When exposing a screen, exposure time varies depending on the type of light, whether there is reflector, distance from the screen, bulb wattage, etc. And if you don’t expose a screen long enough, the image gets washed away. If you expose a screen too long, you will start to lose fine detail of the image.
What kind of light to use? I currently use a 250 watt halogen work light with the UV filter glass, and safety cage removed (you assume all risks if you remove your glass and/or cage from your halogen light). In the past, I have used a 75 watt incandescent bulb in a reflector with a clamp. Whatever you use, make sure you have some way of clamping it, or mounting it above the frame for an extended period of time.
Before you expose real work, you want to run a test to find the best exposure time. First take the light you have, and place it above a frame. You want to place it high enough that the whole screen gets equally exposed, but not so high that the screens will take forever to expose. If you use a spot light, it might focus too much light on the center of the screen, and not as much on the corners. This would lead to uneven exposure.
Once you have the light and placement figured out, we need to figure out some rough times. In the Speedball manual they give some times for a 150W clear incandescent bulb:
Screen size, bulb height, exposure time
8×10, 12 inches, 45 minutes
10×14, 12 inches, 45 minutes
12×18, 15 inches, 74 minutes
16×20, 17 inches, 92 minutes
18×20, 17 inches, 92 minutes
The higher the bulb is above the screen, the longer it needs to be exposed. If the wattage of the bulb is lower, you need to exposure it longer.
When I used the 75 watt incandescent bulb at 17 inches, I had to adjust the time. Since the wattage was half of the 150 watt recommended bulb, I simply doubled the exposure time. This worked fine.
When I started using a 250 watt halogen light at 23 inches, I needed to refigure the exposure time. Since the 250 watt halogen light was brighter than a 150 watt incandescent light. A halogen bulb puts out more light than an equivalent wattage incadescent bulb. So the exposure time would need to drop. And since I was raising the light higher above the screen, the exposure time would need to increase.
So not knowing the best time for exposure, I had to experiment. I didn’t want to expose screen after screen until I got it right.
This is what I did:
I coated a screen the usual way. I printed a transparency with numbers from 10 to 90 in increments of 5. With the light set up, I placed the screen under the light to be exposed. I had a stop watch going. After 5 minutes, I covered the 5 with an opaque piece of paper. After 10 minutes, I also covered the 10. I did this until all of the numbers were covered. Then I washed out the screen. The lowest numbers that were distinctly printed gave me the correct exposure time. The numbers below 40 all got washed away. 50 looked okay, but 60 looked better. So I used 60 minutes as an exposure time for my 250 watt halogen light (without UV filter glass) at 23 inches. With experience, I have since lowered the exposure time to 45-50 minutes.
What kind of emulsion are you using? Do you have a favorite brand that is most effective with your light and timiing?
Comment by Marissa — May 14, 2007 @ 8:40 pm
I use Diazo Photo Emulsion by Speedball. I buy it in the larger 26.2 ounce jars. But there are less expensive smaller-sized jars available. Between coating screens, I store it in the refridgerator.
Comment by Rob — May 14, 2007 @ 10:47 pm
I just did my first screen with Speeball’s Diazo kit. I used direct sunlight (no haze, clouds, etc.) for about 50 seconds. It was slightly underexposed because the wind moved my positive (plexi I had was larger than the screen) but I had my timer set for 60 seconds. I think 60 would have been a little better. Anyway, it worked! I have a screen drying as I type this. I will be cutting a piece of plexi for the frame I have and make a backboard with black felt that will fit it as well, then I can fix everything in place next time.
Comment by Ken — August 21, 2007 @ 8:24 pm
Sunlight is usable, BUT unpredictable. The times you used today, may not work later. There are too many variables such as season, air polution, clouds, time of day, etc. The time needed at noon, may be less than what is needed at 6pm. Just be prepared for some screens that don’t turn out as well as you would have liked.
Comment by Rob — August 22, 2007 @ 7:08 am
Using the Diazo photo emulsion, I took a cue from the instructions that came with the kit and picked up a BBA 250 watt photoflood. I exposed a 10×14″ screen for ten minutes with the bulb twelve inches away. The image washed out almost perfectly, save for a couple of segments of some very thin lines. I’m assuming I could have exposed it slightly longer. In any case, I picked up the the photoflood for less than ten dollars at a camera shop. Seems to do the job nicely.
Comment by Phil — January 6, 2008 @ 6:05 am
Does anyone have a comment about washing out the screen after exposure. One-side wash, both sides – light pressure – harder pressure – water temp- etc…
I really appreciate any advice… artist communities like these are our last resort from big-boxes.
Comment by dave — March 10, 2010 @ 4:39 pm
Dave, when i spray out the screens after exposure, I will have the spray nozzle on shower, and spray both sides. Then I will focus on one side, and spray it with the shower. This will get most of it clear. If there is trouble spots, I will go to flat spray, and spray a little more intensly. As for water temp, I will use cold to luke warm water. Don’t use hot water as it seems to remove emulsion faster and might wreck your exposed image.
Comment by Rob — March 10, 2010 @ 6:26 pm
so i got a bigger screen and image bout 11×14 i have the 150 watt bulb and im trying to determine exposure time last screen i did was a smaller kit size screen and i exposed it for 20 min at 12 inches an it worked great any ideas?
Comment by josh — January 3, 2011 @ 9:44 pm
For a larger screen, you will probably have to back to light off. You want to make sure the edges of the screen aren’t signicantly farther from the light source than the center. Since the light will be farther away, you will need to increase the exposure time. Follow the steps I did in this article.
Comment by Rob — January 4, 2011 @ 8:01 am
i use solar and it takes 7-10 mins.in ghana
Comment by SAMLAMP — January 10, 2011 @ 12:47 pm
Using bright sunlight to expose the Speedball emulsion worked well in Palm Springs, CA. It took me a few tries, but Ken was right, 60 seconds was all it needed at 12:00 noon. The emulsion was still a bit difficult to wash completely clean. So I tried tipping the screen reclaimer bottle into a soaking wet rag and gave one quick rub over the stuck-on area then immediately sprayed with water. It loosened the clogged areas without destroying the stencil. Try it at your own risk, but after four wash-outs I was desperate to get it done.
Comment by Rick — March 26, 2011 @ 9:05 pm
That’s the problem with using sunlight. There is a lack of control. Unless you have a lightmeter or something, you can’t really tell how much sun the screen is getting. It sounds like your screen was overexposed a little bit. Yeah, a light takes longer, but you have more control over the exposure.
Comment by Rob — March 26, 2011 @ 9:31 pm
I just tried photo emulsion for the first time. Overall, I’m really pleased, but on both my screens I think I had a problem with glare from the 150w light bulb. In both cases, I had two transparencies on top of the emulsion, but under the glass. I measured the height of the bulb to 12 inches and set a timer for 45 minutes (following directions). Most of the blocked emulsion washed out both times, but on both screens (exposed separately) I had a circle of over exposure that seemed to be a spot of ‘glare’ where the light would’ve been most direct.
I was able to use a fine bottle brush, time, elbow grease, and my stubbornness to fix it but how can I avoid the problem in the future? I worry about lessening the exposure time since the rest of the screen seemed to be perfect. My fiance suggested glare resistant glass from a hobby shop. Opinions?
Comment by Stephanie — May 11, 2011 @ 10:37 pm
I would raise the light, and increase exposure time. The spot closest to the light will get more exposure than the parts farther away. Take it to an extreme level. Think of the light an inch from the screen. The center would be an inch away. The outsides might be 6 or 7 or more inches away. the center would get the brightest light. So it would be overexposed while the outsides would be underexposed. Moving the light farther away, will even out the exposure.
And you really only need to coat one side of the screen. The more even the coating the better. Too thick may cause exposure problems.
Comment by Rob — May 12, 2011 @ 7:06 am
i am trying screen printing for the first time and unprepared and eager to get my project done, i only have a 40W bulb! any suggestions and exposure time and distance? im not good with these things! thank you!
Comment by Riley — June 29, 2011 @ 3:45 am
A 40w bulb will take a long time. Chances are you will have other light in the roon, like sunlight coming through the windows unless you always expose your screens in a dark roon with just the 40w bulb. I suggest you get a stronger bulb, and go through the steps I outlined above.
Comment by Rob — June 29, 2011 @ 6:52 am
surprisingly, it worked perfectly for me! I put my screen under my 40W lamp in the complete dark and went to bed, set my alarm for a few hours and got up and covered it up and turned off the lamp. rinsed it out this morning and it worked great, but i really do need to get the proper supplies
Comment by Riley — June 29, 2011 @ 12:27 pm