Definition and Scope of Restoration

Restoraton does not mean renewing, beautifying, or rebuilding a historic structure. On the contrary, restoration is a scientific conservation discipline aimed at ensuring the preservation and continuity of original qualities of cultural assets by respecting their historical, artistic, and documentary values.

Fundamental Principles of Restoration

A scientific restoration process is based on certain fundamental principles. First of all, the original material, texture, traces, and historical layers of the structure are preserved... Secondly, the "Principle of Minimum Intervention" is observed... Thirdly, it is aimed that the interventions applied should be reversible in nature without damaging the structure in the future.

What Restoration is Not

Since incorrectly defined and implemented interventions can cause irreversible damage to cultural assets, the boundaries of restoration must be clearly defined. It is not a platform for modernization or purely aesthetic reinvention.

Why Proper Restoration?

An incorrect intervention can irreversibly destroy the historical and documentary values of a cultural asset. Conversely, a proper restoration process increases the biological and physical resistance of the building while preserving its historical authenticity.

1. Confusing Restoration with Renovation

The most basic and common mistake in restoration is treating the process as a "renovation" activity. Interventions carried out with the expectation that surfaces should look clean, smooth, and brand new cause irreversible destruction of the original material and historical layers.

2. Use of Non-Breathing Materials

One of the most frequent technical errors in historic structures is choosing materials that are incompatible with original building systems. Especially cement-based plasters and plastic-based paints show severe incompatibility with traditional lime-based masonry systems.

3. Starting Intervention Without Surface Analysis

Another common mistake in restoration is starting application without sufficient analysis of the current state of the surface. Without understanding the salt index, moisture content, and mineral composition, every application is a blind intervention.

4. Evaluating the Product Separately from the System

The factor that determines success in restoration is not a single product, but a system of components working in harmony with each other.

5. Handling Restoration with Standard Construction Logic

Carrying out restoration applications with typical modern construction reflexes is one of the most critical problems on-site. Traditional masonry requires slow, meticulous, and customized work methodologies.

The fate of a historic building is often decided long before work starts on-site.

1. Analysis and Documentation: "Listening" to the Structure

Proper restoration starts not with intervening, but with listening. Any application carried out without understanding the phases the building has gone through, the interventions it has previously been exposed to, its original materials, and the traces formed over time, will lead to unexpected, irreversible consequences.

2. Fidelity to the Venice Charter: International Principles

Restoration is not an area for personal taste or expectations of "beautification". This is why internationally accepted principles form a common ground for everyone involved.

3. Principle of Minimum Intervention: "Less is More"

One of the most common mistakes in restoration is the desire to make the building "better than it was". However, in most cases, the correct intervention is the minimum necessary.

4. Reversibility: Preserving Options for the Future

Restoration is not a final, unalterable decision; it is like a temporary trust. Any intervention made today must be reversible in the face of tomorrow's advanced knowledge and technology.

A successful restoration project progresses through four main phases: Diagnosis, Strategy, Intervention, and Sustainable Conservation.

I. Preparation and Documentation (Diagnosis Phase)

By conducting measured drawings (rölöve), analytical rölöve, and restitution studies, the health status and historical layers of the structure are revealed.

II. Design and Approval (Strategy Phase)

The restoration project is prepared, conservation decisions are made, and scientific and legal oversight is secured through Conservation Council approval.

III. Application and Intervention (Operation Phase)

Site setup, material analyses, and controlled interventions are carried out based on the minimum intervention principle.

IV. Completion and Sustainability (Conservation Phase)

Long-term protection is ensured through final documentation (As-Built), preservation by utilizing correct functions, and periodic maintenance/monitoring processes.

1. Introduction

The correct identification of deteriorations necessitates intervention decisions supported by analytical evaluation rather than superficial observations.

2. Basic Categories of Deterioration Mechanisms

Physical, chemical, and biological deteriorations and environmental factors directly affect the structural health of the building.

3. Analysis Phase: Right Decisions with Right Data

Through visual examination, moisture/salt analysis, material tests, and multi-layered deterioration criteria, the correct strategy is planned.

4. Intervention Strategies and Timing

The wait-and-see approach, urgent intervention, correct sequencing (Cleaning → Consolidation → Protection), and minimum intervention are observed.

In preserving cultural heritage, restoration is not merely a physical intervention; it is a data-driven, ethically audited, and continuous system management. Modern conservation approaches handle restoration in three interconnected phases: documentation, application, and monitoring.

This cyclical approach guarantees both the technical correctness of the intervention and the preservation of historical authenticity over generations.

Restoration is a scientific, technical, and ethical intervention process aimed at carrying the original value of historic buildings, monuments, and cultural heritage elements to the future. The international framework of the modern conservation approach was determined by the Venice Charter of 1964; preserving cultural heritage was defined not only as a physical repair activity, but also as a historical and cultural responsibility. In Turkey, the legal basis of this field is formed by the Cultural and Natural Heritage Protection Law No. 2863.

To conduct restoration applications in a healthy and scientific manner, it is necessary to act in accordance with certain basic principles.

1. Respect for Authenticity

The value of a cultural asset is directly related to its original material, design, workmanship, plan scheme, and historical context. During the restoration process, these authentic qualities must be preserved, and interventions that alter the identity of the building must be avoided. The original material must be preserved in situ as much as possible; when replacement is mandatory, the new material must be compatible both technically and aesthetically.

Authenticity covers not only physical elements, but also the cultural and symbolic meaning carried by the structure.

2. Minimum Intervention

The basic approach in restoration is to provide maximum protection with minimum intervention. Unnecessary renovations, efforts to make the building look "new", or extensive reconstruction applications can lead to a decrease in historical value.

The minimum intervention principle focuses on consolidation, preservation, and improving the current situation. Interventions must be made only when mandatory and for scientific reasons.

3. Documentation and Scientific Basis

The restoration process starts with a detailed research and documentation phase.

• Rölöve: Measured drawings of the current state.
• Restitution: Scientific proposal regarding past periods of the structure.
• Restoration Project: Application and intervention decisions.

These documents must be based on scientific data such as archive searches, historic photographs, engravings, written sources, and material analyses. Every stage of the intervention process must be recorded and archived. This creates a transparent and traceable information infrastructure for future work.

4. Reversibility

New elements added or techniques applied during restoration must be reversible, meaning they can be removed without damaging the structure when necessary. This principle allows for the implementation of more advanced conservation techniques that may develop in the future.

For example, a system used for structural consolidation should be designed to be demountable if a more suitable method is found in the future.

5. Distinguishability

New additions or completions must be distinguishable from the original parts of the building. Trying to produce "exactly the same as the old" through imitation can mean a misleading reconstruction of the historical process.

However, distinguishability does not mean creating extreme contrasts that disrupt visual integrity. The goal is an intervention that is both honest and aesthetically balanced.

6. Material and Technical Compatibility

Materials used in restoration must be compatible with the original material in physical, chemical, and mechanical terms. For example, using cement-based mortars in a historic masonry structure can cause severe damage to stone and brick in the long term.

Therefore, traditional materials and construction techniques must be researched, and materials of same or similar qualities must be preferred as much as possible.

7. Respect for Historical Layers

Historic buildings often carry traces of different periods. The approach of returning a structure only to its "initial state" can mean ignoring and destroying the historical contributions of subsequent periods.

Conservation evaluation treats the phases of the building as a whole. Every layer is part of the building's historical memory value.

8. Functional Continuity and Preservation by Living

One of the most effective methods in preserving a structure is to keep it functioning. Unused buildings deteriorate rapidly. However, in the process of giving a new function (adaptive reuse), the plan scheme, load-bearing system, and spatial setup of the building must not be damaged.

The selection of function must be compatible with the scale and identity of the structure.

9. Interdisciplinary Collaboration

Restoration requires the joint work of disciplines such as architecture, art history, archaeology, engineering, material science, and urban planning. A healthy conservation process can only be carried out by expert teams.

One-sided interventions lacking scientific basis can cause irreversible losses.

In restoration work, technical and material knowledge alone is not sufficient; every intervention must also be planned and implemented within legal and ethical rules. It must be ensured that the restoration is both respectful to cultural heritage and protectable in the long term.

Therefore, restoration projects must be prepared in accordance with relevant legislation and regulations, evaluated according to international standards, and documented at every stage.

Restoration and Conservation Legislation in Turkey

1. Cultural and Natural Heritage Protection Law (Law No. 2863)
In Turkey, the protection of movable and immovable cultural assets is secured by Law No. 2863. This law is accepted as the legal basis of restoration work and determines the following principles:

• Approval of Conservation Councils: Every restoration project is examined by Conservation Councils and cannot be implemented without approval. This protects cultural assets from unauthorized and unconscious interventions.
• Rölöve, Restitution, and Application Processes: During the restoration of buildings and immovable cultural assets, all stages from planning and measurement to project design and execution are carried out within the framework of the law, ensuring standards are met.

2. Regulations and Directives
Regulations tied to the law explain in detail how restoration applications will be made and audited:

• Project Design, Execution, and Auditing: Technical, material, and execution standards are determined when preparing restoration projects, and applications are audited by expert teams.
• Inventory and Classification: Movable cultural assets are inventoried according to their types and level of importance, and protective measures are determined.
• Area Management and Sit Planning: Planning of conservation areas, environmental arrangements, and archaeological site plans are inseparable parts of the restoration process.

Thanks to these regulations, restoration work becomes a planned, controlled, and traceable process.

International Standards and Principles

1. Venice Charter (1964)

• Respect for original materials, minimum intervention, and reversibility are accepted as fundamental principles in the restoration of historic structures.
• Restoration projects are evaluated and planned according to these principles, thus preserving the historical and cultural value of the building.

2. Nara Document on Authenticity (1994)

• The authenticity of historic structures must be evaluated not only in terms of materials used, but also in terms of cultural context, use, and meaning.
• This document provides guidance in making restoration decisions for structures in different cultures and grants flexibility in application.

3. World Heritage Convention (UNESCO, 1972)

• Ensures the protection of cultural and natural heritage sites on a global scale.
• Countries undertake international obligations regarding the management, protection, and monitoring of heritage sites.
• This convention emphasizes that restoration work must be handled within the scope of global responsibility, not just national interest.

Importance of the Legal Framework in the Restoration Process

• Restoration is not just an aesthetic improvement: Restoration work aims to ensure the structural integrity and long-term preservation of cultural assets, far beyond correcting their appearance. Every intervention must be planned considering material properties, historical value, and environmental factors.
• Legislation and international standards provide guidance: Laws and regulations in Turkey, together with international principles such as the Venice Charter, Nara Document, and UNESCO standards, guarantee the implementation of ethical and scientific rules in the restoration process.
• Obligation of documentation and record-keeping: Every intervention must be detailed, documented, and recorded. This ensures traceability and forms a valuable reference for future restorations or academic researches.

Historic buildings are exposed to many environmental, physical, and human-induced factors during the period from their construction to the present day. The damages and material changes occurring in this process are evaluated within the scope of "building pathology" in restoration science. Correct analysis of building pathology is one of the most fundamental conditions for transferring historic structures to the future by preserving their original identity.

1. Physical and Mechanical Deteriorations

Physical deteriorations occur when building materials lose their structural integrity due to mechanical effects or environmental conditions. Generally, temperature changes, freeze-thaw cycles, wind effects, or ground settlements are the main factors of this process.

• Cracks and Clefts: Formed due to static stresses, ground settlements, or dynamic effects like earthquakes. Capillary cracks can be superficial, while deep clefts indicate structural system weakness.
• Delamination and Peeling (Exfoliation): The material separates in layers and falls off the surface. It is caused by internal pressure differences resulting from moisture movement or salt crystallization.
• Surface Powdering and Disintegration: The crumbling of material into powder due to loss of binding properties. This seriously reduces mechanical strength.
• Material Loss and Erosion: The disappearance of architectural details over time due to particles carried by wind or physical friction.

2. Chemical Deteriorations

As a result of chemical reactions of building materials with environmental factors, permanent changes occur in their mineral structure.

• Salt Crystallization (Efflorescence): The expansion pressure caused by the crystallization of dissolved salts carried by water in pores leads to cracking in the internal structure of the stone.
• Carbonation and Sulfation: Atmospheric gases combining with moisture to form acidic compounds cause chemical erosion and black crust formation on stones like marble.
• Oxidation: Rusting of metal clamps or iron minerals creates volume expansion, cracking the surrounding stone texture.

3. Biological Deteriorations (Biodeterioration)

Occurs with the growth of living organisms on surfaces, especially in damp and shaded environments.

• Microorganisms: Algae, moss, and fungi cause surface pollution and chemical deterioration through the organic acids they secrete.
• Lichen Formation: Their root-like structures settle into the pores of the stone, leading to physical fragmentation on the surface.
• Plant and Root Action: Plants settling into joints generate serious mechanical pressure as they grow, displacing stone blocks.

4. Human-Induced Deteriorations and Incorrect Interventions

Unconscious repair applications can accelerate the natural aging process of the building.

• Incorrect Cement-Based Repairs: Cement, which is incompatible with traditional lime mortars, prevents the wall from breathing and traps moisture inside the structure.
• Incorrect Cleaning Techniques: High-pressure sandblasting or aggressive chemicals destroy the natural protective layer of the stone, the patina.
• Hatalı İşlevlendirme (Incorrect Functional Adaptive Reuse): Loading the structure above its carrying capacity triggers severe structural problems.

Analysis and Diagnosis: The Fundamental Phase of Restoration

For a successful restoration application, the cause behind the damage must be correctly identified. For this purpose, moisture movements, salt types, and mineral structure are analyzed in a laboratory environment. In addition, non-destructive testing methods (NDT) (thermal camera, ultrasonic measurements, radar) provide detailed data on the internal structure of the building.

Surface cleaning is one of the most delicate phases of restoration works carried out during the preservation of historic structures and their transfer to future generations. In restoration applications, cleaning is not merely an aesthetic process; it is also one of the most critical steps for revealing the material properties of the structure, correctly analyzing deterioration processes, and implementing conservation interventions healthily.

Over time, various dirt layers form on the surfaces of historic structures due to atmospheric pollution, biological formations, salt crystallizations, incorrect interventions, or environmental factors. These layers not only create visual pollution but can also cause physical and chemical deterioration of the building material.

Main Purposes of Cleaning in Restoration

• Removal of atmospheric dirt and pollution layers accumulated on the surface
• Cleaning of biological formations (moss, algae, lichen, etc.)
• Giderilmesi (removal) of salt crystallizations and dirt deposits
• Removal of layers created by previous incorrect restoration applications
• Revealing the original surface texture
• Making the material visible for correct surface analyses

Factors Considered in Choosing Cleaning Methods

It is not possible to apply the same cleaning method to every historic structure and surface. To determine the correct method, various criteria must be evaluated:

• Type of building material (natural stone, brick, plaster, marble, etc.)
• Type of dirt or deterioration on the surface
• Physical resistance of the surface
• Environmental conditions of the structure
• Past restoration interventions
• Salination and moisture status

1. Mechanical Cleaning Methods

Mechanical cleaning is the physical removal of dirt and layers on the surface. It is one of the most common methods used in restoration.

• Brushing: Loose dirt layers on the surface are removed using soft or medium-hard brushes.
• Use of Spatulas and Scalpels: Thick dirt layers or hardened deposits are cleaned in a controlled manner with small hand tools.
• Micro-Abrasive (Micro-Blasting): Performed by applying very fine mineral particles to the surface with low pressure.

2. Water-Based Cleaning Methods

Water use in historic structures must be highly controlled. The main techniques are:

• Low-Pressure Water Cleaning: Water applied at controlled pressure ensures the removal of dirt and dust layers on the surface.
• Steam Cleaning: Low-pressure hot steam is effective in removing oily dirt and biological formations.
• Nebulization: Applied in the form of very fine water droplets, this method softens the dirt for safe removal.

3. Chemical Cleaning Methods

Some types of dirt can only be cleaned using chemical agents (cleaners in gel form, poultice methods, biocide applications, etc.). It is of critical importance that the chemical used does not react with the surface material.

4. Laser Cleaning

It is the most advanced technology used in the field of restoration today. While the laser beam vaporizes the dirt layer, the original material is minimally affected. It is especially preferred in historic statues and fine stone decorations.

Common Mistakes in Restoration Cleaning

• Use of high-pressure water
• Uncontrolled sandblasting applications
• Aggressive chemical substances unsuitable for the surface
• Abrasive cleaning with hard wire brushes
• Starting direct application without creating a mock-up (sample area)

In restoration applications, the most decisive element of lasting success is often not the final topcoat material, but how accurately the surface to which this material is applied has been prepared. Surface preparation should not be seen merely as a cleaning process; it must be handled as a comprehensive process of analyzing the current state of the surface, understanding the causes of deterioration, removing weak layers, and establishing a healthy, sound substrate for the system to be applied.

Why Is Surface Preparation a Critical Stage?

Deteriorations observed on the surface are often a result of an underlying problem, not the cause itself. Factors such as moisture movement, salt transport, environmental pollution, freeze-thaw cycles, and biological formations create different types of damage on surfaces over time. Therefore, during the restoration process, we must not only ask "what is visible?" but also correctly answer the question: "why did this deterioration occur?" Surface preparation is the technical counterpart to this challenge.

The Mechanism of Deterioration Penetrates Below the Surface

Problems occurring on structural surfaces are, in most cases, not limited to the outer layer alone. Due to the porous nature of historic masonry, water and dissolved salts penetrate deep into the internal structure of the material. This situation can lead to structural issues such as:

• Blistering and detachment on the surface
• Salt efflorescence (blooming)
• Loss of adhesion
• Structural weakening of the material

gibi (such) problems. For this reason, surface preparation covers not only external cleaning but also rendering the surface load-bearing, stable, and chemically balanced.

The Fundamental Function of Surface Preparation

The primary goal of surface preparation is to ensure that the conservation or repair system to be applied integrates properly with the surface. The success of this integration directly determines the lifespan of the system. Within this scope, the operations performed include:

• Removal of deteriorated, loose, and detached layers
• Purification of salt, dirt, and pollutant accumulations from the surface
• Cleaning and taking control of biological formations
• Balancing the surface in terms of absorption and stability
• Strengthening or consolidating the surface where necessary

The Effects of Incorrect Surface Preparation

Incomplete or incorrect surface preparation can render even the most accurate choice of product and system non-functional. This deficiency is generally invisible in the short term, but over time, it leads to serious consequences such as:

• Decrease in coating performance
• Early surface deterioration
• Adhesion and bonding problems
• Delamination and separation of the system

Sensitivity in Historical and Natural Stone Surfaces

Surface preparation requires a more delicate balance in natural stone, brick, and historical mortared structures. Because in these structures, the aim is not only protection but also the preservation of the material's:

• Breathing capacity
• Original surface texture
• Aesthetic character

korunmasıdır (preservation). For this reason, controlled, material-compatible, and reversible interventions should be preferred instead of aggressive, abrasive methods.

System Success Begins with Surface Preparation

Water repellent systems, mineral-based coatings, and surface protection technologies can only exhibit long-term performance on a properly prepared surface. Otherwise, the system cannot reach its designed technical lifespan. Therefore, surface preparation is not just a pre-application step; it is the fundamental stage that determines the success of the entire system.

Climate change is a dynamic environmental pressure factor that is becoming increasingly decisive in the preservation of historic structures. Temperature fluctuations, changes in precipitation regimes, acceleration of moisture cycles, and salt transport directly affect the behavior of building materials.

The porous and capillary-active nature of historic building materials makes these structures more sensitive to environmental variables. In particular, the acceleration of wetting-drying cycles, thermal stresses, and the increase in biological formations are the main factors accelerating material deterioration processes.

This article presents that climate change is not merely a superficial effect, but a holistic process directly shaping building physics and material behavior; it emphasizes that the conservation approach must evolve from a repair-oriented understanding to an adaptation and resilience-based approach.

1. Introduction: An Unforeseen New Environmental Pressure Layer

Historic buildings have reached the present day by adapting to certain climatic periods over centuries. However, the most critical variable that has emerged in the last century is the rapid climatic deviation, which was not foreseen in the traditional design logic and material selection of these structures.

While traditional risk factors (seismic effects, material fatigue, etc.) are more predictable, climate change can be defined as a dynamic disruptive effect that is constantly accelerating and directly affecting the behavior of building materials.

2. Disruption of Climatic Cycles and Its Impact on Building Physics

Climate change should be evaluated not only as a temperature increase on the structure, but as a shortening of environmental stress cycles.

• Shock effect of the precipitation regime: Sudden and heavy rains increase the rate of water saturation of the building surface and strain the drainage capacity.
• Acceleration of wetting-drying cycles: Frequent wetting and drying processes cause mechanical stresses within the pore structure.
• Relative humidity and salt migration: Moisture changes accelerate the transport of dissolved salts to the surface, leading to micro-cracks due to crystallization pressure.

3. Sensitivity Area of Historic Material

Historic buildings consist of capillary-active materials such as natural stone, brick, and lime-based mortars.

• Volumetric changes: Moisture absorption and desorption processes weaken the binder structure by creating expansion and contraction cycles.
• Thermal shocks: Night-day temperature differences cause stresses between materials with different expansion coefficients.

4. Deep Progression of Deterioration Mechanisms from the Surface

The effects of climatic stress usually start on the surface and deepen over time.

• Surface powdering (friability): Sanding of the surface as a result of the weakening of the binder phase.
• Biological colonization: Increased humidity and temperature accelerate algae and fungi formations.
• Black crust formations: Environmental effects combined with air pollutants create hard and impermeable layers on the stone surface.

5. General Evaluation: Transition from Diagnosis to Strategy

In the face of climate change, the historic building conservation approach should now be handled on the axis of adaptation and resilience enhancement, rather than just repair orientation.

The fundamental question is no longer "which material should be used?", but "how does this structure behave within changing environmental cycles?". A holistic conservation approach is possible with correct diagnosis, precise surface preparation, and integration of systems compatible with the material.

HMSA Glossary of Terms

• Capillarity (Capillary Water Action): The ability of water to move through capillary spaces in porous materials.
• Water Vapor Diffusion: The capability of the material to transmit water vapor to maintain moisture balance.
• Wetting-Drying Cycle: The physical stress process formed by the continuous water absorption and release of the material.
• Crystallization Pressure: The internal stress created by salts crystallizing in the pores.
• Friability (Surface Powdering): The crumbling of the surface as a result of binder weakening.
• Black Crust: A hard, impermeable surface layer formed by air pollutants.

For mineral surfaces in historic buildings that lose their binder, turn into powder, and lose their structural resistance over time, consolidation (strengthening) is an inevitable intervention. However, because this process interferes with the natural cycle of the material, it must be conducted within certain ethical and physical boundaries. This article examines the mechanism of the silification process, the "elasticity modulus" compatibility in consolidant selection, and the risks of "hard crust formation" that excessive intervention can create.

I. Introduction: Tracing the Lost Binder

Natural stone, brick, and lime-based plasters lose their internal bonds over time due to atmospheric effects, freeze-thaw cycles, and crystallization pressure of salts carried by water. The material showing "powdering" (sanding) or "peeling" (exfoliation) is a harbinger of structural collapse. Consolidation is the process of re-establishing mechanical resistance "from the inside out" by injecting a new binder into the pore structure of the material.

II. Silification Mechanism: Mineral Integration

The most accepted method in modern restoration technology is the use of ethyl silicate or potassium silicate-based consolidants. These materials, after penetrating the pores of the material, react with the moisture in the air to form pure silicon dioxide gel.

• Chemical Bond: This gel establishes new mineral bridges between particles.
• Compatibility with Authenticity: Silicate-based interventions exhibit a much higher historical and physical compatibility compared to synthetic resins (epoxy, acrylic, etc.), as they share a similar chemical character with the mineral structure of the material.

III. Ethical and Physical Limits of Consolidation

Although strengthening is a rescue operation, incorrect application can destroy the structure. Successful consolidation must observe three limits:

• Penetration Depth: The consolidant must not remain only on the surface of the material, but must penetrate to the deepest point of the damaged zone. Material remaining on the surface causes "hard crust formation" (case-hardening).
• Preservation of Hygric Properties: The consolidated area must not lose its breathing (water vapor diffusion) capability. Complete blocking of pores traps water under the surface and leads to new pathologies.
• Differential Thermal Expansion: The new binder added must have a thermal expansion coefficient similar to the original material. Otherwise, with temperature changes, the new layer will detach and fall off the original surface.

IV. Risk of Crust Formation (Case-Hardening)

The greatest risk in consolidation is that the outer surface of the material becomes much harder than the inner part. This causes moisture inside the stone to accumulate under the surface as it escapes, and thermal stresses cause this hard layer to peel off as a whole. The intervention must not dramatically change the natural elasticity modulus (E-modulus) of the material, but must only restore the resistance it has lost.

HMSA Glossary of Terms

• Consolidation: The process of strengthening porous building materials that have lost their physical resistance by using a binding agent.
• Penetration: The ability of the consolidant to seep into the depths of the material through gravity or capillary absorption.
• Modulus of Elasticity (E-Modulus): A measure of how much a material stretches under force. In restoration, the E-modulus of the new material must be compatible with the original material.
• Gelation Time: The transition process of silicate-based materials from liquid phase to solid (gel) phase.
• Reversibility: Since complete reverse action is impossible in consolidation, the principle of "re-treatability" is essential.

In restoration, a compatible material is one that can function in a balanced manner with the original components of a historical building in physical, chemical, mechanical, thermal, and aesthetic terms, and can coexist with the building after the intervention is performed.

The most critical threshold here is the principle of "Reversibility", which is one of the cornerstones of conservation discipline. A compatible material must be removable from the structure in the future without causing any damage to the original stone, brick, or mortar of the building if a more advanced technique is preferred.

When this holistic compatibility is not achieved, the following structural problems can emerge over time:

• Capillary and deep cracking,
• Salt efflorescence and blooming on the surface,
• Surface detachments and stone losses due to trapped moisture,
• Increase in microorganic and biological formations.

The Nature of Historical Buildings and Material Conflict

Historical buildings behave very differently from modern reinforced concrete structures. They are like living organisms: they breathe, transfer moisture, have a porous structure, and have established a microclimatic balance over hundreds of years.

When an incompatible, rigid material is added to this delicate balance, the system collapses. We see the most tangible example of this in cement conflicts: when high-strength Portland cement mortar or plaster is applied to a lime-based, flexible stone wall from the 19th century, the new mortar hardens excessively. During structural movement, stress concentrates on the stone. Consequently, while the mortar remains intact, the weakened historical stone fractures and turns to dust.

In other words, in conservation architecture, the “strongest material” is often the most incorrect one, causing the greatest damage to the structure.

5 Basic Characteristics of Compatible Materials

Compatibility criteria, established through modern laboratory analyses (petrographic examinations, XRD mineralogical analyses, and pozzolanic activity tests), are grouped under 5 main headings:

1. Vapor Permeability and Capillary Compatibility (Breathability): Historical buildings must be able to expel the moisture within their structure. If the new material used does not allow water vapor to pass and blocks the pores, moisture becomes trapped inside the wall. When combined with freeze-thaw cycles, this leads to salt crystallization, plaster blistering, and the deterioration of the stone's surface structure. It is therefore critical that the restoration material has a high vapor permeability, similar to the original texture.

2. Mechanical Strength and Elasticity Compatibility: The hardness and compressive strength of the new material must not be higher than the original mortar and stone of the historical building. Excessively hard repair mortars applied over relatively soft and flexible materials, such as natural stone, Khorasan mortar, or historical brick, lead to micro-cracks and edge fractures. The goal is not to concretize the building, but to preserve its original mechanical flexibility.

3. Thermal Movement Compatibility: Every material expands and contracts with temperature changes. However, the thermal expansion coefficients of cement and historical lime mortar or kufeki stone are completely different. Between two contrasting materials moving at different rates, loss of adhesion (bonding) begins over time, gaps open between them, and surface detachments occur.

4. Chemical and Mineralogical Compatibility: Many modern materials (especially Portland cement) contain high levels of sulfates and alkali-soluble salts. When these chemicals react with historical lime systems and stones, they trigger new and destructive chemical processes that disrupt the stone's pore structure. The mineral structure of the new material must not be chemically foreign to the raw material of the original mortar.

5. Visual and Aesthetic Compatibility: Restoration is a cultural act as much as it is a technical one. The new intervention must not overpower the historical depth of the building, destroy its patina, or create an artificial "newly built" feel. However, while doing this, it must not create a "historical forgery" by imitating the original surface identically; it should respectfully present the trace of its own period.

Character Conflict Between Old and New Materials

We can clearly understand why we must avoid cement-based materials in interventions by looking at the contrasts between original mortars and modern materials under the microscope:

Why "Looking Similar" Is Not Enough

One of the most common mistakes in restoration is making decisions solely based on the color and texture of the material. Even if two materials look identical from the outside, their pore diameters, water absorption coefficients, elasticity moduli, and thermal behaviors can be completely different. Therefore, deciding on "compatibility" based on pure observation without performing quantitative (numerical) analyses in a laboratory environment carries significant risks.