone, also known as “artificial stone” or “concrete stone”, increased in popularity due to two of its most valuable features: efficient production of repetitive pieces using industrial processes and the ability to provide custom finishes. It eventually gained acceptance equal to natural stone, when architects decide to use cast stone elements throughout buildings. Cast stone 1 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 was used not only for sills, window frames and cornices, but also for statues, elaborate ornamentation and pinnacles. Symptomatic of the new role and perception acquired by cast stone in the 20th century is an article by Henry P. Warner, president of one of the main manufacturers of cast stone 2, published in 1927 on the journal of the American Concrete Institute: “there is a rapidly increasing number of architects and engineers who are now using the product because of its merits and because they find that they need it and that it has qualities which make it valuable” (as opposed as its mere inexpensiveness), and again “our company is in the production of a material which does not in any way resemble any particular natural stone”. The popularity of cast stone was not limited to the United States or France: a similar pattern occurred in Italy where cast stone composition, texture, grain-size, color and surface tooling first tried to imitate architectural elements made of natural stone, but then acquired its own individuality. This study compares production and use of this innovative material in the United States and in Italy by analyzing samples from 20th century buildings in New York, Milan and Lombardy; it takes into account ashlars, blocks for ornamental purposes as well as slabs used for veneers. 2. Cast stone production and use in two cities 2.1 Examples from New York Early known as artificial stone, the use of cast stone construction elements spread from the USA from the second half of the 19th century. Various attempts were made by using a variety of recipes, mostly including hydraulic and hydrated lime and local natural cements because imported Portland and natural cements from Europe were still very expensive (Gilmore 1871: 49 and Jester 1995: 87). Development of artificial stone manufacturers occurred on the West cost of the United States (mainly the Bay Area) as well on the East coast, being more numerous in the Northeast due to natural cement quarries and producers in the vicinity and harbours to which Portland cement was shipped from Europe (Tomlan 1974: 5). An early example was Frear stone, patented by George Frear of Chicago in 1868, a mix of hydraulic cement, aggregate and shellac; around the same time the American Building Block Company was producing blocks made of common lime and aggregate, also knownDRAFT as Foster process, while Sorel’s artificial stone was manufactured by Union Stone Company in Boston, developed in 1853 based on experimentation by the French chemist Sorel who added hydraulic cement to the mix. In 1868 the Pacific Stone and Concrete Company in San Francisco was producing calcium silicate based blocks under the patent of the Englishman Frederick Ransome while the New York and Long Island Coignet Stone Company produced artificial stone based on the French recipe (Jester 1995: 87-88, Pieper: 1-3, Prudon 1989: 81-84 and Tomlan 1974) 3. Cast stone manufacturing started with production of simple concrete blocks. These first attempts had no aesthetic ambitions and were cast as solid or hollow blocks. Poor quality and failures also characterized this period (Whipple 1915: 9-10 and Gillespie 1979: 30). By the beginning of the 20th century the production of domestic Portland cement, the decrease in price of imported cement and the proven higher quality of cement had 2 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 the effect of abandoning early mixes. This also made the full development of the cast stone industry possible. Cast stone was produced following three main methods: the tamped process, in which the element was cast using a mix of dry consistency and removed from the mould right after; the pressed process which used a slightly more fluid mix pressed by heavy machinery into moulds; and the wet process in which a very fluid mix was poured into moulds and allowed to cure completely. Although more expensive, the wet process was considered the best practice because the units produced with this method were claimed to achieve more strength, increased hardness and improved water tightness 4. To obtain the desired appearance various surface treatments were employed, including abrading the surface on special rubbing beds, tooling using the same traditional tools used for natural stone such as points and variously shaped chisels, and etching the surface by applying or immersing the element in a solution of hydrochloric acid and water. Dry-tamped cast stone presented a surface requiring less work to expose the aggregate, while cement and aggregate fines had to be removed to expose the aggregate in wet-cast units. A smooth surface was referred to as Terrazzo texture5. Sand moulds were used to give a smooth, sandy appearance to the unit (Havlik 1927: 213) as well as to regulate water absorption during the curing process (Whipple 1915: 113). Colour was obtained by using coloured stones as aggregate and pigments in the bulk cement. The main stone used was ground marble which, combined with black copper slag, was intended to replicate granite (Figure 1). Natural granite was also used as aggregate, sometimes with the granite imparting its natural colour. To reduce costs, a common practice was to pour in the mould a first layer (called facing, typically 3/4" to 1 ½" thick or 2 - 4 cm) of cement and costly aggregate and pigments and to fill the remaining portion with a less expensive, coarser mix that would not be exposed to view (Whipple 1915: 126) (Fig. 2). 2.2 Examples from Milan and Lombardy Since the mid-19th century the increase of the cost of stone supply and of stone working encouraged the exploitation of a new material with the same strength of the natural stone but easier to shape in different forms and considerably less expensive. The material called pietra artificiale (artificial stone) or cemento decorativo (decorative cement)DRAFT was made directly on site using moulds made of wood, metal, gypsum or glue. On the contrary, other artificial materials (brick, terracotta) needed factories using expensive machinery. The first cast stone attempts were made using hydraulic lime or magnesium-based lime or gypsum as binders, but the poor results in terms of durability redirected the manufacturers to use Portland cement-based binders. The best results were obtained by Società Italiana dei Cementi e delle Calci Idrauliche (established 1854 in Bergamo) using a Portland cement called Cemento Portland naturale, which used Cretacic marly limestones quarried in Val Seriana (Scanzo, Villa di Serio, Comenduno, Pradalunga), some kilometers north of Bergamo. The Portland cement binder was mixed with different kinds of aggregate to obtain a material with good mechanical properties and able to match the appearance of natural stone; great care was devoted to the preparation of individual components (Fumagalli 1964, 13-62).