The molecule is real: vinegar smell = acetic acid
Vinegar is a water solution of acetic acid (CH₃COOH). When cellulose acetate (including many animation cels and film bases) slowly hydrolyzes, it will release that same molecule into air as a vapor. So when you smell “vinegar,” you are detecting acetic acid in the headspace — not a mystery odor.
Important Clarification: smelling vinegar means degradation is happening, but it does not necessarily mean the cel has reached the critical 'autocatalytic' destruction phase. It can mean: (1) the cel is emitting very low acid levels, (2) storage is trapping vapor (poor ventilation / tight microenvironment), (3) background acidity from other sources, or (4) early-stage behavior not yet in the autocatalytic regime.
Why odor can show up before VS
Your nose can be an extremely sensitive detector, but it isn’t a calibrated instrument. “I can smell it” is often a leading observation (something changed), yet it’s a poor quantitative measure (how much changed).
Key point: many people can detect acetic acid at very low concentrations — often in the sub-ppm range — and sensitivity varies wildly person-to-person. That means you can smell “vinegar” before a cel crosses a practical VS threshold.
Collector translation: the vinegar smell can show up as early as “Stage 0.5” (i.e., detectable odor / early emission), even if the cel is still below what we treat as a practical VS threshold. In our framework, we treat A–D ≈ 1.5 / ~3 ppm as a conservative minimum “autocatalytic concern” line — but you can smell vinegar earlier than that, potentially as early as ~1 ppm (Stage 1 minimum) or even below, depending on the person and the setup.
The "Stage 0.5" Phenomenon
Officially, A–D strips measure Level 0 (Blue) to Level 1 (Green). However, the "Vinegar Smell" can appear before the strip turns fully green.
We call this Stage 0.5: You detect an odor, but the strip is a muddy bluish-green (Teal) rather than bright green. This indicates the cel is off-gassing (<1 ppm) but hasn't yet reached the full Level 1 benchmark.
The reason smell can appear early is geometry + airflow: if you open a sleeve/bag and sniff near the source, you’re not smelling the “average room concentration.” You’re sampling a local boundary layer that can be much higher than the bulk air.
Smell is tricky because it has no reliable scale
Odor intensity does not map cleanly to ppm because it depends on airflow, humidity, temperature, how recently the container was opened, and human perception variability. Two cels could emit the same ppm, yet smell different if one is in a tighter sleeve or if one was just warmed slightly.
Engineering translation: smell can behave like a “lagging indicator” if you only notice it late — but it can also be a leading indicator of bad storage (acid trapping) because you can detect it at levels below your VS threshold. The catch is: you generally can’t map smell intensity back to a reliable “level.”
Use thresholds: smell → confirm with a controlled A–D strip test
Most collectors won’t run ppm instrumentation — and they don’t need to. The practical workflow is: use odor as a flag, then run a standardized A–D test in a small enclosed volume.
Best collector test (simple + repeatable): Place the cel in a new, clean, polypropylene sealed cel bag with an A–D strip for 24–48 hours (don’t flatten it — leave some air space). Then interpret the strip as a threshold indicator (not a precision meter).
| Signal | What it’s good for | What it’s bad for | What to do next |
|---|---|---|---|
| Vinegar odor | Fast “something changed” flag | Severity ranking (no reliable ppm mapping) | Run a 24–48 hr bag A–D test |
| A–D strips | Threshold screening (pass/fail bands) | Fine discrimination inside a band (non-linear) | Use as a decision gate for intervention |
| ppm measurement (optional) | Quantitative comparisons for advanced users | Not accessible to most collectors | Only if you already have the tools |
Smell can indicate bad storage even when VS is not present
If you detect vinegar odor at low A–D levels, the likely story is often not “the cel is doomed” — it’s “the storage is allowing acid to accumulate.” That’s usually a storage design problem, not an immediate “chemical intervention required” problem.
Simple collector decision tree:
1) Smell vinegar → run a 24–48 hr sealed bag A–D test.
2) If A–D indicates a meaningful threshold crossing (e.g., trending into your concern band) → branch to
chemical intervention (scavengers / controlled conditioning).
3) If A–D stays low → branch to storage intervention (reduce trapping, improve airflow strategy, reduce temperature, control humidity).
- Storage intervention: avoid designs that trap vapor in a stagnant pocket (tight backing + no exchange path), especially during warm / humid conditions.
- Temperature: colder storage slows the underlying hydrolysis and reduces emission rate.
- Humidity control: manage moisture availability (absolute humidity matters), because water drives the hydrolysis chemistry.
- When needed: use an acid sink (scavenger) intentionally — not as a superstition — based on A–D results.