Tag Archives: Exposure

Indecent Exposure

Coating the screen can be tricky at first, and my first attempt was, honestly, just plain shit. I’m gonna go through my first few attempts to show how it can go wrong, then go through a better method properly at the end.

NOTE: The mixed emulsion must be left to settle for at least an hour before applying to the screen, just give it a stir to make sure that all the bubbles have dispersed.

The correct tool to use for this job is a coating trough, but you can do essentially the same thing with a squeegee (or bit of card for the squeegee-less fools).

This video is quite handy to show you how to use a coating trough, although the guy that does the narration sounds like the most boring bastard ever and makes screen printing seem about as exciting as playing Bridge with the corpse of a snooker player’s accountant.

The bad thing about that video is that it led me to think that quite a thick layer of emulsion was needed on the screen, which is absolutely not true. I put a shit load of emulsion on my screen, and when I spread it around, it was dripping all over the place like a lusty hound, making a god-awful mess and a screen that was frankly bollocks. Here it is in all its glory:

Crap

If you make this mistake, it’s probably not even worth exposing the screen because it’s just gonna be a waste of time; the results are never going to be good. If  future me had been able to go back and advise myself, I would have slapped myself in the face and told him to wash it clean and start again like a good boy. However, temporarily unaware of my own incompetance, I continued with more blunders and idiocy. Westward Ho!

Once I left this screen to dry, things went from bad to worse; the thick emulsion pooled and dripped through the screen until it resembled the face of a 15 year old World of Warcraft gamer; a blotchy, spotty, hideous mess of shame and regret.

I then exposed the screen in daylight for about a day (probably about 6 or 7 hours):

First exposure

After washing the screen out this was the result:

Balls

Not only was the emulsion thick and patchy, but the screen was overexposed. You can tell this because all the fine lines of detail have been mercilessly obliterated, while the any other lines come out semi-transparent, rather then truly clean (which is obviously what you want, so that ink can pass through). The stencil needs to be cleaned off the screen with Stencil Strip fluid, which is relatively cheap because you dilute it with water 1:30. I think you can use bleach to do this, but I take no responsibility if you do use bleach and your screen turns to trash somehow. Oh yeah, remember to wear some fetching rubber gloves while doing this job.

My next attempt was a bit better. I realised that only a relatively thin layer of emulsion is needed on the screen, and devised a better method.

Firstly, push some drawing pins into the corners of your frame on the bottom side (the side to which the mesh is stapled):

This means that you can work on the bottom side of the screen without it touching the table. After making loads of mess last time I also decided it would be a good idea to tape along the edges of the screen on the inside to minimise the amount crap dripping into the corners of the frame.

Start out on the top side, and dribble a relatively small amount of emulsion in a line across the top of the screen, and then, with a medium-light level of force, pull the squeegee across at a 45° angle so the screen is flooded with a thin layer of emulsion. Repeat this on the bottom side, then flip it over again and finish by doing another coat on the top side. You can do as many thin layers as you like until it’s evenly coated, just remember to use small amounts of emulsion and always finish on the top side, thus leaving a smooth surface to print through later. When you’re done, it shouldn’t look thick and painty like the first one I did above, but rather semi-transparent and evenly coated, not unlike this:

See the difference? Yeah, it’s pretty obvious.

If you’ve done it right, you should be able to dry your screen in front of a fan in about ten minutes. If you have a little fan heater, you can use that; I dried mine in front of a cosy electric fire with a fan pointing at it. You want the screen to dry as evenly as possible, so keep turning it around regularly. It’s actually kinda easier to just prop it up with a fan pointing at it, then you can leave it drying for a while and just turn it a couple times. However, the fire was quite nice:

The emulsion – like most things – will turn from shiny to matte when it’s dry, and if, after a while of drying, there are patches which remain shiny and tacky, then the emulsion was applied too thick in those areas (but don’t worry about it too much, it’s hardly disastrous). Run your finger lightly around the edge to check if it’s dry.

Once fully dry, the screen is ready to be exposed. Still having faith in the magic of daylight, I exposed it a second time by the window, this time for about 4 hours. The result was better, but still not very good:

Again the screen was over-exposed. This meant I had to actually scrub the emulsion to remove it, which results in lots of bits that you actually wanted to keep being scrubbed off as well, leaving me again with a pretty shit stencil. After stripping this one and re-coating it again, I decided to expose it in a controlled manner. To do this, I had to either construct a light box, or come up with a hilariously botched together alternative. Of course I chose the second option:

Plain Awesome

As you can see, I made a reflector for my lamp with some aluminium foil, and then proceeded to entwine its power cord around a dining chair to secure it in position. I quite like it as an object, it looks a bit like the Pixar lamp has gone insane and thinks that government satellites are monitoring its brain waves.

Exposure times vary depending on many factors including type/thickness of emulsion, type/brightness of light, size of screen etc, but Speedball handily provide some guide times in their manual for the popular Diazo emulsion. Here are 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

I only had a 60 watt bulb, which is two and a half times weaker than 150w, so I multiplied the exposure time by 2.5, resulting in a time of 230 mins, or three hours and 50 mins. My lamp was also about 20 inches above the screen so I just gave it a nice round 4 hours.

To expose the screen you need to get something to put underneath it to provide a firm flat surface for the design to sit on top of  (I used a bunch of excellent records – The Locust, Cult of Luna, Lionel Ritchie, you know, the usual). It’s best to cover these with some black material so that no light goes through the screen and bounces back up from underneath. Then you put your screen over this, place your transparency (printed onto either OHP film or tracing paper) the WRONG way round (face down) onto the bottom side (the one with staples), and cover it with a piece of glass, which will hold it down tight against the screen so you get a crisp exposure.

If your glass is light and rubbish, you may have to weigh it down just to ensure the best contact:

Once the exposure time is up, remove the screen and whisk it away to the bathroom, where you can gently spray it with the shower head until the water soluble parts (which have been protected from exposure by the black parts of your transparency) are washed away. It’s best not to scrub the screen, especially if you have fine details; the unexposed emulsion should just run out with ease. If some parts are being stubborn, rub the screen gently with a finger. Once you’re happy with the stencil, you may want to put it back under the lamp for a bit to harden the remaining emulsion, just to make sure it’s gonna withstand lots of printing later:

If you want really good results, the best thing to do (what I should have done from the beginning) is to expose a screen as a test, similar to the technique one uses with photography to determine the correct exposure time. To do this, expose the screen normally, but use an opaque piece of card or paper to progressively cover the screen, moving it further over the screen at regular intervals, say, every 15 mins. Then at the end you will be able to tell exactly what length of time you need to expose your screen for by looking along the screen for the first strip that has exposed properly. Times vary widely, and doing this method will save you a lot of time and effort that is otherwise wasted by doing trial and error. If you are doing trial and error, at least try and keep everything constant, and only change one factor (eg bulb wattage, time, height etc) at a time instead of blindly fumbling around.

Turns out third time was lucky, and I’m pretty satisfied with the screen as it is, so I’m gonna go ahead and see how well this little baby is going to print. Make me proud Screeny.

….note to self. come up with better name for the screen.

Currently listening to: Intronaut – Prehistoricisms, Orff – Carmina Burana