Metal Collection Preservation Climate Control

Metal artifacts and objects in museum collections require precise environmental control to prevent corrosion, tarnishing, and chemical degradation. The relationship between relative humidity, temperature, and metal corrosion rates is well-established through electrochemical principles and forms the foundation for HVAC system design serving metal collections.

Corrosion Mechanisms and Environmental Control

Metal corrosion in museum environments proceeds through electrochemical reactions requiring three conditions: presence of electrolyte (moisture), oxygen availability, and ionic conductivity. Relative humidity serves as the critical control parameter, with most metals exhibiting minimal corrosion below critical threshold values. The HVAC system must maintain conditions below these thresholds while avoiding rapid fluctuations that promote condensation.

Iron and steel artifacts require RH below 40% to prevent rust formation, as ferrous corrosion rates increase exponentially above this threshold. Bronze objects suffer from bronze disease (cuprous chloride corrosion) when RH exceeds 42% in the presence of chlorides. Silver tarnishes through reaction with hydrogen sulfide and carbonyl sulfide at concentrations as low as 0.1 ppb, requiring both humidity control and gaseous filtration.

Environmental Specifications for Metal Collections

Target environmental parameters for metal preservation:

Archaeological metal objects excavated from burial environments often contain soluble salts and chlorides that promote active corrosion. These materials require specialized desiccated storage at RH below 15%, typically achieved through sealed display cases with silica gel rather than relying solely on ambient HVAC conditions.

Gaseous Contamination Control

Atmospheric pollutants accelerate metal corrosion beyond the effects of humidity alone. Sulfur-based compounds (H₂S, SO₂, COS) attack copper alloys and silver. Organic acids from wood products, paints, and sealants promote metal oxidation. Chlorine compounds combine with moisture to form hydrochloric acid. HVAC systems serving metal collections require enhanced air filtration incorporating activated carbon, potassium permanganate-impregnated alumina, or similar chemisorbent media.

Gaseous filtration system design considerations:

• Minimum 85% particulate efficiency (MERV 13)
• Chemisorbent filters for acid gas removal
• Contact time through carbon bed: 0.05-0.10 seconds
• Media replacement based on challenge testing or annually
• Outdoor air pre-treatment before mixing with return air

Supply air delivery should avoid direct impingement on metal artifacts, as velocity effects can concentrate pollutants through boundary layer disruption. Low-velocity laminar flow patterns (face velocity <0.25 m/s at artifact surfaces) minimize this effect.

System Design Strategies

Dehumidification equipment selection depends on load characteristics and control precision requirements. Desiccant dehumidification systems provide superior low-humidity performance (below 30% RH) compared to mechanical cooling-based systems, particularly when space cooling loads are minimal. Hybrid systems using mechanical cooling for temperature control and desiccant wheels for deep dehumidification offer optimal performance for variable load conditions.

Humidity control precision requires:

• Direct digital control with ±2% RH setpoint accuracy
• Proportional-integral control of humidification/dehumidification
• Space RH sensors calibrated to ±3% accuracy
• Vapor-impermeable construction envelope (permeance <0.1 perm)
• Dedicated makeup air pre-conditioning

Storage vault design for highly reactive metals incorporates local environmental control through sealed cabinets with independent conditioning. This approach isolates problematic artifacts from general collection spaces and permits more economical HVAC system sizing for the main storage area.

Seasonal Compensation and Setback

Unlike organic materials sensitive to absolute humidity levels, metal artifacts respond primarily to RH rather than moisture content. This permits modest temperature setback (±3°C) during unoccupied periods without RH adjustment, reducing energy consumption. However, setback strategies must account for thermal mass effects and avoid condensation during recovery periods.

Cold metal surfaces in warm humid environments reach dew point temperatures, causing condensation that rapidly initiates corrosion. HVAC system startup sequences following extended shutdown require gradual temperature recovery (maximum 2°C/hour) with continuous dehumidification to prevent condensation on artifact surfaces.

Monitoring and Documentation

Continuous environmental monitoring through dataloggers provides documentation of preservation conditions and early warning of HVAC system degradation. Recording intervals of 15-30 minutes capture short-term fluctuations that may not trigger alarm conditions but contribute to long-term degradation. Statistical analysis of recorded data identifies patterns requiring operational adjustment before artifact damage occurs.