Why this article exists
"Archival" isn't a label—it's an outcome. Objects age because molecules move, react, and transform: acids migrate out of wood, humidity drives hydrolysis in paper and gelatin, and nanometer-thin water films let metals corrode. If you understand those forces, you can design storage that prevents damage rather than reacts to it.
What follows connects first principles—reaction kinetics, water activity, and pollutant generation—to practical material choices: which papers, plastics, foams, and case materials behave inertly, and why barrier films, silica gel, and adsorbents work. It's written so a hobbyist can act on it, and a designer can turn it into purchase specs and BOM notes.
Read straight through, or jump to a section below.
Deterioration = Chemistry + Transport
Three levers: temperature (energy), relative humidity (water), and pollutants (often emitted by storage materials).
Most decay mechanisms are ordinary reactions—hydrolysis, oxidation, corrosion—that speed up with temperature and available reagents. Relative humidity (RH) supplies water for cellulose hydrolysis and creates ultrathin electrolyte films metals need to corrode. Volatile acids (especially acetic and formic) are generated by wood products and coatings; in closed boxes and cases they accumulate and become active reagents.
Why Wood & MDF Cause Trouble—and How Barriers Fix It
Wood emits volatile acids that accumulate in closed volumes, corroding metals and acidifying organics.
Hardwood hemicelluloses carry acetyl groups that cleave over time, releasing acetic acid; lignin oxidation yields formic acid. Emission rises with temperature and humidity. In confined spaces these acids corrode metals (lead and copper alloys are highly susceptible) and acidify nearby paper and textiles. MDF and some plywoods can be as problematic as oak due to resin systems and wood chemistry.
Two fixes follow from the mechanism: use inert cabinetry (powder-coated steel) where possible, or isolate wood with aluminum-laminate barrier films (e.g., Marvelseal®). Metal foil provides orders-of-magnitude lower vapor transmission than polymer coatings, throttling acid diffusion into the enclosure.
Paper & Board: Why Lignin-Free and When to Buffer
"Acid-free" isn't marketing fluff: lignin and certain hemicelluloses are feedstocks for acids and color bodies. Using lignin-free, alpha-cellulose or cotton rag boards removes that fuel. For most paper artifacts, a modest alkaline reserve (e.g., 2–3% CaCO3) neutralizes acids as they form, slowing hydrolytic chain scission in cellulose.
Exceptions are chemically grounded. Some photographic processes (gelatin silver, cyanotype, certain dyes) are alkali-sensitive. Choose unbuffered but chemically stable papers and require a Photographic Activity Test (PAT) pass for anything contacting images. PAT screens enclosure–photo reactivity (yellowing, redox image change).
Plastics: Why PVC Fails and PET/PP/PE Succeed
PVC releases HCl and plasticizers; PET/PP/PE avoid those mechanisms.
Poly(vinyl chloride) degrades by dehydrochlorination, releasing HCl and forming conjugated polyenes; many PVC products also use mobile plasticizers (phthalates) that migrate. Results: tacky films, greenish deposits on coins, and staining of adjacent paper/photos. The safer families—PET, PP, and PE—lack a chlorinated backbone and plasticizers. For photographic materials, combine these with a PAT pass. For sleeves, envelopes, and pages, these three are museum defaults for a reason.
✅ Safer defaults
- PET (polyester) — stable, no plasticizers
- PP (polypropylene) — chemically inert
- PE (polyethylene) — simple, low-additive
- Acid-free board — lignin-free, buffered (when appropriate)
- Powder-coated steel — no off-gassing
❌ Use with caution / avoid
- PVC — HCl release, plasticizer migration
- Wood/MDF in sealed cases — acetic/formic acid emission
- Unspecified "cardboard" — often lignin-rich, acidic
- Natural rubber — oxidizes, off-gasses
- PSA tapes on artifacts — migration & staining
Foams & Padding: Stability by Polymer Backbone
Cross-linked polyethylene foams (Plastazote®, Ethafoam®, Volara®) are chemically simple, low-additive, and age well—ideal for mounts and drawer padding. Polyurethane foams oxidize and depolymerize; they yellow, crumble, and can emit acidic/oxidative species. Fine for short-term cushioning, risky for long-term contact.
Adhesives & Tapes: Why "Don't Stick It" Is Good Engineering
Pressure-sensitive adhesives are low-molecular-weight polymers and tackifiers. Over time they cold-flow, oxidize, and migrate into porous papers, leaving brown stains that are hard to reverse. Keep PSAs off the artifact. Use stable acrylic PSAs only for enclosure construction (edge-bonding sleeves, box assembly). For paper mounting, use reversible aqueous adhesives (e.g., starch paste) on hinges rather than the image surface.
Microclimates & Sorbents: Controlling the Inside of the Box
Conditioned silica gel sets RH; activated carbon/zeolite capture acids in tight enclosures.
In a tight enclosure, you can set humidity and scrub pollutants. Silica gel works by adsorption: pre-condition to a target RH and right-size for volume/leakage and it will buffer the headspace for months.
For pollutants, activated carbon is broadly effective for organic vapors (including acetic/formic acids); zeolites and activated alumina also perform well for acids in many case studies. Tighter cases and larger adsorbent capacity improve outcomes.
Photographs: Composite Materials, Targeted Safeguards
Photographs combine an image substance (silver or dyes) in a binder (often gelatin) on a support. Image layers are vulnerable to redox chemistry and binder interactions. That's what PAT screens: enclosure–photo reactions that cause image change or staining. Use PAT-passed PET/PP/PE for sleeves and neutral, unbuffered papers for image contact— especially for alkali-sensitive processes like cyanotypes.
Metals & Coins: Starve the Electrochemistry
Corrosion is electrochemistry that needs an electrolyte (water) and a reactant (often an acid). Starve it by keeping RH low and removing or blocking acid sources. That means no PVC, avoiding unsealed wood in closed drawers, and—if wood must remain—wrapping interior surfaces in aluminum-laminate barrier film and adding acid-adsorbing sachets. Lead in particular demands clean air; acetic/formic atmospheres are corrosive even at modest RH.
From Principles to Specs (Buy/Build Checklist)
| Use case | Preferred materials | Notes grounded in the "why" |
|---|---|---|
| Papers, maps, documents | Lignin-free, alpha-cellulose folders/boards; buffered (2–3% CaCO3) | Removes acid feedstock (lignin) and neutralizes new acids; slows hydrolysis. |
| Photographs (all types) | PAT-passed PET/PP/PE sleeves; neutral, unbuffered interleaves | Avoids alkali-sensitive reactions; PAT screens enclosure–photo chemistry. |
| Coins and medals | Inert capsules or non-PVC flips; low RH; no exposed wood in sealed drawers | Prevents HCl/plasticizer attack and acid headspace that drives corrosion. |
| Cases, drawers, furniture | Powder-coated steel; if wood, aluminum-laminate barrier film | Eliminates or blocks acetic/formic acid emission into the case volume. |
| Padding & mounts | Cross-linked PE foams (Plastazote®, Ethafoam®) | Stable polyolefin backbone; low off-gassing vs. degrading polyurethane foams. |
| Headspace control | Conditioned silica gel; activated carbon / zeolite sachets | Sets RH and adsorbs acids; performance improves with tight enclosures. |
| Adhesion | No PS tapes on artifacts; reversible starch paste hinges where applicable | Prevents adhesive migration and staining; keeps reversibility. |
For anything touching photos, log the supplier's explicit "PAT-passed" statement and the polymer family. For boards, capture pH, lignin-free status, and buffer % in your BOM.
Three Short Scenarios
Oak coin drawer → stable microclimate
Replace PVC flips with inert capsules; line the drawer with aluminum-laminate barrier film; add activated carbon and silica gel conditioned to ~35–40% RH. You've removed acid sources, blocked diffusion, and starved corrosion of water.
Family photo box in a closet
Move prints into PAT-passed PET/PP/PE sleeves; avoid glassine; add a small tray of pre-conditioned silica gel. You've eliminated reactive enclosures and stabilized RH where it matters—the headspace around the photos.
Small bronze in a display case
Wrap interior wood with barrier film; size silica gel for volume and target RH; include acid sorbents; Oddy-test foams/paints before installation. You've reduced emission, stabilized moisture, and screened materials before they reach the object.
References & Further Reading
- Image Permanence Institute (IPI): Preservation metrics & resources (temperature/RH & expected lifetime).
- Canadian Conservation Institute (CCI): Agents of Deterioration – Incorrect relative humidity; guidance on metals and pollutant effects.
- Library of Congress: Care, Handling, and Storage (papers, photographs, plastics, enclosures).
- Northeast Document Conservation Center (NEDCC): Preservation Leaflets (enclosures, photo storage, buffering guidance).
- ISO 18916 / Photographic Activity Test (PAT): see IPI overview and supplier certifications; use PAT as a contact-safety screen for photo enclosures.
- Conservation practice on exhibit cases: barrier films (e.g., Marvelseal®) and Oddy testing—see AIC Wiki Oddy Test and British Museum resources.
- PVC degradation & risk to collections: museum and numismatic guidance (e.g., NGC/PCGS) explaining dehydrochlorination, HCl release, and plasticizer migration.
- Foams in conservation: CCI/NPS technical notes on polyethylene vs. polyurethane foams for supports and storage.
- Microclimates & sorbents: NPS/CCI guidance on sizing/conditioning silica gel; literature on activated carbon/zeolite for acetic/formic acid removal.
These sources align the "rules of thumb" with the mechanisms described above.