Mounting and conserving Persian Haft-Rang tiles on soaring domes and iwans demands specialized engineering. Tiles are bedded in traditional Saruj—a water-resistant lime-and-sand mortar reinforced with goat hair and crushed terracotta—or breathable hydraulic lime grout. Conservation requires non-destructive desalination of salt blooms, re-adhering detached tiles with injected micro-lime grouts, and avoiding rigid Portland cements.
At a Glance: Key Takeaways
- Traditional Bedding Mortar: Saruj (slaked non-hydraulic lime, clay, river sand, cattail fluff, and wood ash).
- Thermal Expansion Mechanics: Accommodating extreme desert diurnal temperature swings (-10°C to +50°C).
- Curved Geometry Installation: Tapered joint beveling and segmented radial courses on spherical mosque domes.
- Primary Pathology: Subflorescence of soluble salts (sodium sulfate/halite) causing tile spalling and glaze detachment.
- Desalination Protocol: Repeated cellulose pulp or sepiolite clay poulticing with deionized water.
- Structural Re-Attachment: Low-pressure injection of natural hydraulic lime (NHL 2) micro-grout.
Monumental Mounting Physics: Saruj Mortar and Domes
Cladding a double-shelled masonry dome—such as the 52-meter turquoise dome of the Shah Mosque in Isfahan—imposes extreme physical challenges. The cladding must resist gravity, high desert winds, torrential spring rains, freezing winter snow, and intense solar baking.
Persian builders bedded tiles in Saruj, an ancient water-impervious mortar crafted from slaked fat lime, silica sand, pulverized wood ash (supplying reactive pozzolanic silicates), and vegetable fibers (cattail fluff or goat hair). Unlike rigid modern cement, Saruj possesses high elasticity, slow carbonation over decades, and phenomenal moisture-buffering capacity, flexing with the dome's structural micro-movements without shearing the ceramic tiles.
Salt Decay Pathologies: Subflorescence and Glaze Spalling
The number one killer of historic Persian tilework is soluble salt crystallization. Groundwater or rain dissolves mineral salts (halite, gypsum, thenardite) within the underlying brick masonry. As moisture migrates outward and evaporates through tile seams, salts crystallize beneath the impermeable glassy glaze (subflorescence).
The crystallization pressure of growing salt crystals exceeds 150 to 200 MPa, far exceeding the tensile strength of the quartz ceramic body (5–10 MPa). This hydraulic pressure shatters the quartz biscuit immediately behind the glaze, causing irreplaceable Safavid painted glazes to pop off and disintegrate into dust.
Desalination and Re-attachment with Injected Micro-Lime
Conservation protocols strictly reject pulling intact historic panels down unless structural collapse is imminent. Detached, hollow-sounding tiles are mapped using acoustic tap-testing. Conservators drill tiny 2mm holes through grout joints into the rear voids.
After pre-wetting the cavity with ethanol-water, a ultra-fine, highly fluid micro-lime grout—composed of natural hydraulic lime (NHL 2), micronized limestone dust, and 0.5% cellulose ether—is injected under low pressure (under 1.5 bar). The grout fills voids behind the tiles, restoring 100% structural contact without introducing harmful soluble salts or rigid stresses. Surface salts are extracted by applying wet paper pulp poultices changed repeatedly until conductivity meters register zero ion migration.
Mortar Selection Matrix for Persian Architectural Tile Installations
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Mortar Typology | Compressive Strength | Vapor Permeability | Elasticity (E-Modulus) | Conservation Suitability |
|---|---|---|---|---|
| Traditional Saruj | 3.5 – 5.0 MPa | High (breathable & flexible) | Low (absorbs thermal drift) | Gold standard for historic dome restoration |
| Natural Hydraulic Lime (NHL 2) | 2.0 – 3.5 MPa | Very High | Low to Moderate | Ideal injection grout for hollow tiles |
| Lime-Sand Grout (1:2) | 1.5 – 2.5 MPa | Highest permeability | Lowest stiffness | Recommended for pointing external seams |
| Portland Cement Mortar | 25.0 – 35.0 MPa | Extremely Low (vapor barrier) | Extremely High (brittle rigid) | STRICTLY PROHIBITED; causes rapid tile destruction |
Practical Constraints
Every successful decoration project must account for physical and environmental constraints:
Curved compound geometry of spherical domes requiring tiles to fit without jagged lippage
Mitigation: Taper the rear lateral edges of tiles at 5° to 10° angles and lay courses in exact concentric horizontal rings.
Execution Blueprint & Craftsmanship Sequence
Execute this professional sequence to achieve flawless results without backtracking:
Map Void Cavities with Non-Destructive Acoustic Tapping
Tap every tile lightly with wooden mallet; mark hollow, detached zones on digital CAD elevation map.
Desalinate Salt-Contaminated Panels with Cellulose Poultices
Apply 15mm thick deionized water and Arbocel cellulose pulp poultice over tiles; leave 48 hours to draw out salts.
Drill Injection Port Holes in Grout Intersections
Carefully drill 2mm holes through soft mortar joint nodes into hollow rear cavities using diamond core bits.
Flush Internal Cavity with Alcohol-Water Solution
Inject 50/50 water and denatured ethanol to break surface tension and wet ancient dust inside the void.
Inject Fluid NHL 2 Micro-Lime Grout Under Low Pressure
Pump fluid lime micro-grout through syringe at 1.0 bar pressure until slurry emerges from adjacent weep holes.
Repoint Tile Joints with Breathable Lime-Sand Mortar
Seal injection holes and point joints with 1:2 slaked lime and fine silica sand, wiping glaze clean before set.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Using Portland cement or epoxy to reset detached Persian tiles on historic domes
Pressure washing historic tiled iwans to remove desert dust and bird guano
Project Execution Checklist
Track your physical progress. Checkboxes persist automatically on this device:
Frequently Asked Questions
Why is Portland cement catastrophic for historic Persian tilework?
Portland cement is completely impermeable to water vapor and five times stiffer than the tiles. It traps moisture within the masonry, releasing calcium hydroxide salts that crystallize behind the glaze, causing explosive spalling.
How are Haft-Rang tiles curved to fit spherical mosque domes?
Tiles on domes are typically flat 15cm x 15cm units. The curvature is absorbed by the flexible Saruj mortar bedding layer and slightly tapered radial joints, creating a smooth faceted spherical curve visible from ground level.
