Photo emulsion is a light-sensitive coating applied to a surface, whether that’s photographic film, paper, or a screen printing mesh. At its core, it works on a simple principle: certain chemicals change their properties when exposed to light, and that reaction can be used to capture an image or create a stencil. The term covers two main worlds: traditional photography and screen printing, and the chemistry differs between them, but the underlying concept is the same.
Photo Emulsion in Photography
In traditional photography, a photo emulsion is a layer of gelatin containing tiny particles of silver halide crystals. These crystals are the light-sensitive ingredient. When light hits them during exposure, they undergo a chemical reaction that forms an invisible (latent) image, which later becomes visible during development.
The word “emulsion” is technically precise here. It describes a liquid (gelatin) holding fine particles of another substance (silver halides) without the two fully combining, similar to how oil droplets stay suspended in vinaigrette. To make it, halide salts are combined with silver nitrate and water to produce silver halide crystals, which are then suspended in warm gelatin and spread over film or paper.
This silver gelatin process is distinct from older methods. In albumen and collodion photographs, the silver halides simply sit on the surface of the coating rather than being suspended within it. Silver gelatin is the only one of the three that uses a true emulsion, which is part of why it became the dominant photographic technology for over a century.
From Wet Plates to Dry Plates
Early photographers used wet plate collodion, a process where the entire exposure and development had to happen before the coating dried. That meant carrying a portable darkroom into the field. In 1873, Dr. Richard L. Maddox introduced dry plate negatives coated with silver gelatin emulsion. These plates could be used when dry, were easier to transport, and needed less light exposure. Dry plates were the first economically successful durable photographic medium and remained in common use from the 1880s through the late 1920s, eventually giving way to flexible film coated with the same type of emulsion.
Photo Emulsion in Screen Printing
In screen printing, photo emulsion serves a completely different purpose. Instead of capturing a photograph, it creates a stencil. You coat a mesh screen with emulsion, let it dry, then expose it to light through a transparent film of your design. Where light hits the emulsion, it hardens (crosslinks). Where your design blocked the light, the emulsion stays soft and washes away with water, leaving open mesh that ink can pass through.
There are two main types of screen printing emulsion, and choosing between them affects your workflow, detail quality, and durability.
Diazo (Dual Cure) Emulsion
Diazo emulsion comes in two parts: a jar of emulsion and a small bottle of diazo sensitizer powder that you mix in before use. This type is better for fine detail work like halftone printing. It’s more water-resistant, making it a strong choice when printing with water-based inks. Diazo emulsion also has a wider exposure window, meaning you have more room for error. If the perfect exposure time is 10 minutes, you could be off by 30 seconds to a minute in either direction and still get a usable stencil.
The tradeoffs: diazo emulsion takes longer to expose, has a shorter shelf life once mixed, and the diazo powder itself poses some handling concerns. In powdered form it can become airborne and cause respiratory discomfort or allergic reactions. It also stains skin and surfaces yellow on contact.
Photopolymer Emulsion
Photopolymer emulsion comes premixed and ready to use straight from the container. It generally produces a smoother, more even coat on the screen, which translates to cleaner stencils for solid color prints. It has a longer shelf life than diazo emulsion and is less hazardous to handle.
The exposure window is tighter, so you need more precise timing. One useful advantage: you can post-expose photopolymer stencils (expose them again after washing out the image) to significantly increase their durability. With diazo emulsion, post-exposure makes little difference.
How to Coat and Expose a Screen
The application process is straightforward but requires care. Start with a clean, dry screen. Using a scoop coater (a trough-shaped tool with a thin blade edge), fill it with emulsion and place it at the bottom of the screen, which should be standing upright with a slight backward tilt. Press the blade edge against the mesh, tip the coater forward so emulsion pools against the screen, then pull it steadily upward with firm, even pressure. Stop about an inch from the top, rotate the coater back to catch excess emulsion, and use the blade to smooth the coating to the top edge.
Apply emulsion to the outside (print side) of the screen first, then coat the inside. The pressure from the inside coat drives emulsion outward, building your stencil where it needs to be. One coat is usually enough. For thicker stencils, like those used for white ink or bold jersey lettering, add a second coat to the outside.
Dry the coated screens in a dark, warm room. Fans help with airflow, but keep dust away from the screens. A dehumidifier helps in humid environments. Store screens flat with the print side facing down so gravity pulls the emulsion to the outside surface.
Once fully dry, you expose the screen to UV light. The optimal wavelengths fall between 380 and 420 nanometers. Multi-spectral UV bulbs work best because they activate a broader range of the sensitizer’s chemistry, producing more thorough crosslinking. Single-spectral bulbs may only activate part of the sensitizer, leaving the stencil less durable.
Mesh Count and Detail
The mesh screen you coat with emulsion determines how much detail your stencil can hold and how much ink passes through. Mesh count refers to the number of threads per inch. A 110-mesh screen lays down a thick ink layer and works well for bold designs and block lettering. At 156 mesh, you still get good ink coverage but with noticeably better detail. A 230-mesh screen can hold halftone dots for photographic-style prints, while 305 mesh captures extremely fine detail. The highest counts, 355 to 400 mesh, are mainly used with ultra-thin UV inks for signage and banners.
The emulsion itself doesn’t change based on mesh count, but a finer mesh holds a thinner emulsion layer, so your exposure times and coating technique may need adjusting as you move between screen types.
Reclaiming Screens for Reuse
One of the practical advantages of photo emulsion in screen printing is that screens are reusable. Once you’re done with a design, you strip the old emulsion and start fresh. The process involves applying a stencil remover to both sides of the screen, then either scrubbing with a soft nylon brush right away or letting the chemical soak for 30 to 60 seconds before rinsing. A pressure washer helps clear any stubborn emulsion or ink residue.
The key rule: never let the stencil remover dry on your screen. If it dries, it can create a haze that’s difficult to remove and may interfere with future emulsion adhesion. Use a dedicated brush for reclaiming and keep it separate from other cleaning tools.
Safety Basics
Photopolymer emulsions are relatively low-hazard. Diazo emulsions require more caution, particularly when mixing the powdered sensitizer. Work in a ventilated area and avoid inhaling the powder. Safety glasses or goggles protect your eyes, and protective clothing keeps emulsion off your skin. Coated surfaces can become slippery, so watch your footing in the coating area. For disposal, follow your local regulations for chemical waste, as requirements vary by jurisdiction.