Yüzyıl dönümünde İstanbul?da yapı dış cephelerinde kullanılan yapay taşların mimari değerlendirmesi

The material defined as "artificial stone", frequently used on the façades of the late 19th and early 20th century buildings, is a mixture of binder, aggregate and other additives and may either be applied directly as a coating on wall surfaces or precast in moulds and then attached to façades as decorative architectural elements. One of the effects of the Industrial Revolution in 19th century was a tendency to standardize architectural production, which in turn moved away from time-consuming and costly traditional techniques in search of those in accordance with the dynamic social, economic and cultural structure of the period. One of the resulting solutions was the rapidly mass-produced artificial stones that replaced the traditional stone masonry. The aim of this research is to evaluate the use of artificial stone on the exterior façades of buildings dated to turn of the last century in terms of architecture and conservation science. The program includes a detailed research in written sources focusing on material history, studying their invention, development and use as well as architectural design and construction techniques and styles. The sources which may be categorized as a history of materials and techniques, including books, booklets, reports and patent texts are very colourful, detailed and bear much first-hand information as well as extensive cross-references. It is very interesting to see that not only these early modern scientist but also the authors who are in fact master craftsmen were very well informed about the work of their predecessors as well as that of their colleagues in different countries. Turkey and more specifically Istanbul, located on the periphery of these developments in architecture, techniques and materials, was also affected contemporaneously. However, it is only partially possible to trace this transformation in publications and archive material. The main sources of information are the period buildings themselves. Thus, the use of artificial stone was evaluated with reference to architectural, material and application characteristics. Field-work enabled the selection of ca. 30 buildings for sampling, and the number was reduced to 15 in the final stage, focusing on those buildings reflecting the characteristics of their period. Samples were taken from various types of artificial stone used in architectural programs of different styles. Generally speaking, artificial stones consist of flat plaster coatings applied in-situ and architectural elements with decorative reliefs, some of which may be load-bearing and which may be produced with different types of moulds either in-situ or precast at the workshop. The experiments, conducted according to related national and international standards based on research programs recommended for similar mortar and plaster samples in literature, enabled the determination of the physical, raw material and mineralogical characteristics of the samples. Results indicate that all the binders have hydraulic quality, some being artificial cements and/or natural water limes and others being fat limes mixed with mostly artificial pozzolanic additives, such as brick dust and ash, charcoal or slag as well as some natural pozzolanas such as opal. Some of the limes included magnesite, which is known to enhance the mechanical properties, whereas the identification of calcium sulphate (in XRD and SEM-EDS analyses) enabled the identification cements, as this additive is known to have been used since the 1850s to regulate the setting characteristics of these binders. Those including higher ratios, on the other hand, may be identified as calcium sulphate based binders, which appear frequently in literature. The physical characteristics of the plasters are somewhat different, compared to similar mortars and concretes: The densities and binder/aggregate ratios are generally higher, porosities lower and the size of the aggregates is smaller. The plasters may have single or multiple (two to three) layers. As it may be expected, compared to the lower rough layers, the upper finishing layers are denser, have higher binder/aggregate ratios and include aggregates smaller in size and usually lighter in colour, such as white sands and lime stones as well as marble pieces in some cases. Fibrous aggregates as well as artificial aggregates with pozzolanic qualities are used in almost all samples, including those with hydraulic binders, indicating a continuity of tradition and/or distrust in these new materials. Keywords: Artificial stone, the development of hydraulic binders, historic cements.“Yapay taş” olarak tanımlanan malzeme, 19. yüzyıl sonu ve 20. yüzyıl başı mimarlığında özellikle dış cephelerde sıklıkla kullanılan, bağlayıcı, agrega ve diğer bazı katkılardan oluşan ve doğrudan yüzeylere uygulanan ya da kalıplara dökülerek hazırlandıktan sonra yüzeylere sabitlenen, bir kaplama ve bezeme malzemesi ve tekniğidir. 19. yüzyılda gerçekleşen Endüstri Devrimi’nin de etkisiyle standartlaşan ve serileşen mimarlık üretimi, gerektirdiği iş gücü ve süre nedeniyle maliyeti yüksek geleneksel sistemlerden uzaklaşarak, döneminin dinamikleşen sosyal, ekonomik ve kültürel yapısına uygun malzeme ve uygulama yöntemlerinin arayışına girer. Ortaya çıkan çözümlerden biri, pahalı ve yavaş geleneksel taş işçiliği yerine benzer görüntüye sahip, daha hızlı ve seri olarak üretilebilen yapay taş teknolojisidir. Bu araştırmanın amacı, sözü edilen dönemde yapı dış cephelerinde kullanılan yapay taşları mimarlık ve koruma bilimi açısından değerlendirmektir. Öncelikle malzemelerin gelişimi ve yaygın kullanıma girişine yönelik, mimari uygulama yöntemleri ve üslupları da kapsayan geniş bir kaynak taraması gerçekleştirilmiştir. Kullanımla ilgili incelemeler ise yapı dış cephelerinin mimari, malzeme ve uygulama niteliklerine odaklanmıştır. Arazi çalışması sonucu, döneminin özelliklerini yansıtan yapılar seçilerek, yapı dış cephelerinin mimari düzeni içinde yer alan farklı yapay taş uygulamalarından örnekler alınmıştır. Yapay taş başlığı altında farklı sıva teknikleri ile kabartma bezekli mimari elemanlar ele alınmaktadır. Benzer harç ve sıvalar için literatürde önerilen çalışma programları esas alınarak ilgili ulusal ve uluslararası standartlara uygun olarak gerçekleştirilen deneylerle, temelde hidrolik nitelik taşıyan harçlardan oluşan bu örneklerin fiziksel, hammadde ve mineralojik özellikleri belirlenmiştir. Anahtar Kelimeler: Yapay taş, hidrolik bağlayıcıların gelişimi, erken çimentolar