Law enforcement uses super glue, technically called cyanoacrylate, to reveal invisible fingerprints on evidence. The technique is known as “cyanoacrylate fuming,” and it works by heating super glue until it produces vapors that bond to the oils and moisture left behind by a person’s fingertips. The result is a hard, white polymer coating that makes an otherwise invisible fingerprint clearly visible. It remains one of the most reliable methods for recovering prints from smooth, non-porous surfaces like glass, plastic, and metal.
How Super Glue Fuming Works
When you touch a surface, your fingers leave behind traces of sweat, oils, and amino acids. These deposits are called latent fingerprints because they’re invisible to the naked eye. Super glue fuming exploits the chemistry of cyanoacrylate, which polymerizes (forms long molecular chains) when it encounters moisture. Since latent fingerprints contain small amounts of water and other residues, the cyanoacrylate vapor is drawn to those deposits and builds up along the ridges of the print.
The process typically happens inside a sealed chamber. A small amount of liquid cyanoacrylate is heated on a warming plate, filling the chamber with vapor. Evidence items are suspended inside, and over a period of minutes, the vapor settles onto any fingerprint residue present. The prints gradually turn white and become raised slightly above the surface, making them easy to photograph or process further. Research from the National Institute of Justice found that cooling the evidence surface to around 8 to 10°C while fuming at normal room humidity produces the best results, because more polymer accumulates on the print ridges at lower temperatures.
Where It Works Best
Cyanoacrylate fuming is the go-to method for non-porous surfaces, meaning materials that don’t absorb liquids. These include glass bottles, plastic bags, metal firearms, rubber bands, car windows, and finished wood. Forensic teams have successfully developed prints on trash bags, sandwich bags, sheet protectors, bubble wrap, and textured plastics using this method. For porous surfaces like paper or cardboard, other chemical techniques work better because the fingerprint residue soaks into the material rather than sitting on top where vapors can reach it.
Enhancing Prints After Fuming
The white prints left by super glue fuming are often visible enough to photograph, but they don’t always show strong contrast against light-colored or reflective surfaces. To solve this, forensic examiners apply fluorescent dyes after fuming. The most common are rhodamine 6G, rhodamine B, and fluorescein. These dyes soak into the polymerized super glue and glow brightly under specific wavelengths of light, making even faint ridge details pop against the background.
This two-step process has proven powerful even on old evidence. In one notable case, investigators revealed a fingerprint on a plastic bag from a double homicide that was roughly 30 years old. They used cyanoacrylate fuming followed by rhodamine 6G staining, then visualized the print under green laser light at 532 nanometers. The combination was enough to recover usable ridge detail from decades-old residue.
Lab Chambers vs. Field Kits
In a forensic laboratory, fuming typically takes place in a purpose-built chamber with controlled temperature and humidity settings. These chambers hold evidence on hangers or shelves, regulate airflow, and give examiners precise control over how much vapor the evidence receives. Overexposure is a real concern: too much cyanoacrylate can fill in the valleys between fingerprint ridges, turning a detailed print into a featureless white blob.
Crime scene investigators don’t always have the luxury of transporting evidence back to a lab before fuming. Portable fuming kits exist for field use, including collapsible chambers that break down into flat panels and fit into a carrying case. These can be assembled in under five minutes without tools and include a small heating wand and portable humidifier. Some departments also use handheld fuming wands to treat surfaces that can’t be moved at all, like car dashboards or door handles at a crime scene. The tradeoff is less environmental control, which can affect print quality, but field fuming captures prints before evidence degrades from handling or weather.
Safety Precautions
Cyanoacrylate vapor is an irritant. Breathing it in can cause coughing, wheezing, and irritation of the nose and throat. With repeated exposure, some people develop an asthma-like allergic response, where future contact triggers shortness of breath and chest tightness. The recommended exposure limit set by occupational health authorities is just 0.2 parts per million averaged over an eight-hour shift, which is an extremely small concentration.
Because of these risks, fuming should always happen in an enclosed chamber with proper ventilation, or outdoors when using field kits. Lab chambers are designed to contain and exhaust the vapors safely. Forensic technicians who work with cyanoacrylate regularly may use supplied-air respirators when ventilation alone isn’t sufficient. The fumes also bond quickly to the eyes and mucous membranes, so eye protection is standard during any fuming operation.
Why Super Glue Over Other Methods
Fingerprint powders (the classic dusting technique seen on TV) work well on fresh prints on smooth surfaces, but they’re messy, can obscure fine details, and don’t perform as reliably on textured or curved objects. Cyanoacrylate fuming coats prints uniformly regardless of surface shape, which makes it far more effective on items like soda cans, gun casings, and crinkled plastic. It also preserves the print under a durable polymer shell. Once fumed, a print is physically stabilized and much harder to accidentally wipe away during handling or transport.
Another advantage is sequencing. Super glue fuming is typically done as an early step in fingerprint processing because it doesn’t interfere with later techniques. After fuming, examiners can still apply fluorescent dyes, powders, or other chemical treatments to squeeze more detail from a print. Starting with powder, by contrast, can contaminate the surface and limit what other methods can accomplish afterward.