So we talked about how useful the inks are on getting a positive human identification (Click here to read about it before you continue with the following..)
This week we will go on and talk about inks from different period, or even removed would also help positive identification, which is particularly helpful in human identification in disasters. And for this part, the ink used for tattooing is the crux of the whole study.
Back in 19th Century, the ink manufactured for tattooing is obtained from burning a liter of oil soot. And 300g of the oil was combined with fresh urine (!). (If you are shocked, you are not alone). Though the manufacturing of the ink looks as raw and rough as it is, it has successfully been kept till today, and the collected specimen are now exhibiting in Romania.
Tattoo ink penetrates as deep as dermis of human skin (though sometimes varies the thickness of the skin too). It is especially useful when the epidermis (outermost layer) of skin is destroyed in fire for instance. Tattoos are usually divided into two types, “amateur” and “professional.” It depends on depth the ink was injected. “Amateur” tattoo would have less densely packed pigment when compared with the professional ones. Before the application of medical lasers, surgical removal is the only option to remove tattoos. Since amateur tattoos have various depth of ink injection, it is more difficult to remove amateur tattoos than professional tattoos. Yet, it is the otherwise with the application of medical laser because of the density of ink.
Tattoo can also use to identify gang members, religious belief and some previous engagement or convictions. It is because tattoos can show the memberships of groups and gangs, when a person left the group, they may have the tattoo removed. Evenly so, removed tattoos could be traced with X-rays, infrared and lasers.
The theory behind using lasers to remove tattoo is to heat up the pigment particles, have them mover away from the dermal cells. “Pigments …migrates away from the tattoo site due to skin cells dividing or dying. Therefore, the depth of the ink in the dermis may also potentially be used to age a tattoo, another useful tool in human identification.” (Clarkson and Birch) That is to say, in any postmortem circumstances, movement of ink pigment could be observed in lymph nodes, which indicates the presence of tattoos even though it is invisible on the epidermis.
X-ray can also use to indicate not only the existence of tattoo, but also the time frame of the tattoos inked. How so? “The older inks would be more visible than modern inks. This expectation is based on the known metal content in older tattoos ink.” (Clarkson and Birch) The metal component would reflect itself on the X-ray if detected.Yet, it was banned by the EU that the new types of ink should be metal free. Thus, by taking radiography images, it could possibly identify the time period, namely before or after the EU restriction for the individual to get the tattoos done.
The alternative to “remove” the tattoo is by doing “cover-up” tattoo. With the help of infrared, it can show the proximity of color deposit onto the skin. Studies show that ink would deposit further into the dermis over time. This way, the latent tattoos are visible under a particular wavelength, and thus enhance the visualization for the forensic context.
Resources:
Clarkson, Helen. and Wendy Birch. 2013. “Tattoos and Human Identification: Investigation into the Use of X-Ray and Infrared Radiation in the Visualization of Tattoos.” J Forensic Sci, September 2013, 58(5).
Karsai, S., G. Krieger, and C Raulin. 2009. “Tattoo Removal by Non-Professionals–Medical and Forensic Considerations.” JEADV.