# How long can you leave coated screens in the darkbox?



## Harold Boss (Aug 8, 2017)

I seem to have wasted three screens.

I made them up about two weeks ago and left them in the darkbox. I went to expose them today and only got a faint image. None of the emulsion would fall out despite blasting with a hose.

The only explanation I can think of is that the emulsion has been left too long in the darkbox and is no longer suitable for exposure.

This was a big surprise because the box is nice and dark, and was kept indoors in a shed anyway. It was only around two weeks and I vaguely remember doing it this way in the past.

Is it normal for coated screens to have a shelf-life even when kept in total darkness?

I'm using what is the most common Murakami emulsion. The screen mesh is also a good middle of the road size for t-shirts. I'm exposing the screens in the daylight for around 40-60 seconds.

I've performed the exact same sequence of events and got a good result, the only difference is that I left the screen in the box for just one day.

But I don't quite understand why they would go bunk just for being left two weeks in the dark, that's not intuitive to me at all. So I thought I would post here to see if coated screens do indeed have a shelf life that is that short.


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## NoXid (Apr 4, 2011)

I've left Saati PHU screens in dark storage for that long, and longer.

Hmmm, when emulsion expires, it fails to expose, so it should all wash out if it got old and expired (the whole bucket would be expired). So sounds more like it became exposed. I'm told excess heat can cause emulsion to expose. If not exposed to light or too much heat, I don't know.


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## Harold Boss (Aug 8, 2017)

yes it does seem like they were inadvertently exposed somehow. i've got a 25watt red lightbulb and the darkroom isn't the darkest... but i already made up a screen in exactly the same way and it worked fine. so... what gives? anyway i'll try again tomorrow and let you know of the result.


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## NoXid (Apr 4, 2011)

Given the time of year, could it have gotten too hot? It was 105 here for several days last week. My shop is insulated and has a concrete floor, so the temp stays more moderate. But before I did the insulation, that space would really bake in the summer ... probably would have been 120 in there last week if not for the insulation.

I've seen 95-100 mentioned as the temp not to exceed, when such admonitions include an actual temperature.


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## berkeley (Jun 14, 2014)

It sounds like you are using a diazo based on your exposure time. From what I've read, diazo emulsion has a one month shelf life once mixed, and coated screens can be stored for up to a month. You didn't mention the specific emulsion you were using, but based on the top result for Murakami diazos (http://www.murakamiscreen.com/documents/SP-1400.pdf), it also sounds like you should be using a yellow safe light. This may or may not be the problem, but it is best to follow direction to the T when trouble-shooting. I don't use diazos so I don't know, but wonder if the emulsion was 2 weeks into the shelf life when coated if that would shorten the coated storage time? I would call your distributor and ask some questions, and if they don't know the answers, call the manufacturer directly.

As an aside, I use a photopolymer, which has a shelf life of a year. Just recently got curious about some odd-sized screens that had been coated and stored in my dark room for about 4 years, and they exposed and washed out fine. I was surprised!


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## Harold Boss (Aug 8, 2017)

So I tried again today with freshly coated screens and again abysmal results. :-(

Seems like the screens are overexposing in only a few seconds. I should have done a step test but it's frustrating to think that you need to do a step test every single time.

I think exposing in the sun is just too difficult. I'm cursing the guy who recommended that approach to me.

I'm going to get hold of some blacklights if possible, that worked well in the past.


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## Harold Boss (Aug 8, 2017)

Thanks so much for the responses and advice, it's very much appreciated.

I was also wondering if anyone had tried using the UV blacklight bulbs rather than the tubes? Seems like the bulbs are much easier and cheaper to find these days, 40w bulbs seems to be the maximum you can get.


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## NoXid (Apr 4, 2011)

I would use a slow emulsion if using the sun as the light source, as that allows a bit more margin for error. So a Diazo type. Also, obviously one must consider time of day, angle of sun, relative angle of the screen, etc in order to repeat the same exposure time. For most other DIY light sources, I would use a fast Poly/SBQ type of emulsion as the UV light is likely to be weak. 

A single twist UV bulb would be a very weak source. My first DIY exposure unit used 4 40watt UV tubes, and it was fairly weak. 7 or 8 40w UV tubes would be decent, especially with a poly emulsion. The most common low budget DIY light source is a 500w halogen work light (with the glass removed). Still weak, but a better pattern of light dispersion than a twist bulb.

Uhm, one last thought. Leave a screen in your dark box for a few days and then see if the emulsion will wash out (don't expose it). Just to verify that something isn't going wrong at that stage of the process. Hmmm, also ... are you covering up the backside of the screen when you are exposing it to the sun? Else ambient light is going to expose it from the back. Washing it out under a safelight? Sorry, just tossing out ideas--apologies if some are lame


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## dynamikgraphics (Jul 21, 2013)

Harold Boss said:


> So I tried again today with freshly coated screens and again abysmal results. :-(
> 
> Seems like the screens are overexposing in only a few seconds. I should have done a step test but it's frustrating to think that you need to do a step test every single time.
> 
> ...


Exposing in the sun = PITA. It's a bad idea from every angle. For one, there's no way to get the film flush against the screen, meaning you can't get a crisp, sharp exposure. Secondly, the sun will bombard your screen with UV light, exposing it within seconds. It's the quickest way to over-expose a screen. 

If you don't have an exposure unit, you can make a homemade one:

1) Get a cardboard box that's 6" - 8" deep and at least 2' x 2'. 

2) Cut a hole on the side of the box near the bottom, large enough to thread a cord through. 

3) Pick up 2 fluorescent auto trouble lights from an auto parts store, then place them in the box with the lights facing up. Keep the lights spaced apart so they they will emit light evenly on the glass. Thread their cords through the hole and plug them into an outlet. 

4) Find a sheet of glass to place over the box, like out of a giant picture frame. Alternatively, you can pick up a sheet of frosted plexiglass from Lowe's, but this tends to buckle under the weight of a screen. 

5) To expose, turn the lights on, place the glass over the box, tape your film positive to the glass, then lay your screen down on top. Cover the screen with a black cloth like a tshirt (this prevents the backside from exposing). Be sure to place some books on the inside of the screen to weigh it down - this keeps the mesh flush against the glass to sharpen exposures. Exposure times vary, but I've done one for 5-6 minutes with this particular setup. 

6) Rinse out the screen either indoors or in a shaded area. Whatever you do DO NOT let it get exposed to direct sunlight; it will overexpose immediately.


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## Harold Boss (Aug 8, 2017)

I continued thinking about this problem and I think it's probably due to the transparencies. The earlier transparencies were nice and black held up to the sun, these later ones look decidedly grey. I'm surprised that it could cause such drastically different results, but seems to be the best explanation I can find. So most likely the screens were ok in the darkbox after all.


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## dynamikgraphics (Jul 21, 2013)

Definitely have to have a rich black on your transparencies, otherwise light will bleed though. Especially since you're exposing in the sun; like I said, sunlight is a brutal onslaught of UV light, so it only takes mere seconds for a screen to expose. 

For comparison, a metal hallide exposure unit uses UV light, and exposes a screen within 40 seconds. That's under less UV than sunlight.


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