Historic restoration and conservation · Lesson 2 of 4

Historic restoration and conservation: materials, standards and design constraints

What the Standards require when historic metal is replaced, when a substitute is acceptable, galvanic separation, thermal movement by metal, copper weights, the limits of what AFS forms, and who reviews.

You will be able to: State the replacement standard for historic metal, choose between in-kind and substitute metal with the reviewer, separate dissimilar metals, work a thermal-movement figure from published values, and check a piece against what AFS can form.

13 min · 11 slides · 12 questions

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Slide 1 of 11

The replacement standard

Rehabilitation Standard 6 says deteriorated historic features will be repaired rather than replaced.

Where replacement of a distinctive feature is unavoidable, the new feature must match the old in design, color, texture and, where possible, materials.

Replacement of missing features must be substantiated by documentary and physical evidence.

Sources

The Secretary of the Interior's Standards for the Treatment of Historic Properties with Guidelines for Preserving, Rehabilitating, Restoring & Reconstructing Historic Buildings, Part 1: Preservation and Rehabilitation (2017) - U.S. National Park Service, Technical Preservation Services

Glossary

In kind: Replacement with the same kind of material as the original, as the Guidelines recommend when an entire metal feature is too deteriorated to repair.
Compatible substitute material: A different material that closely matches the visual and physical properties of the historic one and may be considered when using the same material is not feasible.
Galvanic corrosion: Corrosion of the less noble of two different metals placed together without a separating material, for example copper corroding steel.
Thermal movement: The change in a piece's length with temperature, equal to the expansion coefficient times the length times the temperature change.
Girth: The total flat width of a formed piece before bending, hems included, which AFS limits to 48 in.