A cel is paint on a sheet of film. Stacked without an interleaf they stick together, and then the image is a lump. In the overview Security for collectors it is a special case of paper and plastic at once.

What breaks without a thief

Heat, light, pressure, the wrong sleeve. You store them singly, dark, cool, and flat, with an interleaf recommended by someone who knows cels, not with the first clear film you grab. A sheet that outgasses takes the paint with it.

When someone comes

Small and light, once it is framed or in a folder. Famous scenes get recognized. You do not hang the most expensive cel in the hallway. It lies flat in the cabinet if the climate there does not get hot. A certified safe that heats up in a fire is a risk for the sheet. The document cabinet with a steady temperature is often the better place. You settle that on the piece, not as an article of faith.

The folder

Production, scene, whether the backing is original, dimensions, proof, photo. Backing and sheet are two lines when both share the price.