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Article

Art Nouveau Buildings, Examples of Innovative and Sustainable Approach—Case Study: Oradea, Romania

by
Mariana Ratiu
1,*,
Emil Traian Gligor
2,
George Florentin Tamas
2,
Ana Cornelia Peres
3 and
Mircea Bogdan Tataru
1
1
Faculty of Managerial and Technological Engineering, University of Oradea, 1 Universitatii Street, 410087 Oradea, Romania
2
Faculty of Construction, Cadastre and Architecture, University of Oradea, 4 Barbu Stefanescu Delavrancea Street, 410058 Oradea, Romania
3
Faculty of Environmental Protection, University of Oradea, 26 Magheru Street, 410048 Oradea, Romania
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(13), 5952; https://doi.org/10.3390/su17135952
Submission received: 29 March 2025 / Revised: 24 June 2025 / Accepted: 26 June 2025 / Published: 28 June 2025
(This article belongs to the Special Issue Architecture, Urban Space and Heritage in the Digital Age)
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Abstract

In the current context, where sustainability, energy efficiency, and reduction of environmental impact are omnipresent themes, the need and interest for conservation, restoration, and reuse of heritage buildings is a topic of real interest. The city of Oradea, engaged in the last decades on the path of a sustainable urban regeneration, has managed to bring back to life part of the valuable Art Nouveau architectural treasure it inherited from previous generations. This paper presents a part of a much wider research and studies on Art Nouveau buildings in Oradea, in the context of the needs arising from the many problems identified with the start of conservation and restoration works. After an introduction on the link between heritage buildings and sustainability and a review of the current context in Oradea related to this topic, one of the emblematic buildings under renovation, the Ullmann Palace, is presented, as well as the results of some physical-mechanical tests carried out. This is followed by some discussions on the innovative and sustainable character of Art Nouveau buildings and the importance of carrying out high-quality conservation and restoration works that are environmentally friendly, safe, and sustainable.

1. Introduction

Art Nouveau, a style that emerged in the late nineteenth and early twentieth centuries, is an artistic movement that is increasingly recognized and appreciated for its ornate patterns, its organic shapes, flowing lines and decorative elements inspired by nature, vibrant colors, and complex and intricate but very pleasing decorative elements, at the same time, its principles inspiring contemporary architecture in unique ways [1,2,3]. The Art Nouveau architectural legacies, which combined organic forms with the new technologies of the time, provide inspiration for today’s sustainable architecture, through the use of energy-efficient designs and materials that minimize environmental impact, creating a balance between innovation, sustainability and harmony with the environment, which aligns with modern green architecture practices. Art Nouveau buildings are thus true examples of good practices, their design and construction representing modern and innovative approaches for those times, a fusion of international innovations and local traditions [4], their beauty and durability withstanding the test of time.
In the context in which, in recent times, the interest in the conservation and restoration or rehabilitation of cultural heritage has increased, as well as against the background of awareness and assumption of environmental issues and in order to achieve the objectives of the European Green Deal [5], there is a need to find solutions and ways of ecological and sustainable conservation and restoration. Given that significant amounts of mineral and energy resources are consumed for the construction, use and renovation of buildings, with buildings absorbing 40% of total energy consumption at European level, the European Green Deal, from 2019, provides, among other measures, for the construction and renovation of buildings in an energy- and resource-efficient manner, as well as increasing the renovation rate of the real estate stock. In this context, a holistic approach to conservation, renovation, rehabilitation and reuse is needed, starting from the monitoring of the state of conservation, the development and adoption of ecological, efficient, sustainable and safe strategies, the use of traditional materials and methods, but adapted to the present and to each heritage objective, benefiting from the available digital tools and technologies, so that conservation and restoration to be environmentally friendly, sustainable and energy efficient, but also safe.
In this regard, numerous studies claim the need for the correct conservation and restoration of the architectural heritage of the Art Nouveau style [6], for consistent restorations, without loss of historical memory [7], but at the same time using new discoveries in the field, such as, for example, the coating of materials with nanoparticles, in order to better preserve them and increase their durability [8]. Regarding the safety, environmental conditions, and indoor air quality in old buildings, there are studies that investigate these conditions [9], so that they are useful and taken into account in the process of renovation, modernization, or reuse of buildings.
According to the International Institute for Sustainable Development (IISD), “sustainability is the foundation for today’s leading global framework for international cooperation, the 2030 Agenda for Sustainable Development and its Sustainable Development Goals”. Environmental sustainability is a crucial proactive approach for the survival of the current and future generations, requiring a responsible commitment from all to the entire planet, to the conservation of natural resources and ecosystems, and to maintain ecological balance [10]. Today, modern sustainability principles are expressed in a set of five basic principles, systemic in nature, coming from five different domains, but which are at the same time interdependent and interrelated, integrated in a balanced way. The flow of resources (material domain) to circulate in an economy that respects the ecological processes of the planet (economic domain), ensuring the maintenance of diversity of all forms of life (life domain), maximizing the level of freedom and potential of all people (social domain), in a universal ethic (spiritual domain). Through such integration and interaction, through responsible and committed choices and actions, the concept of sustainability can be put into practice, either at a general or at a particular level, in specific areas or situations [11].
Heritage and sustainable development are naturally very closely linked. A comprehensive literature review on the connection between the concepts of heritage buildings, sustainable development, and urban regeneration is presented in [12]. Heritage buildings, in themselves, comply with current principles of sustainable development, particularly in the cultural, social, and ethical fields, but also in the material and economic fields [13]. Sustainable conservation of these buildings will lead to long-term sustainability, giving many future generations the chance to enjoy their inestimable value. In [14], some well-known heritage buildings from around the world are presented where sustainable conservation and restoration practices have been implemented.
The sustainability indicators for historic buildings are grouped into three key areas of sustainable development: environmental (climate change, recyclability, reversibility), social (aesthetics, preservation of cultural heritage), and safety and resilience (safety factor of existing structural masonry, displacements) [15]. Environmental, economic, and social sustainability are supported and stimulated by cultural sustainability, including a high-quality built environment, Baukultur, as recognized and assumed by the Davos Declaration of 2018. Art Nouveau buildings fit perfectly into this trend of modern creativity, with detailed, traditional, and local construction methods, as well as innovative techniques [16,17]. As Stephen Gist said, “Sustainable architecture looks to the future by looking at the past”. Ancient building techniques and the use of local materials are considered sustainable practices, deeply rooted in history and constituting the basis for the realization of buildings that stand the test of time [18]. The theoretical and practical evolution of architectural heritage conservation in the context of sustainable development is extensively presented in [19].
Considering all this, and taking into account the wealth of Art Nouveau buildings that Oradea has, thus occupying a well-defined place on the map of cities with Secession/Art Nouveau architecture in Romania and Central Europe [20], as well as the interest shown by local authorities in the preservation and restoration of cultural heritage, but also the already evident impact in increasing the number of visitors and reviving tourism in the area, the need for studies and research in this field has emerged.
Based on the previous, the general objective of this paper is to highlight and demonstrate the innovative and sustainable character of Art Nouveau buildings and the importance of carrying out quality, ecological, sustainable, and safe conservation and restoration works. After a presentation of the context that shows the wealth of the Art Nouveau heritage in Oradea, one of the buildings under renovation, the Ullmann Palace, is presented, and the results of the physical-mechanical tests carried out on materials taken from this emblematic building are shown. This is followed by some discussions on how Art Nouveau buildings can be examples and models for an innovative and sustainable approach, and conclusions.
The novelty of this work is represented by the integrated approach to the architecture of Art Nouveau buildings and the materials used in their construction, which are innovative and sustainable. Also, the realization and presentation of the tests carried out on the sampled materials, in order to identify as accurately as possible their physical-mechanical properties and their origin. This is the context in which, during the restoration of the Ullmann Palace, which is still ongoing, difficulties were encountered in identifying the type of material and the manufacturer. In order to finalize the restoration, extensive studies and research are needed to provide relevant information to facilitate effective and sustainable conservation and restoration interventions. This work is part of a wider series of works that have been and will be presented, exhaustive research being in progress, for different buildings, already under renovation or to be included in this process, precisely in order to be useful in choosing the best solutions, for each case.

2. Context

Under the Interreg Central Europe European funding programme [21], a series of cooperation projects have been approved for a smarter, greener, and better-connected Central Europe. Through such a project, funded by the Interreg Danube Transnational Programme, entitled “Sustainable protection and promotion of the Art Nouveau heritage in the Danube region”—Art Nouveau [22], a complex database, “Art Nouveau Danube digital repository”, was created. As shown in Figure 1, the extent of the Art Nouveau phenomenon in this region, including in Romania, mainly in the North-West area and, especially, in Oradea. More than 6% of these digital objects, that means 1045 Art Nouveau buildings and artifacts, out of a total of 17,000 inventoried and digitized in seven countries from the Danube region, are found in Oradea.
The successful implementation of this first project, the splendor, beauty and breathtaking craftsmanship of the numerous Art Nouveau monuments, led to the obtaining of funding and the implementation of the next project, “Strengthening the cultural identity of the Danube region by building on common heritage of Art Nouveau”—Art Nouveau 2 [24]. The objective of this new project was the responsible and sustainable use of the Art Nouveau cultural heritage of the Danube region, so that it constitutes a vehicle for the consolidation of the regional cultural identity, for stimulating economic growth and sustainable development of the area, as well as for diversifying and improving the region’s tourism offer and services. The “Art Nouveau digital catalogue” has been developed, comprising online Art Nouveau resources managed by public libraries and archives in Europe and the partner countries in the project.
Also, through the research carried out within these projects, a series of scientific papers were published [25], which highlighted the existing situation in each of the countries participating in the project, identified solutions, and issued recommendations. Case studies were presented for Austria [26], Bulgaria [27], Croatia [28], Hungary [29], Romania [30,31], Serbia [32], and Slovenia [33,34,35]. In the case of Oradea, part of the recommendations made were implemented by the municipality through specific policies, strategies, and actions, contributing to what Oradea is today in terms of Art Nouveau heritage and beyond.
Oradea is the only city in Romania that is part of the Réseau Art Nouveau Network (Figure 2), a network that includes 28 cities in Europe with a rich Art Nouveau heritage, including important metropolises such as Barcelona, Brussels, Budapest, Paris, and Vienna. Here it is celebrates World Art Nouveau Day, on 15 June, through a series of special events organized by the Foundation for the Protection of Historical Monuments in Bihor County, Oradea Heritage.

3. Case Study: Oradea, Romania

3.1. The Wealth of the Art Nouveau Heritage in Oradea

The municipality of Oradea, located in the north-western part of Romania, the tenth largest city in the country, has been committed to sustainable development since 2000, through the implementation of Local Agenda 21, developing progressively and integrated, innovatively and sustainably, so that it is currently catalogued as an example of good practices at national level, both in terms of attracting European funds and implementing projects financed through them, and for the urban progress and innovation achieved. Increasing the standard of living of the citizens of Oradea is a goal assumed by the municipality in the Integrated Urban Development Strategy of the city and the adjacent metropolitan area, as well as in the strategic document Oradea Green City, 2020–2030. In the last 25 years, the city’s face has visibly changed through adequate urban planning and regeneration [37], the creation of an efficient and good quality infrastructure, the capitalization of existing local tourist attractions and the creation of new ones, the improvement, protection and valorization of the city’s cultural and green heritage. Today, Oradea is a thriving and welcoming city, full of life and color, where you can relax, taking advantage of the benefits of thermal water, or enjoy pleasant walks visiting remarkable tourist and cultural attractions.
Oradea is an architectural mix where, along with baroque buildings, a stellar-medieval, romantic, classical and eclectic bastion [38], there are 89 Art Nouveau style objectives, buildings and monuments, of which 26 are classified as historical monuments, 25 proposed with value for classification as historical monuments and 38 with undeniable architectural value, according Visit Oradea [39]. Because it owns and has enhanced, in recent years, the most valuable Art Nouveau architectural treasure in the country, with a high density of Art Nouveau buildings, each with its own uniqueness and personality, Oradea is often called the “Art Nouveau Capital of Romania” or the “City of Palaces”.
In Oradea, the Art Nouveau Museum opened in 2020, the first of its kind in Romania, in the Darvas House—La Roche, an emblematic building, a true architectural jewel in the Art Nouveau-Secession style, with impressive stained glass windows, built between 1909 and 1912. The museum can also be accessed and visited virtually, thanks to a digitization project [40] through which a virtual tour was created with a complex series of interconnected panoramic images, graphic, textual and audio information, thus representing a first and successful innovative approach in terms of accessibility and attractiveness for tourists and the general public. In fact, the offers and information available online, well thought out and promoted, are, lately, premises for the development of the region [41,42].
The renovation of Art Nouveau buildings belonging to the Jewish cultural heritage, a community well represented in the past in Oradea [43], has led, in recent years, to the transformation of Oradea into an emerging and very dynamic tourist destination [44]. The organization of numerous cultural events that also include renovated Art Nouveau buildings, such as the “Night of Museums,” leads to the improvement of the educational and cultural level of the community [45]. The enhancement of the exceptional Art Nouveau architectural heritage, as can be seen in Figure 3, has made Oradea to constitute and be promoted as one of the best Art Nouveau destinations in Europe, by European Best Destinations [46], the impressive collection of monuments and heritage buildings being extensively presented by Oradea Heritage [47].

3.2. Ullmann Palace

The Ullmann Palace is a building located adjacent to the 1 December Park, one of the areas in full development at the beginning of the 20th century, being a representative symbol for the Jewish community in Oradea. The architecture of Oradea in the years 1900–1914 fit into the architecture of Central Europe, taking over the Secession current, spread in Vienna and Hungary [48]. At the end of the 19th century, due to the permanent economic rise, the Jewish community in Oradea played an important role in the public life of the city. Among the most important representatives, the Ullmann family stood out, having trade as its main object of activity. Sandor Ullmann, son of the merchant Izidor Ullmann, transforms his father’s commercial enterprise into the first share-based company in Oradea, based on the ground floor of the palace. During the Second World War, the palace was part of the perimeter of the Jewish ghetto. Here and in the neighboring buildings, Jews were merged, and later they were deported to extermination camps. With the end of those tragic events, the Ullmann Palace regains its original function. The changes brought by the communist regime in terms of the economy and the need for housing caused the palace to undergo repartitioning [49]. At present, the building is classified as a category B architectural monument, the category of the historical monuments of local interest, according to the National Heritage Institute [50].
The Ullman Palace was built in 1913 according to the plans of the architect Franz Lobl from Oradea. From a planimetric point of view, the building is organized around an inner courtyard, with the main facade facing 1 December Square [51]. The construction consisted of a basement, a high ground floor, 3 floors, and an attic. According to the project, the basement was provided with storage spaces, and on the ground floor, there were shops and a large warehouse. The upper floors were intended as housing for wealthy families, including the commander, who occupied the entire second floor. Also, on the second floor, the commissioner’s family benefited from a prayer room, a ritual bath (mikvah), and a terrace garden [52]. The apartments for families were designed with generous spaces, all positioned overlooking the street, and those for servants were modest apartments, positioned towards the inner courtyard. In fact, the building reflects the characteristics of the architecture of the early twentieth century, when, against the background of intensive urbanization, industrial development and the growth of the urban population, there was an overcrowding of buildings and the appearance of some with multiple floors, organized, primordially, to satisfy practical needs and then adding artistic connotations [53].
Through its architectural conception, the Ullmann Palace is one of the most representative Secession style buildings in Oradea. The building has mainly decorations on the main facade facing the 1 December Square, the facades facing the inner courtyard being more austere. The high ground floor of the main facade is clad on the outside with blue-green glazed ceramics, which contrasts with the upper floors, whose facade is smooth, painted in white and grey (Figure 4). In this upper register, the two pyro granite bas-reliefs in the area of the bay windows stand out, depicting the seven-armed menorah and the lions of Judah, molded in Zsolnay precious ceramics [54], elements that indicate the owner’s belonging to the Jewish cult [55,56], the building being one of the few non-religious buildings in Oradea that obviously illustrate the belonging to the Jewish faith.
The Ullmann Palace, today, can be found on some tourist routes, especially the presentation of the Secession buildings, although it is only partially renovated. The building was included in the building rehabilitation program, launched in 2014, by the Oradea City Hall, owner of 51% of the building, but the private owners of the other apartments opposed the rehabilitation, due to the very high estimated costs. Thus, in an attempt to make the building safe and stop its degradation, a tender was launched for the rehabilitation of the facades and roof of the Ullmann Palace, to which, however, only one bidder was presented, the investment being financed from the local budget, within the Multiannual Program for carrying out the protection and intervention works on the buildings with cultural-architectural value located in the Urban Ensemble—Historic Centre of Oradea, approved by HCL no. 718/2016. Among the works included in the rehabilitation of the facade is the restoration of the glazed tiles on the ground floor of the building, an operation that has again sparked controversy, this time between the City Hall and the Culture Commission, upon notification of some citizens. Unfortunately, this is the current state of the building; the interior spaces and courtyards will be rehabilitated at a later stage.
Given these circumstances, a rigorously documented study is essential to guide a correct, high-quality, and sustainable restoration. This wider investigation aims to characterize the composition and microstructure of the materials employed in the building’s original construction. In this preliminary phase, this study has focused on the physical-mechanical properties of the floor tiles in the shared interior spaces and of the tiles located on the ground-floor façade.

4. Materials and Methods

One of the objectives of this paper is to highlight and demonstrate the innovative and sustainable character of the materials used in the construction of Art Nouveau buildings. To this end, some information on the nature and origin of some of the materials is presented, followed by a combination of experimental techniques used in the research to evaluate the properties of the materials used.
The materials used for the flooring of the interior, common spaces of the building, were taken from damaged areas, where small pieces were already detached, so that the integrity of the work was not affected, which led to the obtaining of samples that do not fully comply with the dimensional requirements provided by the standards. But, for the reason of not intervening destructively on the work, as can be seen in Figure 5a that it was done at a certain time, probably through an improper repair, it was considered that, at least at this stage of the research, the evidence is sufficient to carry out the estimated attempts to be made.
Regarding the nature and origin of the material from which the floor tiles are made, we still do not have clear evidence; we are in the phase of research in archives and microscopic analyses. In terms of the materials used for the floors in Art Nouveau buildings, there are few studies and information. It is known that tiles were the most common material used, in various compositions and models, produced usually by local producers, but also imported. Similar tiles have been identified, possibly produced in the Carl Ullmann Riga cement factory, which operated between 1899 and 1931 [58], when numerous Art Nouveau buildings were constructed, representing almost a third of the entire inventory of the city center [59], respectively used at Zsolnai Allami Forealiskola Zilina [60].
In a paper on the floors inside the Art Nouveau buildings in Oradea [51] it is specified the existence, in that period, of a promotional catalogue, for advertising, of the Rosenberg Izso Cement and Building Materials Warehouse, edited by the Sonnenfeld Institute of Graphics in Oradea, at the beginning of the twentieth century, which contains 64 models, some with different color positions. It emerges from this work that for the floors of the buildings, among which the Ullmann Palace is exemplified, mosaic marble slabs would have been used, obtained by pressing in high-pressure hydraulic presses, from the most tenacious marbles, thus forming, on the surface, a very hard and wear-resistant layer, slabs of very good quality, produced locally and at a moderate price compared to imported ones, namely metlachit plates (the dry-pressed porcelain slab, named after the town of Mettlach, Germany) brought from Budapest. In fact, against the background of the urban development process at the beginning of the twentieth century and the emerging need for construction materials, the number of cement manufacturing plants has increased, located, for practical and economic reasons, even in the vicinity of construction sites [61].
At first glance and based on the expertise of some members of the research team, we claim that it is a pressed burnt clay, a clinker arranged between two layers of kaolin, in the case of floor tiles (Figure 5b). The blue-green tiles with which the openings on the ground floor of the facade of the building are decorated are in the Viennese Secession style [52], small pieces of the remains during the renovation process being used for the tests carried out (Figure 5c,d).
The clinker is a product that is highly resistant to mechanical, thermal, or chemical shocks, from which floor tiles or exposed bricks are usually produced. It is made of clay, a natural and non-toxic material, burned at very high temperatures, over 1000 °C, enriched with porcelain, chamotte, feldspar, etc., and molded in different shapes. This results in tiles with high hardness and outstanding strength, with a dense texture and smooth surface, a perfect combination of robustness and elegance, very suitable for outdoor applications and heavily trafficked areas. Clay ceramic floor tiles have a high resistance to abrasion and corrosion, their colors have stability over time, they are very good sound and thermal insulators, ensuring good energy performance, they have a low absorption rate, which disfavors infiltrations, moisture and mold, maintaining a healthy and comfortable indoor environment, and increases weather resistance, being able to remain intact after over 100 freeze-thaw cycles. They have a compressive strength of up to 200 MPa, at a thickness of 47 mm, i.e., almost twice as high as concrete blocks with a thickness of 80 mm [62]. The composition and density of the material, favored by the production process, give the clinker floor a very high resistance to wear, making it extremely difficult to scratch or damage. In addition to its superior technical performance, which gives them strength and durability, being made of clay, a natural material, ceramic floor tiles are nontoxic, environmentally friendly, ecological, and sustainable, both in terms of material and use.
In turn, kaolin is a natural ceramic of the aluminosilicate type, which, in contact with water, becomes plastic, forming pastes that can be shaped. Although initially, these materials are porous, hygroscopic, and fragile, the combustion process results in compact and glassy structures with good insulating properties [63].
Building materials, in particular natural stone, widely used in the construction of monuments and heritage buildings, suffer degradation over time caused by aggressive environmental factors such as extreme temperatures, acid precipitation, and atmospheric pollution [64]. Water is considered the main cause of degradation [65], and to counteract this effect, protective solutions are used, which include environmentally friendly hydrophobic treatments and nanoparticle products that improve material properties and prevent biological colonization [66]. Recent studies [67] highlight the effectiveness of treatments based on natural products and the need to estimate the behavior of the stone over time by accelerated aging tests in climatic chambers. The importance of climatic parameters (temperature, humidity) and the concept of “biosusceptibility” in assessing the durability of stone is emphasized [68]. Analysis of the impact of high temperatures and sudden cooling on stone reveals better strength of materials with superior mechanical properties [69]. The evaluation of these phenomena is essential for the protection of built heritage [70].
Sepiolite is a clay mineral, a hydrated magnesium silicate, with properties such as low weight, high porosity and specific surface area, high adsorption and absorption capacity, and is effective in filtration and purification. Thanks to these characteristics, it is a versatile material used in many fields, including construction. Through modern processing methods, sepiolite is considered an innovative material, contributing to the improvement of stability, mechanical and thermal resistance of various materials [71].
A 7% nanosepiolite solution was used to coat the samples, which were then subjected to ultrasonic sonication for 5 min. This option was chosen as optimal considering other sonication research [72,73,74], as well as the results of other options tried during the research. To determine the size, distribution, and Zeta potential of the sepiolite nanopowders, the dynamic light scattering (DLS) method was applied using a Malvern nanosizer, ZEN3690 Zetasizer Nano ZS (Malvern Panalytical Ltd., Malvern, UK).
An SDJ03S climate chamber, manufactured by Shanghai Jianheng Instrument Co., Ltd. (Shanghai, China) and available in the Bihor Science and Technology Park, was used to simulate the environmental conditions. It allows setting the temperature between −12 and 100 °C and the humidity between 20 and 98% R.H., in both fixed and programmable modes. The chamber is equipped with an LCD touch screen for temperature and humidity control. Constructed of stainless steel, it has a heating system with nickel-chrome alloy wire and SSR relay, humidification with steam generated by an automatic boiler, refrigeration system with Secop compressor and DUPONT R134 (DuPont de Nemours, Inc., Wilmington, DE, USA) environmentally friendly refrigerant. The air is recirculated by a fan and distributed to the test area by blowers located at the bottom of the enclosure. Figure 6 shows images from the simulation process, with the samples inside the chamber and the settings displayed on the touch screen at two different times and for two different combinations of temperature and humidity conditions, respectively. The whole simulation process lasted 24 weeks, following a pre-established program, depending on the variations of temperature and humidity conditions recorded in the area.

5. Results

The determination of the physical-mechanical properties of the materials subjected to the tests, floor tiles and tiles from the facade of the building, was done using test methods provided in the European and Romanian standards in force, using the equipment and equipment available in the laboratories of the University of Oradea, respectively in the Bihor Science and Technology Park.

5.1. Compression

The compressive strength test was performed on an automatic testing machine, Controls Pilot 4, model 50-C5642 (Controls S.R.L., Milan, Italy), manufactured in 2012, with a maximum compressive capacity of 2000 kN, intended for compression tests of construction materials, including reinforced concrete, according to the UNE EN 1926:2007 Natural stone test methods—Determination of uniaxial compressive strength [75]. It was not possible to comply with the requirements regarding the number and dimensions recommended by the standard for the samples, being tiles with a thickness of 15 mm, taken from a historic, protected building. After setting the test parameters, respectively, dimensions, area, and mass, the specimen load was applied with automatic control, with a constant pressure speed of 0.5 MPa/s, registering a compressive strength of the floor tiles of 44.77 MPa, at a maximum load of 71.6 kN.

5.2. Density and Porosity

Based on the standard UNE EN 1936:2006 Natural stone test methods—Determination of real density and apparent density, and of total and open porosity [76], the test procedure with the Le Chatelier method was applied, obtaining an apparent density of the floor tiles of 1915.11 kg/m3 and a total porosity, covering the volume of closed and open pores, of 12.992%, and for the facade tiles, density of 1967.94 kg/m3 and total porosity of 19.566%.

5.3. Water Absorption

According to UNE-EN 13755:2008 Standard, the water absorption at atmospheric pressure [77] Ab is calculated by the equation Ab = 100(ms − md)/md, where: md—mass of the dry specimen, in grams; ms—mass of the saturated specimen (after immersion in water until constant mass is reached), in grams; Ab—water absorption at atmospheric pressure, expressed as a percentage.
After drying the samples in an oven at a temperature of 105–110 °C and saturating them with water to a constant mass, an average water absorption at atmospheric pressure of 3.72% for the floor tiles and 7.1% for the façade tiles was calculated.
Following the standard test procedure, six cube-shaped specimens from the façade tiles were saturated with water to constant mass after drying in an oven at 105–110 °C. As shown in Figure 6, the samples during the simulation in the climatic chamber were three in their natural state (samples 1, 2, and 3), and the others were coated with 7% sepiolite (samples 4, 5, and 6).
The values obtained in the course of the absorption test procedure are shown in Table 1, calculating a water absorption at atmospheric pressure of 7.1% for the samples in their natural state and 5.99% after sepiolite coating. After simulating the environmental conditions in the climate chamber, the calculated values for water absorption were 6.56% for the uncoated samples and 6.28% for the sepiolite-coated samples, respectively. It can be seen that, percentage-wise, the natural state samples have a higher water absorption at atmospheric pressure, decreasing the most for the sepiolite-coated samples. Following simulations of the environmental conditions in the climatic chamber, the absorption values are in between the two, being lower for the sepiolite-coated samples than for the uncoated ones. These results show the protection offered by nanoparticle coating in terms of water absorption.

5.4. Abrasion

To determine the abrasion resistance of floor tiles, the Böhme test method was applied, considered the most popular and used abrasion test method, but only for one sample, available, not for three, as required by the latest edition of the EN 14157:2017 standard [78]. The wear resistance by the Böhme method was determined as a loss in volume, calculated on the basis of mass changes. To do this, the specimen was weighed accurately before the test and after every four test cycles, and it was subjected to a total of 16 abrasion cycles, each consisting of 22 rotations, as can be seen in Figure 7. Before each cycle, 20 g of standardized abrasive material (grinding powder—artificial corundum—White Fused Alumina Section Sand) was evenly sprinkled on the test area of the rotating disc, and the area was then carefully cleaned each time. After each test cycle, the specimen was rotated, progressively, horizontally, at 90°, the tested surface being always the same.
Figure 8a presents the graphical representation of sample mass loss during abrasion testing, and Figure 8b shows the sample surface of the floor tiles at the end of the abrasion test cycles.

6. Discussion

Regarding the materials subjected to physical-mechanical tests, the results show their high density and low porosity, especially of sandstone, which leads to a low degree of water absorption. However, this level was further lowered by the application of a protective nanosepiolite solution, which has the ability to improve other properties of building materials. At the same time, the rather high values of the compressive strength and, especially, abrasion wear of the floor tile samples explain their good condition after more than 100 years of use. These results confirm the sustainability of the materials used in the construction of Art Nouveau buildings and reinforce the claim that these buildings are true models of an innovative and sustainable approach.
By discussing on innovation and sustainability of Art Nouveau buildings, a multitude of elements of the Art Nouveau style can be aligned with modern principles of sustainability, especially in terms of conservation, longevity of materials and energy efficiency, through the connection with nature and the use of high-quality artisanal pieces, many of them locally produced. From these perspectives and many others, Art Nouveau buildings can currently be considered true models of an innovative and sustainable approach in architecture and construction. By applying innovative digital management technologies to the restoration process [79], it will align with the five main directions set out in the European Framework for Action on Cultural Heritage: inclusion, sustainability, resilience, innovation, and partnership [80].
Art Nouveau buildings often contain valuable, unique materials such as decorative wrought iron, stained glass, and ceramic tiles. The integration of natural materials, such as wood, stone, or brick, and wrought iron from local, sustainable, and efficient sources, provides healthier indoor environments with improved air quality, which also leads to a reduction in energy consumption by decreasing the time required for ventilation. Intricate designs, inspired by nature, could be reimagined, and repaired and reused, recycled and sustainable materials can be used in the restoration, from sustainable sources, such as reclaimed wood, bamboo, natural stone or recycled metals, or from ethical sources, concrete or other green materials, which provide aesthetic continuity with the original style, while promoting sustainability, by reducing waste and energy consumption. Art Nouveau’s emphasis on unique, artisanal pieces challenges today’s culture of mass production and fast fashion, emphasizing quality over quantity. Using artisanal products promotes sustainability by valuing products that last, as high-quality and long-lasting items reduce waste and excessive consumerism. In addition, the local production of goods reduces the carbon footprint associated with the transport of goods, in the context in which, currently, the highest degree of pollution comes from the transport activity, especially by road.
Art Nouveau celebrated the beauty of nature, often in the form of organic patterns and shapes. This deep appreciation of the natural world can be linked to the modern sustainability trend, which advocates for environmental conservation. Designers and artists during the Art Nouveau period sought to integrate natural motifs into everyday life that can inspire sustainable and eco-friendly designs today. The organic, biophilic design, obtained by inserting the floral and organic motifs of Art Nouveau, also contributes to increasing the quality of life of the people who live in such buildings, reflecting man’s connection with nature and constituting reasons for joy and relaxation. At the same time, inspired by the organic and natural forms of Art Nouveau, contemporary buildings can incorporate green, living roofs that not only support biodiversity and water management but also reduce the effects of the urban heat island. Solar panels with artistic features can also be designed and integrated into the design of buildings, maintaining the aesthetic elegance of Art Nouveau buildings, complementing their existing curves and motifs. A case study on the application of energy efficiency measures in the restoration of a historic building is presented in [81].
Just as the curvilinear forms of Art Nouveau seem to mimic the environment, using modern techniques and digital tools, architects today can design buildings that respond dynamically to the local climate, help create innovative forms that maximize airflow, passive cooling, and energy efficiency.
Art Nouveau buildings often included lush gardens with flowing water features, fountains, and outdoor spaces that harmonized with the building’s curves and natural themes, being thought to be water-efficient. Modern reinterpretations can use xeriscaping processes, with rainwater harvesting systems and drought-resistant plants to minimize water consumption, reducing or eliminating the need for irrigation. Such sustainable landscaping leads to ecological urban integration, creating green urban spaces that promote biodiversity and mitigate pollution, and at the same time contribute to sustainable urban development by maintaining cultural landscapes.
The energy efficiency of traditional Art Nouveau buildings is also evident through the use of large windows and natural lighting, with Art Nouveau buildings often having extended windows that connect the interior spaces with natural light. Modern adaptations of this feature can now be enhanced with energy-efficient windows that maximize natural light while minimizing heat loss [82]. The paper [83] demonstrates, through a case study of the architectural rehabilitation of a heritage building, that it is possible to reduce the environmental impact almost to zero, through the sustainable use of thermal comfort installations and organizing activities according to the orientation of the building and the season.
The use of thick walls and vaulted ceilings, often found in Art Nouveau buildings, is again a factor that leads to energy efficiency. Currently, this characteristic can be adapted by incorporating materials with mass thermal capacity, which help regulate interior temperatures, reducing the need for mechanical heating or cooling systems.
By combining the natural forms and beauty of Art Nouveau with modern sustainability techniques, architects can create buildings that not only reflect this elegant style but also prioritize awareness of environmental protection and sustainability. The fusion of historical aesthetics with innovative environmental practices can lead to innovative and functional architectural projects that stand the test of time. Integrating Art Nouveau aesthetics into buildings such as green hotels or eco-friendly residences, where the use of materials, energy systems, and nature-based solutions blend seamlessly with decorative elements, represents a way of adaptive reuse and ensures their sustainability while maintaining their artistic integrity.
The adaptive reuse of historic buildings is a good example of sustainability through conservation. Instead of demolishing old Art Nouveau buildings, adaptive reuse offers a sustainable approach to preserving their aesthetics while modernizing them with modern, energy-efficient technologies. This approach also leads to a reduction in the carbon footprint associated with new construction, which has a significant impact on the environment in terms of resources, energy, and waste [84]. By preserving and modernizing old buildings, including Art Nouveau, the environmental cost of demolition and new construction is avoided. Additionally, incorporating smart technologies such as energy-efficient heating and ventilation systems, lighting, and automatic shading can bring Art Nouveau buildings into the 21st century while retaining their original charm. Through adaptive reuse, historic Art Nouveau buildings are put back to use for modern needs, such as, for example, transforming old buildings into modern office or commercial spaces, art galleries or other cultural venues, hotels, or even residential units, while retaining their original architectural charm. This reduces the environmental costs associated with new construction and promotes sustainability by using the existing built environment and preserving it for future generations.
In addition to environmental and resource protection aspects, the sustainability of Art Nouveau buildings is also focused on cultural aspects, these buildings being considered true architectural and cultural treasures. Their preservation ensures the continuity of cultural identity and history, as Art Nouveau represents a significant movement in urban design and development. In some cities, the construction and restoration of Art Nouveau buildings has been and is part of broader urban regeneration efforts, promoting sustainable tourism and revitalizing neighborhoods, while preserving heritage [85,86]. Renovated Art Nouveau buildings are certainly among the objectives visited by tourists practicing a new specific form of tourism, in recent times, arch [87], which involves visiting buildings and other architectural works. The development of tourism in a certain area, favored by several other factors related to well-being, infrastructure, security, and the environment, as well as by certain factors of a local nature [88,89], will determine and support the sustainable development of that area.

7. Conclusions

Art Nouveau represents a remarkable period in European architecture, with its rich details and deep connection with nature, still celebrated today. These buildings remain iconic representations of the artistic and architectural aspirations of the time, being spread throughout Europe, including Romania, and especially in Oradea, called the “Art Nouveau Capital of Romania”, following the recent process of renovation, conservation, and enhancement of heritage buildings.
The preservation of Art Nouveau buildings is not only about preserving beautiful structures, but it is also an opportunity to demonstrate the synergy between historical and cultural heritage and environmental protection. Sustainable restoration, adaptive reuse, and eco-friendly materials can transform these historical treasures into ecological, sustainable, and safe spaces, combining architectural conservation with environmental objectives, helping not only to protect history and the environment but also the health and well-being of people and the entire planet. Art Nouveau’s respect for nature and local tradition provides a valuable foundation for today’s eco-friendly and sustainable design. Integrating these ideas with modern technologies and materials can help create innovative, more eco-friendly, and sustainable designs, while celebrating the beauty of organic forms.
The sustainability of Art Nouveau buildings depends to a large extent on proper maintenance, proper restoration, and careful modernization, in order to preserve their value and increase their energy efficiency. Their preservation ensures both environmental sustainability in the form of adaptive reuse and socio-cultural sustainability, continuing to serve as iconic landmarks for future generations.
The results of the physical-mechanical tests presented in this paper for the tests carried out on the two types of materials from the heritage building presented as a case study, confirm the durability and sustainability of the materials used in the construction of Art Nouveau buildings and reinforce the claim that these buildings represent true models of innovative and sustainable approach.
Further investigations are needed and will be carried out in order to establish the exact origin and composition of these materials. Investigations will also be extended to other materials, both from this historic building and from others to be restored in the coming period.

Author Contributions

Conceptualization, M.R. and G.F.T.; methodology, M.R.; software, M.B.T.; validation, E.T.G.; formal analysis, A.C.P.; investigation, M.R., E.T.G. and M.B.T.; resources, M.R. and G.F.T.; writing—original draft preparation, M.R.; writing—review and editing, A.C.P.; visualization, M.B.T.; supervision, E.T.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding. The APC was funded by the University of Oradea.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

This research was done using the equipment available in the laboratories of the University of Oradea, respectively, in the Bihor Science and Technology Park.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Art Nouveau Danube digital repository—“Reprinted with permission from ref. [23]. [2025] [Art Nouveau Danube digital repository]”.
Figure 1. Art Nouveau Danube digital repository—“Reprinted with permission from ref. [23]. [2025] [Art Nouveau Danube digital repository]”.
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Figure 2. Members of the Reseau Art Nouveau Network—“Reprinted with permission from ref. [36]. [2025] [Réseau Art Nouveau Network]”.
Figure 2. Members of the Reseau Art Nouveau Network—“Reprinted with permission from ref. [36]. [2025] [Réseau Art Nouveau Network]”.
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Figure 3. Iconic Art Nouveau buildings in Oradea: (a) at 1900—“Reprinted with permission from ref. [20]. [2025] [Mircea Pasca]”; (b) today.
Figure 3. Iconic Art Nouveau buildings in Oradea: (a) at 1900—“Reprinted with permission from ref. [20]. [2025] [Mircea Pasca]”; (b) today.
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Figure 4. Ullmann Palace: (a) in the past—“Reprinted with permission from ref. [57]. [2025] [VATRA Association of Heritage Artisans and Creators]”; (b) in the present.
Figure 4. Ullmann Palace: (a) in the past—“Reprinted with permission from ref. [57]. [2025] [VATRA Association of Heritage Artisans and Creators]”; (b) in the present.
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Figure 5. Materials subject to testing: (a) part of the interior pavement repaired improperly at some point; (b) ground floor of the facade during renovation; (c) pieces of floor tiles taken for testing; (d) piece of exterior tiles for testing.
Figure 5. Materials subject to testing: (a) part of the interior pavement repaired improperly at some point; (b) ground floor of the facade during renovation; (c) pieces of floor tiles taken for testing; (d) piece of exterior tiles for testing.
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Figure 6. Climate chamber simulation process.
Figure 6. Climate chamber simulation process.
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Figure 7. Abrasion testing with the Böhme disc machine: (a) sample during abrasion testing; (b) the display panel of the test apparatus at the end of a cycle of 22 rotations.
Figure 7. Abrasion testing with the Böhme disc machine: (a) sample during abrasion testing; (b) the display panel of the test apparatus at the end of a cycle of 22 rotations.
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Figure 8. Result of the abrasion testing: (a) graphical representation of sample mass loss during abrasion testing; (b) floor tile sample after abrasion testing.
Figure 8. Result of the abrasion testing: (a) graphical representation of sample mass loss during abrasion testing; (b) floor tile sample after abrasion testing.
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Table 1. Mass variation of samples tested at atmospheric absorption.
Table 1. Mass variation of samples tested at atmospheric absorption.
SampleNatural Stone Samples (Uncovered)Samples Covered with Sepiolite 7%
Before Climatic ChamberAfter Climatic ChamberBefore Climatic ChamberAfter Climatic Chamber
m [g]md [g]ms [g]m [g]md [g]ms [g]m [g]md [g]ms [g]m [g]md [g]ms [g]
112.912.813.712.812.613.5------
217.317.118.317.116.918.0------
316.616.417.516.416.217.2------
417.717.518.8---1817.718.717.617.418.5
516.416.217.4---16.616.317.316.316.117.1
614.514.315.3---14.714.415.314.414.215.1
Total mass
[g]		95.494.310146.345.748.749.348.451.348.347.750.7
Absorbtion Ab [%] 7.1 6.56 5.99 6.28
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MDPI and ACS Style

Ratiu, M.; Gligor, E.T.; Tamas, G.F.; Peres, A.C.; Tataru, M.B. Art Nouveau Buildings, Examples of Innovative and Sustainable Approach—Case Study: Oradea, Romania. Sustainability 2025, 17, 5952. https://doi.org/10.3390/su17135952

AMA Style

Ratiu M, Gligor ET, Tamas GF, Peres AC, Tataru MB. Art Nouveau Buildings, Examples of Innovative and Sustainable Approach—Case Study: Oradea, Romania. Sustainability. 2025; 17(13):5952. https://doi.org/10.3390/su17135952

Chicago/Turabian Style

Ratiu, Mariana, Emil Traian Gligor, George Florentin Tamas, Ana Cornelia Peres, and Mircea Bogdan Tataru. 2025. "Art Nouveau Buildings, Examples of Innovative and Sustainable Approach—Case Study: Oradea, Romania" Sustainability 17, no. 13: 5952. https://doi.org/10.3390/su17135952

APA Style

Ratiu, M., Gligor, E. T., Tamas, G. F., Peres, A. C., & Tataru, M. B. (2025). Art Nouveau Buildings, Examples of Innovative and Sustainable Approach—Case Study: Oradea, Romania. Sustainability, 17(13), 5952. https://doi.org/10.3390/su17135952

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