Acid Emission in Animation Cels

1. What “acid emission” actually means

Collectors often talk about “vinegar smell”, “offgassing”, or “acid escaping the cel”, but the concept is rarely defined clearly. In preservation science, we use the term emission to describe:

Working definition Acid emission is the movement of acetic acid vapor from inside the cellulose acetate (CTA) film into the surrounding environment.

This is completely different from acid generation, which is the chemical creation of acetic acid inside the film due to hydrolysis. Generation happens inside the polymer; emission happens across the surface of the polymer.

Understanding emission is critical because it controls how quickly acidic environments form, how much stress the paint layer experiences, and how well any storage method actually protects the cel over time.

2. How acid moves inside a cel before it ever reaches the air

Acetic acid formed inside the CTA base doesn’t instantly “appear” in the room. It first has to move through a series of layers:

• The bulk cellulose acetate film
• Any paint layers on top of the image side (and their binders)
• Coatings or inks, if present
• Any backing materials such as paper, foam, or microchamber board
• The thin layer of air touching the cel surface

This movement is driven by diffusion, and diffusion depends on:

• Temperature of the film and surrounding air
• Moisture content of the acetate (how plasticized the film is)
• Degree of degradation and internal acidity
• Film thickness and paint coverage
• The concentration difference between the film interior and the surface

A lightly degraded cel has a very different diffusion profile from a heavily vinegar-syndrome cel. Two cels can show similar AD strip colors and still have very different paths for acid to escape.

3. Once acid reaches the surface: the boundary layer problem

Every solid object in air has a boundary layer—a thin, relatively stagnant region of air directly touching the surface.

Inside that boundary layer:

• Gas exchange is slower
• Diffusion dominates over bulk airflow
• Acid concentration is highest right at the cel surface

If the surrounding environment is poorly ventilated or partially sealed:

• The effective boundary layer thickens
• Local ppm of acetic acid rises
• More acid remains near the cel for longer
• The paint and backing live in a more aggressive environment

Frames, binders, sleeves, and boxes all shape that boundary layer. In practice, the cel doesn’t live “in your room” – it lives inside a small microenvironment that sits between the film and the rest of the world.

4. What determines the emission rate?

Several interacting factors control how quickly acetic acid leaves the film and enters the air:

Temperature

Higher temperature increases molecular motion and raises the diffusion coefficient. All else equal, a warmer cel emits acetic acid faster.

Relative humidity and film moisture

Moisture plasticizes CTA. A slightly more flexible, hydrated polymer gives acid more freedom to move. At higher humidity, both generation and emission can increase, though the relationships are not purely linear.

Degradation stage (internal acidity)

A more degraded cel has more free acid available to move. That’s why a strong vinegar odor is a late symptom—there’s a substantial internal reservoir feeding emission.

Paint and backing geometry

Heavy paint coverage can choke certain diffusion paths while leaving others relatively open. A cel with 80–90% paint coverage may emit very differently from one with 30–40% coverage, even at the same overall degradation level.

Microenvironment resistance

A sealed box, a semi-closed frame, and an open shelf all offer different amounts of resistance to the outward flow of acid. That resistance shapes how much acid accumulates near the cel and how fast the system reaches a new equilibrium.

5. Why emission is often misinterpreted

A lot of common advice about vinegar syndrome implicitly confuses emission with overall health. A few typical assumptions:

• “If it smells less, it must be better.”
• “If I ventilate it, I’m helping it breathe.”
• “Microchamber removes acid instantly.”
• “Leaving a sleeve open removes the danger.”

In reality:

• A cel can smell less simply because its environment is already saturated with acid, so the gradient is weaker.
• Ventilating a cel can sometimes increase hydrolysis by reducing the resistance at the boundary layer and constantly pulling acid—and moisture—out of the film.
• Microchamber materials are slow, steady stabilizers, not instant acid vacuums.
• Open storage trades one problem (buildup) for another (uncontrolled moisture and continuous emission into the room).

Without separating generation, emission, and accumulation, it’s very easy to draw the wrong conclusion from what your nose or an AD strip is telling you.

These same misunderstandings show up when collectors try to use AD strips to decide whether a cel is “getting worse” or “getting better”. To see why, we need to look at what AD strips actually measure.

6. AD strips: what they actually measure (and why it matters)

AD strips are one of the simplest and most widely used tools in the cel hobby, but they are also one of the most misunderstood. In preservation terms, an AD strip is not a degradation test – it is an emission test.

AD strips only respond to vapor-phase acidity, meaning:

Key point An AD strip measures what escapes into the air around the cel, not what is happening inside the film.

This distinction is critical.

A cel with severe internal degradation but slow diffusion may produce a relatively mild AD result. A cel with modest internal acid but very open diffusion pathways (for example, limited paint coverage and more exposed base) may show a stronger AD result.

AD strips can tell you things like:

• How much acetic acid is entering a test container
• How aggressive the microenvironment is around the cel or test piece
• How different storage setups influence local vapor chemistry
• Whether scavengers or materials are changing atmospheric conditions over time

But they cannot tell you:

• How much acid remains inside the film
• The internal pH of the acetate
• Whether hydrolysis is accelerating or slowing
• Whether the cel has chemically stabilized
• Whether the cel is “safe” in the long term

This is why AD strips live squarely in the Emission pillar of the Preservation Framework. They are extremely useful for understanding what the environment is doing, but they cannot be used on their own to define a cel’s underlying chemical condition.

Understanding what AD strips measure sets up an important distinction in the broader risk model: protecting the environment around the cel is not the same thing as lowering the cel’s baseline risk.

7. Emission and the Cel Nexus Risk Score

In the Cel Nexus Preservation Framework, overall risk is driven by the interaction of four factors:

• Generation – how fast acetic acid is produced inside the film
• Emission – how quickly that acid leaves the film and enters the air
• Accumulation – how much acid builds up in the microenvironment over time
• Control – how well the environment mitigates or reshapes those behaviors

A common misconception is that improving emission control—for example, by adding scavengers or upgrading a storage container—automatically lowers the cel’s fundamental likelihood of degradation. It does not.

Risk perspective Emission control is protective, not curative. It prevents the environment around the cel from accelerating damage, but it does not change the internal rate of acid generation.

This has several important implications:

• A cel that is chemically unstable remains unstable even if its atmospheric acidity is temporarily suppressed.
• Emission control can reduce cross-contamination and paint stress, but it cannot, by itself, move the cel into a lower-likelihood tier.
• Two cels in similar microenvironments may show different emission behavior and similar AD readings, yet share the same underlying risk if their temperature, humidity, and internal condition are comparable.

In other words, emission control is a risk mitigator, not a risk reducer. It shapes the external context the cel lives in, but the internal chemistry remains the core driver of baseline likelihood in the risk score.

This distinction becomes important when we talk about tier classification, stabilization versus correction, and what it actually means to “improve” a cel’s situation over time.

8. How emission is managed in preservation science

Conservation labs and archives don’t just “let things breathe” and hope for the best. Emission is managed by shaping:

• Concentration gradients near the film surface
• Air exchange rates in the microenvironment
• Diffusion paths through backing materials
• Where vapor is allowed to go once it leaves the film
• How much moisture and temperature fluctuation the system will tolerate

Tools used to influence emission include:

Scavengers

Materials designed to reduce acetic acid concentration by adsorption or neutralization. Different families include zeolites, activated carbons, impregnated carbons, and composites like microchamber papers and boards.

A detailed comparison of these materials is covered in the companion article: “Zeolite vs Activated Carbon vs MicroChamber: What Actually Removes Acetic Acid?” (coming soon).

Passive, controlled ventilation

Small, predictable leakage paths can prevent extreme buildup without stripping moisture too aggressively. The goal isn’t “lots of airflow” but a controlled balance between retention and release.

Diffusion barriers and liners

Sleeves, foils, and laminates can slow or redirect where acid goes once it reaches the film surface, shaping the boundary layer and the path toward any scavengers in the system.

RH stabilizers

Materials like Art Sorb are used to keep humidity within a narrow band, which in turn makes emission behavior more predictable and less spiky during seasonal or daily changes.

Engineered microenvironments

Boxes, frames, and binders designed as systems rather than containers can tune all of the above: how acid is generated, how it moves, where it accumulates, and what ultimately removes or neutralizes it.

9. Why emission control alone isn’t enough

It’s important to be clear about what emission control can and cannot do.

Key idea Controlling emission reduces atmospheric acid around the cel, but it does not stop acid generation inside the film.

Managing emission helps protect the paint, backing boards, and neighboring cels by keeping the external environment less aggressive. It also reduces the risk of cross-contamination in shared storage. But if the polymer itself is already in a degraded, autocatalytic state, emission control alone cannot reverse that chemistry.

This is why emission is just one of the four pillars in the broader Cel Nexus Preservation Framework: it interacts with generation, accumulation, and control, but doesn’t replace any of them.

10. Where emission fits in the preservation framework

In the full framework, we look at a cel’s environment through four interconnected lenses:

• Generation – how fast acetic acid is produced internally (temperature, RH, chemistry)
• Emission – how quickly that acid leaves the film and reaches the surrounding air
• Accumulation – how much acid builds up in the microenvironment over time
• Control – how we use materials, design, and engineering to keep the system in a safer state

This article populates the Emission branch. The temperature and humidity discussion in the Generation blog (T & RH) lays the groundwork for how acid is created, and future articles on Accumulation, Control, and the Risk Score will connect these ideas back to practical storage and display decisions.

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