Cyanoacrylate, the main ingredient in super glue, reacts primarily with salts found in sweat residue on fingerprint ridges. Specifically, it is the anionic eccrine salts, particularly lactate and alanine, that initiate the chemical reaction. This is the basis of “super glue fuming,” one of the most widely used forensic techniques for revealing latent fingerprints on surfaces like glass, plastic, and metal.
The Specific Chemicals That Start the Reaction
For years, it was assumed that water in fingerprint residue triggered cyanoacrylate polymerization. Research from the University of Tennessee has shown otherwise: the active initiators are anionic eccrine salts, not water. The two key players are lactate (a carboxylate) and alanine (an amino acid), both naturally present in sweat.
Eccrine sweat, the type produced by glands concentrated on your fingertips, is about 99% water with only 0.3 to 1% dissolved substances. That small fraction contains inorganic salts, lipids, and amino acids at concentrations of roughly 0.3 to 2.59 grams per liter. Despite being present in tiny amounts, these compounds are chemically potent enough to kick off the polymerization of cyanoacrylate vapor on contact.
The two most important chemical groups in these salts are carboxylate groups and primary amine groups. When researchers tested each one in isolation, surfaces with carboxylate groups were significantly more effective at initiating the reaction than surfaces with primary amines. This means the lactate in your sweat is doing most of the heavy lifting.
How the Reaction Builds a Visible Print
The reaction is a type of anionic polymerization. When cyanoacrylate vapor contacts the salts left behind in a fingerprint, those salts act as nucleophiles, meaning they donate electrons to the highly reactive cyanoacrylate molecule. This triggers a chain reaction where individual cyanoacrylate molecules link together end to end, forming long polymer chains called polycyanoacrylate. The process repeats rapidly, building up layer after layer of hard, white material along the ridges of the fingerprint where sweat residue was deposited.
The result is a stable, white-colored fingerprint that is physically tough and resistant to degradation. Unlike the invisible residue left by a finger touching a surface, the polycyanoacrylate deposit is permanent and clearly visible against most backgrounds. This durability is one reason the method is so popular in forensic labs: once developed, the print can be photographed, transported, and further processed without risk of smearing or fading.
Why It Only Works on Certain Surfaces
Cyanoacrylate fuming is a technique designed for non-porous surfaces like glass, plastic, metal, and coated cardboard. On these materials, fingerprint residue sits on top of the surface where the vapor can reach it. Porous surfaces like untreated paper or fabric absorb the sweat and oils into their fibers, pulling the chemical initiators below the surface where cyanoacrylate vapor cannot effectively contact them. For those materials, forensic examiners rely on other techniques like ninhydrin or DFO staining.
The Role of Humidity
While water itself is not the primary initiator, humidity in the fuming environment still matters. The recommended relative humidity for cyanoacrylate fuming is around 80%, with an effective range of roughly 75 to 84%. Moisture in the air helps the cyanoacrylate vapor circulate and interact with the fingerprint residue more efficiently. Too little humidity can result in faint or incomplete development. Too much can cause the vapor to polymerize indiscriminately on the surface, creating a white haze that obscures the fingerprint detail rather than highlighting it.
Making the Print Easier to See
The white polycyanoacrylate deposit is often visible on dark surfaces, but on light or reflective materials it can be difficult to photograph. Forensic labs commonly follow up fuming with fluorescent dye staining to improve contrast. The dye absorbs into the porous polymer coating on the ridges and glows under specific wavelengths of light, making the ridge detail far easier to capture.
Several dyes are standard in forensic work. Rhodamine 6G fluoresces under green light at 530 nanometers and is considered one of the most effective secondary stains. Basic Yellow 40 fluoresces under blue light and works particularly well with cyanoacrylate-developed prints. Ardrox is a liquid dye that performs well under UV lamps and forensic light sources. MBD fluoresces across a broad range of wavelengths (415 to 535 nanometers), making it versatile across different equipment setups. Some labs use a combination called RAM, which blends Rhodamine 6G, Ardrox, and MBD for maximum flexibility.
Safety Considerations
Heating cyanoacrylate to produce vapor carries a specific risk: overheating can generate hydrogen cyanide, a highly toxic gas. Research has confirmed the presence of cyanide when cyanoacrylate adhesives are heated beyond the temperatures needed for normal fuming. This is why the process is performed inside sealed fuming chambers with controlled heat sources and proper ventilation, not in open workspaces. The chambers allow examiners to regulate temperature and exposure time precisely, keeping the reaction within safe and effective limits.