Abstract
The selection of appropriate resins and consolidants is a critical aspect of stone conservation for architectural heritage. This research evaluates the efficacy of Paraloid B-72 (an acrylic resin) and an epoxy resin for the consolidation of the trachyte stone used in the historic Khosravi Leather Factory building, which now serves as the central campus of the Islamic Art University of Tabriz. These polymers were selected for their documented performance, accessibility, and cost-effectiveness. Two application methods—total immersion (saturation) and brush application—were tested on trachyte stone samples. To determine the most suitable conservation strategy, a comprehensive analysis was conducted. The mineralogical and elemental composition of the stone was characterized using X-ray Diffraction (XRD) and X-ray Fluorescence (XRF). Key physical properties (capillary water absorption, saturation, porosity, and density) and durability indicators (resistance to thermal shock, freeze-thaw cycles, and salt crystallization) were measured before and after treatment. The results demonstrated that the brush application of Paraloid B-72 provided the most favorable balance of consolidation and preservation of the stone's original properties. Based on these findings, this method was identified as the optimal treatment for the in-situ conservation of the building's stonework.

Introduction

1.1. Site History and Significance
The Khosravi Leather Factory, constructed in 1931 and commencing operations in 1932, stands as one of Tabriz's earliest and most significant industrial complexes. Following a period of restoration, the expansive 36,000-square-meter facility was repurposed to house the Tabriz Islamic Art University and is still undergoing phased development. Its historical importance is further highlighted by the presence of early mechanical installations and the city's first electrical generator on-site.
 
1.2. The Conservation Problem: Deterioration of Trachyte Stonework
A primary challenge in the preservation of this historic structure is the deterioration of the trachyte stone used extensively in its construction. Located in Tabriz, a region characterized by a cold climate with numerous freeze-thaw cycles, the stonework exhibits significant weathering. Common forms of decay observed include granular disintegration (powdering), micro-cracking, and loss of cohesion. This progressive degradation necessitates urgent preventive conservation measures to ensure the building's long-term structural and aesthetic integrity.
 
1.3. Consolidation as a Conservation Strategy
Stone consolidation is proposed as the key intervention strategy. Its primary objectives are to restore the mechanical integrity of the weathered stone, improve its durability against environmental stresses, and stabilize it sufficiently to withstand any further conservation treatments. An ideal consolidate must possess several critical properties: low viscosity for deep penetration, flexibility to accommodate thermal expansion, high water resistance, and long-term chemical stability. Furthermore, because consolidation treatments are largely irreversible, the amount of material applied must be carefully controlled to be the minimum necessary.
 
1.4. Research Scope and Objectives
Among the available options, organic polymer resins are widely used in stone conservation. This study focuses on two common types: Paraloid B-72, an acrylic copolymer, and a standard epoxy resin. Both are recognized for their strong adhesive properties and ability to create a hydrophobic, solid network within the stone's pores. However, a potential drawback is their tendency to reduce porosity and alter the pore-size distribution, which can negatively affect the stone's natural moisture transport behavior.
       Therefore, this research combines a literature review, fieldwork, and rigorous laboratory testing to systematically evaluate and compare the performance of these two consolidants on the specific trachyte stone of the site. The ultimate goal is to identify the most suitable material and application method that provides effective consolidation while minimizing adverse side effects, thereby establishing a best-practice protocol for the building's conservation under local climatic conditions.
 

Materials and Methods

To evaluate the efficacy of selected consolidants on “Sprahkhan” stone, twenty-six cubic samples (5 cm × 5 cm × 7 cm) were sourced from the stairways of the Central Campus at the Islamic Art University of Tabriz. The samples were subsequently allocated into three distinct groups: a control group of untreated stones (n=6), a group treated with a 7% (w/v) solution of Paraloid B-72 (n=6), and a group treated with a 7% (w/v) epoxy resin solution (n=6).¹ All specimens were stored under stable laboratory conditions for a period of one to two months prior to testing. The experimental program was designed to compare the treated and untreated samples through a series of laboratory analyses, including material characterization, physical property measurements, and durability assessments. Two methods were employed for the application of consolidants: total immersion and brush application. For the total immersion method, specimens were fully submerged in the respective 7% consolidate-acetone solutions until saturated, after which they were left to air-dry. For the brush application method, the consolidate solutions were applied in multiple successive layers with a soft brush to ensure the formation of a uniform and continuous coating on the stone surfaces.
 
Table: Summary of Analytical and Experimental Methods




Category


Test Method


Objective / Application


Standard or Source




Material Characterization


XRD (X-ray Diffraction)


Identification of the mineralogical phases in the rock samples.


--




XRF (X-ray Fluorescence)


Semi-quantitative and qualitative analysis of the rock's elemental composition.


--




Thin Section Petrography


Examination of the rock's texture, mineralogical composition, and microstructure.


--




Physical Properties


Capillary Water Absorption


To determine the rate of water absorption and penetration into micropores as a function of time.


ASTM D1585 (2004)




Water Absorption and Saturation


To calculate the percentage weight gain resulting from the saturation of the rock's internal pores with water.


ASTM C127-88




Wetting and Drying Cycles


To assess the rock's stability against cyclic moisture and thermal variations (28-day duration).


ASTM D5313 (2004)




Freeze-Thaw Cycles


To determine the rock's resistance to stresses caused by the formation and thawing of ice crystals within its pores.


ASTM D5312 (2004)




Salt Crystallization Test


To measure the rock's resistance to stresses induced by the crystallization of salt (sodium sulfate).


ASTM D5312 (2004)




Analytical Software


SigmaPlot


Plotting charts and conducting statistical analysis of quantitative data from physical and durability tests.


Version 2016




 

Results and Discussion

3.1. Stone Characterization
Prior to consolidation tests, the trachyte stone was characterized to establish baseline properties. The stone exhibited a dense, hard texture. Mineralogical analysis (XRD) identified quartz, plagioclase, and anorthite as the primary mineral phases. Elemental analysis (XRF) confirmed that silicon (Si) and aluminum (Al) were the predominant elements, consistent with the composition of trachyte.
3.2. Performance of Consolidation Treatments
The study evaluated the performance of two consolidants (Paraloid B-72 and Araldite epoxy) applied via two methods (total immersion/saturation and brush application). Physical and durability tests, including capillary water uptake, water loss measurements, and accelerated aging cycles (thermal shock, freeze-thaw, and salt crystallization), were conducted on treated and untreated (control) samples. The results consistently demonstrated that both acrylic and epoxy resin treatments significantly improved the physical properties and durability of the trachyte stone compared to the untreated samples. However, a clear performance hierarchy emerged. Brush application with Paraloid B-72 yielded the highest stability and strength enhancement. This finding aligns with the practical application of this method in the successful restoration of a wall section at the Islamic Art University of Tabriz.
3.3. Discussion and Long-Term Considerations
While polymer consolidants are effective, a key challenge is ensuring deep penetration and uniform distribution. Solvent evaporation during the curing process can cause resin migration towards the surface, potentially compromising long-term performance. The superior results of the brush application method in this study suggest it allows for more controlled application, minimizing this risk compared to saturation. A critical consideration for these materials is their limited lifespan. It is well-documented that organic polymers like Paraloid B-72 and epoxies may begin to lose their effectiveness after approximately ten years. This degradation is often accompanied by aesthetic changes, such as discoloration and yellowing, resulting from oxidation, UV radiation exposure, thermal fluctuations, and chemical interactions with the stone substrate. The need for periodic re-application and the management of polymer decay present a valuable and ongoing area for future research in heritage science, particularly for the conservation of porous building materials.
 

Conclusion

The primary objective of this research was to identify an effective consolidation strategy for the trachyte stonework of the Tabriz Islamic Art University campus, based on its specific material properties and the local climatic conditions. The study concluded that the application method is as critical as the choice of consolidate. Under the environmental conditions of Tabriz, brush application consistently outperformed the total immersion (saturation) method. Among the tested materials, Paraloid B-72 demonstrated superior performance on the trachyte stone compared to the epoxy resin. Based on the comprehensive laboratory results, a clear efficacy ranking was established:

First Choice: Brush application of Paraloid B-72
Second Choice: Brush application of epoxy resin
Third Choice: Saturation with Paraloid B-72
Fourth Choice: Saturation with epoxy resin

       These findings were validated through a successful in-situ pilot application on a one-square-meter section of the faculty's courtyard wall. The optimal treatment protocol—application of a 7% Paraloid B-72 solution in acetone by brush during the dry season (August)—proved effective. Future work will involve long-term monitoring of the treated area, including water absorption and permeability tests, to assess the durability of the consolidation over time. This research underscores the necessity of tailoring conservation strategies to the specific type of stone and its environmental context to ensure responsible and effective heritage preservation.
 
Acknowledgments
The authors extend their sincere gratitude to the Dean of the Faculty of Applied Arts, Dr. Baqerzadeh Kasiri, for administrative support. We thank the workshop supervisors, Mr. Shakouri and Mr. Hassanzadeh, and engineers Delkhaah and Khalghi for their technical assistance. We are also grateful to Dr. Ranjbarzadeh and the director and staff of the Tabriz Meteorological Organization for providing valuable data. Finally, we would like to acknowledge Ms. Namani for her dedicated collaboration on this project.