Picking up from our Vinegar Syndrome explainer: once film acidity approaches roughly 0.5, the chemistry crosses into autocatalysis. This post translates that chemistry into what you’ll actually see over time, using AD strip color as the most relatable signal.
For the underlying chemistry behind hydrolysis and autocatalysis, see our Hydrolysis and Vinegar Syndrome guides — this post shows how those reactions look in the real world over time.
CTA is made by adding acetyl groups to cellulose (acetylation). Hydrolysis removes those acetyls and produces acetic acid, pushing the polymer back toward cellulose-like behavior (more hydrophilic, stiffer, more brittle) while the acid formed accelerates further deacetylation (autocatalysis). The table below shows what this reverse process looks like to collectors.
AD strips are a proxy. They don’t measure pH directly—they report the presence of acetic acid vapor that correlates with rising film acidity inside the cel base.
| AD strip | Approx. film acidity (ml 0.1N NaOH / g) | What’s happening | What you’ll see | What to do |
|---|---|---|---|---|
| Blue | 0–0.1 (IPI Level 0) | Acetyl groups largely intact; hydrolysis minimal; free acid low. Hydrolysis has technically begun, but at an extremely slow rate. | Looks normal. Faint vinegar odor only when tightly sealed or stacked (and often not at all). | Move to cool storage; add acid scavengers; take baseline photos for comparison. |
| Blue → Green | ~0.2 (IPI Level 1) | Early deacetylation: more acetyls cleaving; acetic acid beginning to accumulate. Most acid is still bound in the polymer, so smell may be faint or absent. | Still visually stable. Odor may be noticeable when opening a sleeve or box, but lack of smell does not mean no decay. | Intervene now for maximum benefit—cool storage + scavengers can hold this stage for decades. |
| Green | ~0.5 (IPI Level 1.5) | Inflection point: autocatalysis begins. Both feedback loops (acid autocatalysis + hydrophilicity) start reinforcing each other; internal acceleration has started. | Corners lift; subtle warping/rippling, especially in unpainted areas. Odor may be mild or intermittent. | Prioritize cooling; minimize handling; isolate from other cels to avoid cross-contamination. |
| Yellow-Green | ~1.0 (IPI Level 2) | Significant loss of acetyl content; base becomes more hydrophilic; plasticizer loss stresses paint. Moisture uptake rises, causing swelling and dimensional stress. | Paint tacks/sticks to sleeves or backgrounds; fine cracking in large paint fields. Odor often becomes consistent at this point. | Stop sleeve swaps; add release barriers; aggressive cooling and monitoring. |
| Yellow | ~2.0 (IPI Level 3 onset) | Advanced deacetylation + chain scission; cellulose-like stiffness; heavy offgassing. Strong vinegar odor is common here—but acceleration began earlier. | Severe curl/shrink; flaking paint; cel feels stiff, brittle or oddly tacky. | Focus on slowing collapse; triage high-value pieces; handle only when essential. |
| Deep Yellow / Orange | ≥2.0 (IPI Level 3+) | Acetyl content effectively exhausted; comprehensive chain scission. DS very low—polymer behaves closer to cellulose (stiff, hydrophilic, brittle); network collapse. | Crumbling or shattering acetate; widespread paint detachment. | Artwork functionally lost; document condition; consider conservation decisions. |
Film acidity anchors follow the Image Permanence Institute (IPI) A-D Strip calibration benchmarks (free acidity measured as milliliters of 0.1N NaOH per gram of film).
Cellulose (–OH, hydrophilic, stiff)
⟶ Acetylation (adds –O–COCH₃) ⟶ CTA (flexible, clearer, thermoplastic)
CTA unit: Cellulose–O–COCH₃ (up to ×3 per glucose)
⟶ Hydrolysis (aging): Cellulose–O–COCH₃ + H₂O ⟶ Cellulose–OH + CH₃COOH
Degradation path:
1) Loss of acetyls (deacetylation) → polymer trends back toward cellulose-like chemistry
2) Acetic acid produced (free + bound) lowers internal pH → autocatalysis
3) Chain scission reduces molecular weight → stiffness ↑, brittleness ↑
4) Water affinity ↑ → swelling/warping; paint stress and adhesion issues
5) At high acidity, network collapse → terminal behavior
Takeaway: Vinegar Syndrome is the reverse of acetylation. Reset removes acid (free & some bound) and cold storage slows further hydrolysis—but neither recreates acetyl groups. Damage is cumulative and progressive over time.
For deeper details, see: