Exposing the Screens:

All you need is ultraviolet light, which you can get from the sun, a halogen light bulb, and certain types of florescent lights. Grow lights for plants will work too.  We’ll start with the cheapest method, the sun. The idea to place the film positive in front of your light source and right up against the screen. The better contact you have, the sharper the image will be. You’ll want to put it on backwards, As if you laid the positive on the ground, and set the screen on top. If you are looking at the inside (squeegee side) of the frame, the artwork looks as it should when printed. You can use clear tape, but it’s hard to get the film to stay right next to the screen across the whole image. Short of some sort of static electricity experiment, you’ll need a can of spray tack or a piece of glass.  If you use spray glue, try to get the kind that is made for repositioning, kind of like a sticky note.  Very lightly coat the film positive and adhere it to the outside of the screen. Too much glue and it will gum up the screen, making it harder to wash out the image area. Not enough glue and you won’t have contact everywhere, and the light waves will refract around the dark areas and make your lines fuzzy.

 

A better way to expose a screen in the sun is with a small piece of glass that is jot much larger than the image area. The weight of the glass will hold the positive against the screen. The larger the glass the less tight the fit since the weight of the glass will cause the screen to stretch a little. If you are using this method, it’s best to expose the screen around noon when the light is as close to perpendicular to the frame as possible. Also, make sure the inside of the screen is covered so that ambient sunlight doesn’t get through to the emulsion that way. If you are using a light bulb of some sort you can make a quick and dirty exposure table that works quite well.  Put the light bulb on the ground, shining up. Suspend some glass over he sawhorses, using a couple of long pieces of  wood. Make sure the glass is thick enough hold the weight of the screen plus the weight of a couple of books. Tape the corners of the positive onto the back of the frame, then lay it on the glass. Place books or newspapers or a plastic bag filled with a little sand (or cat litter) spread evenly over the image area. The added weight will push the screen against the positive, and the positive against the glass. If you cover the entire image area, you contact will be perfect while at the same time prohibiting the light from exposing the crucial image area of the screen from behind. It’s win/win as long as your glass is strong enough.  You can use a storm window or plexi-glass something similar, as long s it doesn’t block UV Rays like auto glass or expensive double pained windows.  Professional tables are made with a rubber blanket and vacuum that sucks the air out and compresses the positive against the screen and glass, but it’s the same principal. For most uses this will work just as well.

The amount of time you expose the screen depends on how strong the light is as well as the particular emulsion. Plan on the first one not working. In fact, the best way to get a good exposure guide is to make a test positive with long repeated areas containing lines and or type of varying thickness.  Secure the positive to the screen and cover most of it with a thin piece of cardboard in one minute intervals you will move the cardboard area over so that al little more of the screen is exposed. By the end of the process you will have a screen that test many different exposure times at once instead of having to do a ton of trial and errors. Be sure to keep track of how many intervals you had, and keep each interval the same amount of time.  The shortest exposure times will likely show nothing at all on the screen, so you’ll have to count backwards by subtracting intervals from the total time.  You can choose smaller intervals, but you will likely be able to tell from minute to minute or judge whether you need half minutes based on how well it washes out. 

Wash out the screen with a garden hose. First stream water over both sides of the screen until fully wet. Parts of the image should become visible at once. Actually, if you hold the screen at the right angle in the light you should be able to see the image faintly even before you get it wet.   If you are using a vellum positive, you will also see a faint outline of the paper, but don’t worry about it.  Continue soaking the screen on both sides, after a minute or two the light (unexposed) areas should start to dissolve, revealing the screen. You can use some pressure from the hose, but try not to blast it really close to the screen. Use a spray head that has a flat setting, where the water comes out in a flat, fan-like jet. This will give you good pressure while still spreading out the water so you'll have less chance of blowing out your detail. Also, if the temperature outside is cold you will need to use warm water or soak the image with warm water from a bucket in the beginning.  As you spray out the screen, the underexposed areas will all wash away, while the image in the overexposed areas will not dissolve. Find the strip that looks the best and was easiest to wash out without too much pressure. Sometimes you can force a slightly overexposed area to come out with more water pressure or warmer water, but be careful because that can degrade the stencil quality.  If one strip starts to lose detail as you wash it out, and the adjacent strip is too hard to wash out, just split the time difference.  Hold the screen up to a light and look through the image area, tilting the screen back and forth a little. This will let you make sure that the image area is completely washed out. Sometimes it will start to dissolve and may appear to be washed out, but it will have a very thin coating left, or you may have missed some of the details. Pick the exposure time that you think is best, grab another screen and your artwork, then expose one for real.

A fully washed out screen may continue to release a kind of soapy film or light suds under a hose. Just hold the screen up so it drips past the image area, and set the screen somewhere to dry. It doesn’t have to be kept light safe anymore. You can use the same drying rack you made for coating screens as long as you don’t let the water drip on the unexposed screens. Once you have chosen or guesstimated the proper exposure time with the test sheet you are ready to do it with the real art. You might want to try a test print with the exposure sheet artwork first to get a clearer idea of which area was exposed best. More on how to reclaim a screen for a different graphic later.

Changing the distance form the light to the screen will change your exposure time. If you are outside and using the sun, the time of day and time of year can also change exposure times. As a light bulb gets older it can also affect exposure times. Emulsion also has a shelf life and will start to behave erratically or stop functioning entirely after time or improper storage. Consistency is key.

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