Deterioration and Archival Storage

May is Preservation Month, and here at TSLAC we’re celebrating by launching several new environmental monitors throughout our building.  Providing a stable environment is one of the best steps you can take toward safeguarding collections.  By controlling temperature and relative humidity, you can slow the clock on natural aging processes and often avoid conservation treatment altogether.

How does environment matter from a conservation perspective?  Simply put, it impacts materials’ mechanical and chemical stability.  Mechanical damage may occur when fluctuations in temperature and relative humidity cause materials to expand or contract.  Rapid cycling of temperature and moisture is especially problematic.  This can lead to cracks, tears, and breaks.  One household example of mechanical damage can occur when pouring very hot water into a cold glass causes the glass to break.  This dramatic damage can also occur gradually over time if archival materials are improperly stored.

Mechanical damage: transcription disc.

Mechanical damage is one factor at work in the pictured transcription discs. The discs’ outer coating (cellulose acetate plastic) and inner core (glass) have reacted to their storage environment differently, causing severe cracks and losses. (Photo by Steve Kantner.)

Chemical damage occurs when materials change or weaken at the molecular level.  Chemical damage, in its many forms, generally speeds up in warmer, wetter conditions.  Without sufficient heat or moisture, some deterioration processes may never start.  Familiar examples of chemical damage driven by environmental conditions include yellow, brittle paper and silver mirroring in black and white photographs.  In archives, a particular concern is iron gall corrosion, the process by which historical ink can eat through paper.

Chemical damage: iron gall ink

Chemical degradation has caused the iron gall ink in this document to weaken its paper support. As a result, brown halos appear around heavily-inked areas, and losses are visible in the large letter “G”.

Archival environmental standards for paper-based materials are 45 – 55% RH, 65 – 72 degrees Fahrenheit.  While it’s OK for these levels to drift gradually across seasons, short-term fluctuations should be minimized.  Remember these environmental impacts as you safeguard your treasures.