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I made the original post in this thread, and thought I'd add an update. I received my replacement slides from Toyo in October 2016. They're made of plastic, not aluminum, and they weigh 18 grams...

So MAC Group got back to me and let me know that Toyo did their own testing and determined that the slides had been milled too thin, hence the translucency. They're going to replace my slides.

Doremus, I can't see sunlight through the dark slides outside, but I can see light from an LED headlamp coming through in a darkened room.

Patrick, I've seen the problem on at least one side of each of my 8 holders so I'm assuming they're all the same. Every one of my dark slides I've deliberately exposed to sunlight as a test has...

Toyo says it may be a manufacturing defect so they're sending me new dark slides to test.

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So I did another test to try to figure out the exposure threshold where light gets through the closed dark slide. I did 3 shorter exposures in direct sun (7500 footcandles) then 3 exposures in the...

So I did a test to try and isolate the problem, and as it turns out, the Toyo dark slides are definitely not opaque, and sunlight will definitely pass through them, as Christopher Barrett suggested...

Yes, it was a Toyo holder. My holders are brand new so it doesn't seem like they should be leaking. If the sunlight is getting through the dark slide itself that would account for the patchy texture....

It shows up on some of the Porta sheets I just had processed by a professional lab, so it's definitely an exposure problem.

Doremus, the fact that you think it looks like a light strike makes me want to investigate the idea that it happens during exposure. If it's an exposure problem it's intermittent, because I've had...

Joker, I've been handling the negs by the edges - I may have touched the picture area occasionally but not over the entire frame. I've been loading the tank with clean hands. The sheets were loaded...

Michael, Stephen and Jim, thanks for the suggestions. If it's the agitation technique that's the problem, could it only affect certain sheets in the tank, like I'm seeing?

One detail I forgot to...

I'm just getting started with 4x5 and I've been having an issue with a blotchy pattern on some of my negatives. My previous film darkroom experience was processing and printing B&W 35mm.

I'm...