After the screen as been coated, and dried, it is ready to be exposed. Your coated screen is like unexposed film. You need to leave it in the dark until you have everything set up to expose it.
The first light I used was a 75 watt clear bulb with a reflector. The Speedball instructions gave the exposure time for a 150 watt clear bulb at 17 inches above the screen as 1 hour 32 minutes. Since the bulb I was using had half the wattage, I doubled the time to 3 hours and 4 minutes, and this worked.
It just took way too long, so I decided to get a brighter light. I went to Home Depot and bought a 250 watt halogen work light for $8.50. It even came with a clamp and a couple bulbs. Since it is the UV rays that affect the photo emulsion, I removed the UV filter safety glass. I also removed the cage, as it cast shadows on my screen, and I was concerned it would expose parts of the screen less than others. *** If you remove the glass, or the cage, you do so at your own risk! *** I mounted the light 23 inches above the screen (I have it clamped to the side of a card table), and I exposed my screen for 60 minutes.
How I figured this time out was to print a transparency with time ranging from 10-60 in increments of 10. I exposed the whole thing similar to how I would burn a regular image, except that after 10 minutes, I covered up the 10, then after 20 minutes, I covered up the
20, etc. After I had exposed the screen for the full 60 minutes, I then washed it out. The parts of the screen that were under-exposed got rinsed away. The image started to be set at about 40 minutes. I have since bumped the exposure time up for transparencies that are taped
together, because I found that I would get pinholes along where the edges of the tape were. When I went to 60 minutes, I got a clean image, with no pinholes at the tape, and transparency edges.
There are several variables that affect how long exposure time should be. They are the type and brightness of the light, the distance from the light to the screen, and I am guessing that different brands of photo emulsion, may take different amounts of exposure.
Choose whatever light you are going to use and mount it to point down at your screen. I have a piece of cardboard that I have painted flat black, and I place it below the light. The screen will sit on this. The reason I do this is to minimize the amount of light that passes through the
screen, and is reflected back up causing some detail to be lost in the exposed screen.
Next, I place the screen on the black cardboard under the light. I position my positive on the screen where I want it to be burned in. I have a piece of Plexiglas that I lay over the positive to hold it tightly to the screen. For smaller screens, and use regular plate glass. Be careful not to cut mesh in screen with sharp edges of either glass, or Plexiglas. I then turn on the light, and start a timer. I use a stopwatch that I bought for a couple bucks at a thrift store, but a
kitchen timer would work well too. I expose my screen for the appropriate time.
When the exposure time is up, turn off the light, remove the Plexiglas, and transparency. The emulsion is still photo sensitive at this point, so you want to be care about exposing it to bright light, especially sunlight. You will probably be able to make out your image on the
screen. Take the screen to a large sink, or bath tub. Using lukewarm water, wet both sides of the screen. Continue to the spray the screen (about as hard as a comfortable shower, not a fire hose!!!!), until the emulsion has been washed away, leaving your image. Place your screen somewhere to dry in a horizontal position.