Scope: Photo, Stamp & Memorabilia Storage (keepsake-storage) — archival-grade enclosures and containers for storing photographs/negatives, postage stamps, trading cards/collectibles, and other personal keepsakes (textiles, documents, medals). Excludes general office document filing/binders (separate leaf: binders) and coin/currency-specific holders (no sibling leaf found, folded into general_memorabilia as mixed_or_hard_object).

Seed:
1. The "thread count vs weave" equivalent here: "acid-free" is the word every listing leads with, but buffered-vs-unbuffered is what actually decides safety — buffering helps B&W photos and plant-fiber textiles but can damage color photos, cyanotypes, and protein-fiber textiles (silk/wool/leather).
2. The audio-engineer persona would notice first that "archival quality" marketing is rarely backed by the actual measurable bar (passing ISO 18916 PAT) — almost no retail listing states PAT compliance even when brand spec pages do.

Sources:
[gaylord_photo] https://info.gaylord.com/resources/guide-archival-storage-of-photographic-materials — buffered/unbuffered by photo type, PVC vs polyester/PP/PE, lignin <1%, PAT/ISO 18916 — fetched excerpt
[wiki_pat] https://en.wikipedia.org/wiki/Photographic_Activity_Test — PAT = ISO 18916, formerly ISO 14523 — fetched excerpt
[wiki_stamphinge] https://en.wikipedia.org/wiki/Stamp_hinge — hinge mechanism, gum contact, mid-20th-c shift to mounts — fetched excerpt
[jaysmith_mounts] https://www.jaysmith.com/Resource/Behind-The-Scenes/2014-07-31-Stamp-Mounts.html — split-back (Showgard) vs open-3-side (Hawid) mount styles — fetched excerpt
[startstamp_mounts] https://startstampcollecting.com/stamp-mounts-or-hinges/ — mounts vs hinges collector preference, black backing — fetched excerpt
[gaylord_textile] https://info.gaylord.com/resources/guide-archival-storage-of-textiles — buffered only for plant-fiber, unbuffered for protein-fiber/dyed, cedar chests unsuitable — fetched excerpt
[talas_tissue] https://www.talasonline.com/Acid-Free-Unbuffered-Tissue — unbuffered tissue for silk/wool and color photos — fetched excerpt
[cardshellz_guide] https://www.cardshellz.com/blogs/the-bunker/card-protection-guide — toploader pt scale 20-360pt, rigid PVC, side-loading pages — fetched excerpt
[ultrapro_onetouch] https://ultrapro.com/products/one-touch-x-for-sleeved-cards — non-PVC UV-blocking magnetic holders, pt sizes — fetched excerpt

Branches: product_type fork supported by gaylord_photo (photo materials differ from textile/stamp/card rules) and wiki_stamphinge/jaysmith_mounts (stamp mechanism is wholly different, hinge vs mount). storage_intent fork supported by cardshellz_guide/ultrapro_onetouch (UV-filtering only matters for display, not dark storage or handling).

Noise: brand/price/rating — excluded per rule. "Capacity (photos/stamps/cards per unit)" — size-dependent, not an expert-divides axis without pricing data this run, left out. "Color of box/album" — cosmetic, not expertise. Optical disc storage (ISO 18925, mentioned on gaylord_photo) — out of scope, this leaf is photo/stamp/memorabilia, not media/CD storage. Humidity/temperature of the storage room — environmental advice, not a product facet (no SKU attribute).

Corrections (2026-10-26):
- Removed `archival_box_access_style` from `makes_irrelevant` on the photo, trading_card, and frequent_handling_or_trading branches — contradicted `photo_enclosure_style=loose_in_archival_box`, cardshellz_guide's "place sleeved cards in acid-free storage boxes", and the facet's own why_experts_care; left the stamp branch's makes_irrelevant unchanged (defensible, stamps live in albums/stockbooks).
- Removed `iso_18902_stated` from `governing_preservation_standard.values` and its definition — neither cited source (gaylord_photo, wiki_pat) mentions ISO 18902; facet now only covers PAT/ISO 18916.
- Fixed `card_holder_thickness_pt.range` from {min:20, max:360} to {min:35, max:260} — 20pt is the bare card's thickness, not a holder size (smallest holder is 35pt, both sources); 360pt was single-sourced to cardshellz_guide, ultrapro_onetouch's own catalog corroborates only up to 260pt.
- Added facet `photo_negative_film_base` (cellulose_nitrate / cellulose_acetate / polyester_safety_film / unknown_or_not_film) under the photo branch, cited to gaylord_photo and new source [wiki_nitrate] — nitrate/acetate negatives are a safety-relevant gap (flammability, cold-storage need) that `photo_size_format_fit=negatives_or_slides` had no further branching for.
- Added `sleeve_plus_semi_rigid_holder` value to `card_protection_layer` between penny_sleeve_only and sleeve_plus_toploader — evidenced in cardshellz_guide (PSA requires a semi-rigid "Card Saver" over a toploader for grading submission); reworded the facet's question, definition, and why_experts_care to fit.
- Added vocabulary entries: Polypropylene, Polyethylene, Polystyrene, Penny sleeve, Semi-rigid holder (Card Saver), Magnetic holder, Cellulose nitrate film, Cellulose acetate film, Safety film.
- Changed `memorabilia_item_material_class.backfill_cost` from low to medium — its own backfill_note says a wrong default "can cause real damage," which doesn't fit a low-cost default-and-move-on backfill.
- Reworded the `storage_intent` cold-start question to "Archival dark storage, UV-protected display, or frequent handling?" so the wording itself (not just the rationale) reads as archival/collectible-specific.
- Rejected: the semi-rigid holder addition and the negative/film-base facet keep their existing single/double sourcing rather than seeking a third source for the pt-range max — psacard.com and other toploader-size pages returned 403 and web search quota was exhausted this run; capping at the ultrapro-corroborated 260pt was the safer fix the checker itself suggested as a fallback.