iridescent spots in between layers of cel?

[MajinTangerine]

Hey all,

I have recently noticed this on my favorite vegeta cel, so I wanted to get some expert opinions. The cel is two layers, and in between the layers, there are some spots or iridescence. Is this a common thing on multi layer cels?

Thanks all!

[Jadeduo]

Without seeing a picture this could be moisture that can collect between layers which you can generally blow out if you are super gentle. Or it could be the infamous dirt mildew there is not much you can do about that without a professional. If they are unstuck you could break out a mild solution of dawn and water and scrub it gently off with cotton balls and qtips.

Sense I put together a post about this on his update thread I believe.

Jadeduo

[sensei]

viewtopic.php?f=61&t=28751&p=339564&hilit=moire#p339564

It has come up before, and my sense is that it is a result of air (or possibly fumes emitted by the cels, which are chemically active) getting caught between the layers. It's thin enough that it creates a moire pattern, similar to that formed by an oil slick in a puddle. It can be banished fairly quickly by inserting a thin piece of paper between the layers, forcing the blank parts apart enough to vent the air/gas/water vapor. I'd leave a thin strip of microchamber paper in to soak up whatever it is, but in my experience it's no more than a cosmetic issue.

[MajinTangerine]

Thanks very much!

Here is a picture of the area, I will try the paper solution!

[Jadeduo]

That definitely looks like moisture, Sensei is right the paper method should clear that up quickly. Make sure you get some microchamber paper in that bag it will help curb that in the future!

JD

[sensei]

I'd put a bit in between the layers. It looks like you've got a good bit of blank space, and it would be good to keep any byproducts of the cel's reactions soaked up and not affecting the rest of the plastic. Just cut the paper to fit the blank space. If you put a sheet in with the cel, put it on the front side, as the paint can start to stick to it if you put it on back.

[frozentime]

Actually, that is more like classic thin film interference, instead of Moire. The latter is actually a low frequency pattern created by two or more higher frequency structures with a chirp or slight offset. Thin film interference occurs when you have either a solid/liquid film or an air gap between to surfaces that is close to the wavelength of light.. or is thicker, but only changing by about as much as a wavelength (~1 micron) over a visible distance.

There are multiple possibilities for this with cels. If the sheets are rough and clean, you will get no pattern. If the sheets have very, very low roughness they normally would stick together very well. This also shows no fringing, but micron scale particles can create gaps that introduce the fringing. As someone (I think sensei) mentioned earlier, you can also have some outgassing that creates trapped pockets. This can create a similar artifact, but would normally eventually diffuse, unless the outgassing rate is similar to the diffusion rate. If out-gassing is creating this, simply separating the layers and putting them back together would remove the pattern for at least a short period of time. Unfortunately, there are a few other possibilities that are leas appealing, but also likely. I've heard of some japanese collectors using cleaners that can leave a thin film and create this type of issue. I hope that some of the nastier cleaning substances I've heard used (like benzene) are too volatile to leave a film. Any oily residue that is thin enough between smooth plastic can also create the pattern, even without an air gap, in much the same way a soap bubble does, as long as it has a significantly different index of refraction to the celluloid. In general though, I think this is a little less likely than an air gap since cellulose acetate has an index of about 1.47 and most cleaning agents are between 1.47 and 1.49. That's generally too close to create bright fringing without an air gap. Water can show visible fringing at 1.33 (though it isn't bright), so any oil-like contamination would need to be in that range. Otherwise, an air gap is the most likely cause.

[LWK]

I corrected some cels with this today, but the layers were more accessible for the repair. A really gentle dry cloth helps. I also did a distilled water wipe for this. I wrecked some air brushing with that though, so be really careful, that stuff is super gentle.