Not all emulsions respond the same when coating screens: some emulsions require adjustments in pressure and speed, depending on the emulsion thickness and the type of mesh you're working with on your screen. The end result should give a nice glisten that makes a great stencil for your next screen printing job. Darkroom expert Colin Huggins is on the case.
DIFFERENCES IN EMULSION
There are a couple of different types of emulsions screen printers can use. Presensitized emulsion is highly sensitive, exposes quicker, has a longer shelf-life, and captures fine detail beautifully. Dual-cure emulsion relies on diazo to make it sensitive to light. Adding diazo creates a six-week shelf life for dual-cure emulsions. The emulsion you choose may depend on your darkroom setup.
Pro Tip: Keep emulsion in a cool place, like a refrigerator, to extend the shelf life of dual-cure emulsion to eight weeks.
RELATED: WHICH EMULSION IS RIGHT FOR ME?
DIAL IN THE DARKROOM
Every darkroom requires a few things to keep it in the best condition possible. First, the darkroom should be warm, dry, and light-safe. Install yellow or red UV-safe lights, block out all other lights, and invest in a dehumidifier to keep the air in the darkroom dry. A hygrometer will help you keep track of the darkroom conditions. It measures temperature and humidity, so you’ll know exactly what needs to be adjusted.
Emulsion dries quicker with a breeze. Keep a fan next to your drying rack to blow across the screens. Make sure the fan is blowing clean air. Sweep and mop your darkroom regularly to avoid the buildup of lint, dust, and debris. If you’re worried about dust getting on your screens, secure the mesh of a screen on the back of the fan. This will keep dust particles out and keep your screens clean.
The best darkroom is around 75°-80°F and under 40% humidity. No UV light shines into the darkroom from doors, windows, or lights. If this sounds like your shop, a sensitive emulsion like Baselayr Complete is a great option. For darkrooms that aren’t as dialed in, a dual-cure Baselayr Long Lasting is much more forgiving and will still create a great stencil.
MIXING DIAZO INTO DUAL-CURE EMULSION
Mixing dual cure emulsion takes a few steps. Here’s a walkthrough:
- Grab a bottle of distilled water and let it reach room temperature. Find a stir stick like a popsicle stick or paint stick.
- Fill the bottle of diazo with the distilled water to the fill line. Replace the cap and shake well for at least 30 seconds.
- Once the diazo has dissolved into the water, pour it carefully into the bucket of emulsion.
- Using a stir stick, slowly mix from the bottom up until the diazo is completely combined with the emulsion. Take this process slowly to keep as much air out of the mixture as possible.
- Clean up the bucket of emulsion and replace the lid. Set the mixed emulsion aside for three hours to allow any air to settle out.
Once the emulsion has been mixed and settled, you’re ready to coat a screen.
COATING A SCREEN
Coating a screen is an art and may take a bit of practice. First, let’s talk about the tool needed: a scoop coater.
A scoop coater has two sides — a round edge and a sharp edge. The sharp edge is designed to cut the emulsion more, giving the printer more control over the amount of emulsion they coat on the screen.
The round side is more active; it shoves the emulsion and creates a thinner stencil. Due to having less control on the rounded side, printers who feel comfortable in their coating skills will have more luck creating a smooth, glistening coat with the round side.
When applying the scoop coater to the screen, you can either put more pressure on the back of the scoop coater or on the front. If you put more pressure on the back, it allows the coater to cut more. If you put more pressure on the front, it will influence how much emulsion is pushed through the mesh and how much is cut.
Pro Tip: There are two types of scoop coaters: the standard coater and the Monster Max. Colin prefers the Monster Max scoop coater. Use whichever coater works best for you. The information shared here works for both scoop coaters.
You can coat a screen in two ways. One way is to hold the screen in one hand and use the other hand to apply the emulsion via the scoop coater. In this version, use even pressure on the front and back of the scoop coater. Start the screen at an angle and as you bring the scoop coater up the screen, then slowly straighten the screen until it's upright when you finish the emulsion application.
When you flip it over to do the other side, turn it 180° so what was the top of your first side of the coating is now on the bottom. On the side that touches the t-shirt, apply two coats of emulsion. On the squeegee side, apply one coat.
You'll know if you coated a screen beautifully if the glistening effect happens. The glistening effect happens when you hold your screen up to a light and it reflects, no dull areas exist. A glistening screen means the emulsion on top and bottom of the threads has fully encapsulated the threads. It's important that the emulsion is wrapped around the threads because that's how it holds onto the screen.
When you're done, put the screen in your drying rack. Make sure to lay it with the squeegee side up, and t-shirt side down. The emulsion needs to settle on the t-shirt side to make it a little thicker for the stencil. When you're on the press, you'll be able to lay down a good layer of ink.
COMMON DARKROOM QUESTIONS
During the darkroom live stream, Huggins answered several frequently asked questions. Here’s a breakdown:
QUESTION 1: WHY SHOULD YOU NOT MIX EMULSION WITH A METAL SPATULA?
Using a metal stir stick to mix emulsion can add unwanted minerals into the emulsion. For this same reason, Huggins urges printers to use distilled water when adding diazo into any emulsion. A wooden stir stick and distilled water will keep the emulsion pure.
QUESTION 2: WHAT’S THE PURPOSE OF BASELAYR COMPLETE DIAZO?
Baselayr Complete is a presensitized emulsion and does not need diazo to properly crosslink and create a great screen. However, Baselayr Complete offers an optional diazo. Diazo isn’t just for cross-linking. It’s also an anti-halation additive.
What does that mean? Essentially, an anti-halation additive helps light to not scatter during exposure. When a screen is exposed, light hits the screen and wraps around it. The diazo helps keep this light scattering to a minimum so you can resolve all that super-fine detail.
QUESTION 3: IS THERE A RELATIONSHIP BETWEEN COATING SCREENS AND HAZE FOLLOWING RECLAIM?
Short answer: yes. First, let’s define haze. Emulsion haze is caused by an emulsion that did not break down properly, while image stain is caused by pigment being locked in the knuckles of the mesh. For this question, Colin refers to emulsion haze.
Emulsion haze can be caused by many things: chemical reactions with the ink, the cleaning solution used, or underexposed emulsion. By the time you get to the reclaim process, many chemicals have touched that screen. Work backward to figure out which process caused the emulsion haze.
There is tons to learn about the darkroom process. There's always more to know from creating the perfect darkroom to exposure tips. Check out the full live stream for all of Colin’s tips, and subscribe to our YouTube channel for more content.