Influence of Ambient Conditions on Building Materials: Study of Material Degradation in the Archaeological Areas of Prague Castle

Influence of Ambient Conditions on Building Materials: Study of Material Degradation in the Archaeological Areas of Prague Castle

Abstract

 

The conservation of archaeological sites presents unique challenges due to their exposure to environmental factors such as moisture, salts, and temperature fluctuations. This study focuses on the Third Courtyard of Prague Castle, where historical masonry and timber structures experience severe salt contamination and climate-induced degradation. The primary objective is to analyze the effect of ambient conditions on material deterioration using hygroscopic moisture content (HMC) testing, ion chromatography (IC), and ECOS-RUNSALT modeling.

 

Additionally, this research evaluates material properties such as porosity, density, and capillary absorption, which influence moisture and salt movement within the structures. Results indicate that opuka stone, the primary building material, exhibits high vulnerability to salt-induced damage, while timber elements suffer from biological degradation. The study provides recommendations for humidity control, efflorescence removal, and ventilation improvement to mitigate long-term structural deterioration. The findings contribute to a broader understanding of how environmental conditions accelerate material decay in historic sites, offering conservation strategies for similar cases.

 

 

1. Introduction

1.1 Historic Preservation and Archaeological Sites

Historic preservation is a crucial discipline focused on safeguarding cultural heritage from deterioration, ensuring that future generations can study and appreciate historical structures. Architectural conservation involves maintaining monuments, archaeological sites, and historically significant buildings through restoration and monitoring.

 

Among heritage structures, archaeological remains – such as buried foundations, cellars, and walls – face distinct conservation challenges. Unlike standing buildings that have been continuously maintained, archaeological sites are often excavated after centuries of burial, exposing them to new environmental stresses.

Key threats to historic masonry include:

• Moisture infiltration from rainwater, groundwater, and atmospheric humidity.
• Salt contamination, which accelerates deterioration through crystallization and hydration cycles.
• Temperature fluctuations, which cause expansion and contraction of materials.
• Biological growth, including fungi and moss, which weaken structural integrity.

 

Understanding the relationship between ambient conditions and material degradation is essential for developing conservation strategies that effectively prevent damage to archaeological structures.

 

 

1.2 Prague Castle Third Courtyard Site

The Third Courtyard of Prague Castle is an enclosed, subterranean archaeological site featuring remains from the 9th century onwards. The site consists of:

• St. Bartholomew’s Chapel: A medieval stone structure with surviving masonry walls and a paved floor.
• The Palace: A residential structure with a large stone cellar and staircase, indicative of a high-status building.

 

Since its excavation in the 1920s, the site has been subject to moisture ingress, salt deposition, and environmental fluctuations, contributing to severe stone and timber degradation. Modern interventions, including the installation of reinforced concrete support structures, have altered local environmental conditions, often exacerbating material deterioration.

 

1.3 Research Techniques and Objectives

This research aims to analyze material deterioration due to environmental conditions using:

• HMC Testing: Determines the relative humidity at which salts absorb moisture.
• Ion Chromatography (IC): Identifies types and concentrations of salts.
• ECOS-RUNSALT Modeling: Predicts salt crystallization and solubility cycles.
• Climate Data Analysis: Evaluates temperature and humidity fluctuations affecting material stability.

By integrating laboratory experiments, field monitoring, and computational modeling, this study provides a comprehensive understanding of how ambient conditions impact historic structures and offers practical solutions for long-term preservation.

 

 

2. Salt Contamination in Historic Masonry

2.1 Introduction to Salts

Salts are one of the most destructive agents in historic masonry. Once dissolved in water, they infiltrate porous building materials through capillary action and diffusion. When the water evaporates, the salts crystallize, exerting significant pressure on pore walls, which ultimately weakens the material’s structure. Over time, this leads to flaking, scaling, and loss of cohesion. The severity of salt damage depends on the type of salt, environmental conditions, and material properties.

 

2.2 Properties of Salts

• Solubility: Highly soluble salts travel further into masonry and crystallize upon drying.
• Hygroscopicity: Some salts absorb moisture from the air, remaining in solution and accelerating decay.
• Single vs. Mixed Salts: Salt mixtures behave unpredictably, often lowering the relative humidity at which crystallization occurs.

 

2.3 Sources of Salts & Moisture

Salt contamination originates from multiple sources, categorized as natural, environmental, and anthropogenic. In archaeological sites, salts accumulate gradually over centuries, often becoming concentrated in specific areas.

At Prague Castle, primary sources include:

 

 

Groundwater and Soil Contamination

1. 1. • Water in contact with masonry carries dissolved salts from soil into porous materials.
      • Rising damp transfers nitrates, sulfates, and chlorides into walls.

 

Atmospheric Deposition

1. 1. • Pollutants from Prague’s urban environment contribute to sulfate and nitrate deposition.
      • Rainwater carries dissolved atmospheric gases, forming acidic solutions that interact with masonry.

 

Historic Construction Materials

1. 1. • Cement-based materials used in past repairs release alkaline salts.
      • Lime mortars and old plasters can introduce salts over time.

 

Modern Interventions

1. • The reinforced concrete ceiling of the site contributes to calcium leaching, forming thick lime crusts on stone surfaces.
   • Improper cleaning methods (acidic washes, chemical treatments) leave residual salts that worsen damage.

 

2.4 Mechanisms of Transport

Salts migrate through materials via:

• Capillary Action: Water wicks up porous stone, carrying dissolved salts.
• Diffusion: Salt solutions move to areas of lower concentration.
• Evaporation & Recrystallization: Moisture loss causes salts to crystallize, exerting pressure on materials.

 

2.5 Mechanisms of Decay

The major decay mechanisms associated with salts include:

• Crystallization Pressure: Expanding salt crystals exert tensile stress on masonry, leading to cracking.
• Hydration Swelling: Some salts expand upon hydration, causing internal stress.
• Thermal Expansion: Salt-contaminated materials experience differential expansion and contraction, leading to micro-cracking.

 

 

3. Material Classification

3.1 Masonry

Opuka stone, the primary masonry material at Prague Castle, is highly porous and extremely susceptible to salt attack. Its mineral composition includes quartz, calcite, and clay minerals, making it vulnerable to moisture absorption. Physical characterization tests confirm that opuka stone has a high capillary absorption rate, meaning it readily absorbs and retains salts.

 

 

3.2 Timber

Timber elements, found primarily in subterranean wooden structures, have suffered from fungal colonization, salt contamination, and chemical treatments. Consolidation with carboxylic acid in past conservation efforts has altered its properties, leading to surface hardening but internal degradation.

 

 

3.3 Brick, Reinforced Concrete, & Soil

Brick elements exhibit lower salt absorption than stone but suffer from surface encrustations. Reinforced concrete, used in the protective covering, leaches alkaline salts, contributing to secondary contamination. The soil itself is a significant source of salts, continuously replenishing contamination in the masonry.

 

 

4. Damage Survey & Characterization

A combination of laboratory and field techniques was used to assess the impact of ambient conditions on material degradation.

1. Sampling and Material Testing: Samples were collected from key locations in the site.
2. Hygroscopic Moisture Content (HMC) Testing: Evaluated how salts absorb moisture under controlled conditions.
3. Ion Chromatography (IC): Identified specific salt concentrations in different materials.
4. ECOS-RUNSALT Modeling: Simulated salt crystallization cycles under varying humidity levels.
5. Climate Monitoring: Long-term data on temperature and relative humidity was analyzed to correlate salt activity with environmental fluctuations.

 

 

 

5. Experimental Methodology

5.1 Sampling

45 samples of stone, timber, mortar, and soil were collected from key locations for analysis.

 

 

 

5.2 Stone Characterization

Tests included:

• 48-hour Water Absorption: Measures moisture uptake capacity.
• Bulk Density & Open Porosity: Determines water retention potential.
• Capillary Absorption Coefficient: Analyzes moisture transport rate.

 

 

5.3 Timber Characterization

Samples were tested for fungal growth, consolidation effectiveness, and structural integrity.

 

 

5.4 Hygroscopic Moisture Content & Salt Content Analysis

• Sample Preparation: Standardized for consistency.
• HMC Testing: Determines the RH at which salts deliquesce.
• Ion Chromatography: Identifies salt compositions.
• ECOS-RUNSALT Modeling: Modeled salt behavior under fluctuating humidity conditions.

 

 

6. Experimental Results & Discussion

The experimental methodology provided valuable data on the physical properties of materials, their salt contamination levels, and their response to environmental conditions. These results offer insights into the mechanisms driving degradation in the Third Courtyard site and help develop conservation strategies.

 

48-hour Water Absorption at Atmospheric Pressure

This test evaluates the maximum amount of water the stone can absorb under natural conditions.

• Average absorption: 12.3%
• Range: 5.98% – 18.10%

These results suggest that opuka is highly absorptive, meaning it can retain significant moisture, which facilitates salt migration and crystallization cycles.

 

 

Key Findings:

Higher absorption values indicate increased susceptibility to salt attack.
Samples with pre-existing cracks absorbed more water, implying that structural damage accelerates moisture intake.

 

 

Bulk Density and Open Porosity

This test measures the stone’s density and the percentage of open pores, which influence moisture transport.

• Average bulk density: 2.01 g/cm³
• Average open porosity: 24.6%

A high open porosity value confirms that opuka readily absorbs and retains water, supporting capillary rise and salt transport.

 

 

Capillary Absorption Coefficient (CAC)

Capillary absorption dictates how quickly water rises through masonry via capillary action.

• Samples classified as “Highly Absorptive”
• Rate: 2.6–5.8 kg/(m²·√h)

This confirms that opuka allows rapid moisture ingress, which exacerbates salt deposition in evaporation zones.

 

 

Water Vapor Diffusion Resistance

This test examines the breathability of the stone, crucial for moisture release.

• Resistance Factor (μ): 334.7
• Higher values indicate reduced breathability

This suggests that opuka has moderate permeability, meaning it absorbs moisture faster than it releases it, leading to water retention and prolonged salt exposure.

 

 

Drying Behavior (One-Dimensional Drying Test)

• Water evaporates unevenly, leading to localized damage
• Slower drying stones accumulate salts over time, worsening internal stress

 

 

6.2 Timber Characterization

Timber degradation was assessed through biological cultivation and microscopy.

Biological Colonization & Microscopy

• Timber samples exhibited fungal growth, indicating high moisture content.
• Microscopy revealed cell wall deterioration due to salt crystallization inside wood fibers.

 

Discussion of Timber Results

• Heavily consolidated wood absorbed less moisture, confirming that past chemical treatments altered its properties.
• Unconsolidated timber showed increased fungal colonization, proving that salts encourage biological decay.

 

 

6.3 Hygroscopic Moisture Content & Salt Analysis

A critical part of the study involved measuring hygroscopic moisture content (HMC), salt content, and microclimate conditions.

 

Ion Chromatography (IC) Results

Major salt types detected:

• Chlorides (NaCl) – Highly soluble, moves easily through masonry.
• Sulfates (CaSO₄, Na₂SO₄) – Forms thick crusts, causes scaling.
• Nitrates (KNO₃, NaNO₃) – Hygroscopic, accelerates degradation cycles.

These results confirm that multiple salt sources are active, creating complex deterioration patterns.

 

 

Local Climate & Salt Crystallization Cycles

Temperature and humidity fluctuations were recorded over a 2-year period.

• Frequent RH fluctuations between 60–90%
  Threshold crossings triggered salt deliquescence and recrystallization

This means that salts dissolve and re-crystallize repeatedly, causing mechanical stress and damage acceleration.

 

 

Discussion of Climate Impact on Salt Crystallization

Key Observations:

• High RH encourages salt dissolution, leading to deeper penetration into masonry.
• Low RH causes rapid salt crystallization, resulting in increased internal pressures and damage.

Solution Implications:

• Humidity control is essential to slow deterioration.
• Climate stabilization measures must be implemented to reduce crystallization cycles.

 

7. Conclusions & Recommendations

The study demonstrates that salt contamination, moisture fluctuations, and environmental conditions are the primary drivers of material degradation at the Prague Castle Third Courtyard site.

 

7.1 Damage Survey & Mapping

The damage survey revealed that:

• Stone walls exhibit extensive salt efflorescence and flaking.
• Timber structures show biological decay and chemical alteration.
• Reinforced concrete components contribute to salt leaching.

 

7.2 Material Properties

Opuka’s High Vulnerability

• Highly porous, allowing rapid salt absorption.
• Slow drying rate, leading to long-term moisture retention.
• Frequent salt crystallization cycles accelerate surface scaling.

 

Timber’s Biological Sensitivity

• Salt-contaminated wood shows higher fungal growth rates.
• Previously consolidated timber reacts differently to moisture, affecting its preservation needs.

 

7.3 Salt Content and Local Climate

The microclimate data suggests:

• Frequent RH fluctuations activate salt crystallization cycles.
• Salt mixtures amplify the risk of damage by altering equilibrium humidity levels.

 

Recommendations

• Monitor Relative Humidity: Stabilize humidity levels to prevent deliquescence cycles.
• Salt Removal Strategies: Use gentle desalination techniques to reduce contamination.
• Improve Ventilation: Reduce humidity accumulation in enclosed spaces.
• Long-Term Monitoring: Continue climate data collection to refine preservation strategies.

 

 

Final Remarks

This study provides a comprehensive understanding of salt-induced degradation at the Prague Castle Third Courtyard site.

Key Takeaways:
✅ Opuka is highly susceptible to salt damage due to its porous nature.
✅ Frequent humidity fluctuations worsen crystallization cycles.
✅ Conservation must focus on environmental stabilization, salt mitigation, and protective measures.

 

Future Research Needs:

• Developing improved desalination methods.
• Assessing the impact of protective coatings on historic materials.
• Exploring alternative humidity control solutions for enclosed sites.

With proper conservation efforts, the long-term preservation of Prague Castle’s historic structures can be secured for future generations.