Hydroxide Rehydration Techniques

Chemical rehydration of mummified remains to restore skin pliability and friction ridge detail. This method can restore friction ridges not visible prior to soaking.

video/mp4 8 HYDROXIDE REHYDRATION FINAL.mp4 — 277824 KB

Video Transcript

Bryan T. Johnson, Major Incident Program Manager, Latent Print Unit, FBI Laboratory Division: Welcome to another installment of the Postmortem Fingerprinting video series.

In this video, we dive into some of the hardest deceased to print—the mummified cases. These are often rendered as unprintable simply because the fingers cannot be accessed without removing digits. Other than burn cases, these are the least likely cases for practitioners to pursue fingerprints on.

In this video, we will cover the process of utilizing two different chemical hydroxide solutions to recondition not only the hands but also the fingers and the fingerprints they're in. While these techniques would be considered advanced and require some specialized chemicals, this video is meant to dispel any rumors of difficulty. Given the right training and chemicals, the road to success can be measured by the ability to pour liquid into a bucket. We hope to encourage people in the field that removing fingers to rehydrate is often an unnecessary step. As is the case with many of these techniques, they're not always instant, but they may be the only chance there is to identify a decedent.

Lastly, we would like to thank the District of Columbia Office of the Chief Medical Examiner for their collaboration and participation in this series, ensuring that everyone watching is provided with the tools needed to identify unknown deceased everywhere.

This video will be for examining a case exhibiting mummification. As always, the first step is to examine the hand to determine if it is printable or if there is any damage present. Looking at this hand, it is obvious that it is not very pliable. The skin is very difficult to move, the fingers do not bend, and in this case it would be very difficult to fingerprint the person without further processing. In some cases, the fingers can be slowly bent back to expose the volar pads, the surface where the fingerprints are located. In doing so, it is usually observed that they are deeply wrinkled, indented, and very hard to the touch. The skin cannot be manipulated to allow for printing. Examining the other fingers shows the same signs of mummification and lack of ability to be printed as is.

Because the fingers are not bendable, it would be very difficult to even photograph them since the end joint is not accessible to capture straight on. Photographs of fingerprints should always be one-to-one at a 90-degree angle to the volar pad and captured with the scale. Having noted that there is no easy way to capture the prints, the next step is to rehydrate the hands in a rehydration chemical solution such as sodium hydroxide or ammonium hydroxide. In order to accomplish this, many offices will quickly resort to removal of the hands or the digits. It may speed up the process in some cases and allow for the hydration to be done without the body present, making it logistically more desirable, but it is important to note that the fingers do not need to be removed to be rehydrated. It is actually best practice not to remove the digits if at all possible. If the choice is made and approved by the medical legal authority to remove the fingers, simple cutting tools, saws, or clippers can be used to cut them off at the lowest joints.

It is important to never remove just the top joint of the finger as that can potentially damage the recordable skin. Likewise, when rehydrating, the site of the cut can become partially compromised so the base of the finger is the best place to cut. For starting the rehydration, a simple bucket or beaker will do. As long as the container is mildly chemical resistant, it will be effective. For this process, the point is to submerge the fingers in the liquid. But when the deceased is laying on a gurney, the hand will not be able to be in the solution without spilling it. In these cases, the shoulder should be propped up a little bit so that it stays inside the liquid. The angle needed will be dependent on the deceased. To achieve the proper amount of lift under the shoulder, something like an autopsy headrest can be very useful. The shoulder is propped up so that the hand can now gently rest down into the upright bucket. This will keep the hand inside the solution and allow the hydration process to take place.

The amount of time required to process a hand or individual digits depends wholly on the chemical solution used. Certain chemicals can take quite a while so the hand will need to stay in a solution for a number of hours or even potentially days. If the deceased is refrigerated during that time, it will drastically slow down the hydration process. It is also very important to ensure the fingertips are always fully immersed in the solution. For instructional purposes, one hand will be soaked in a 3 percent sodium hydroxide solution and the other in a 2 percent ammonium hydroxide solution with sodium bicarbonate. This will demonstrate the difference in rehydration they have over time. By employing a second headrest under the other shoulder, both of decedent's hands can be soaking at the same time and ensure that the fingertips are being rehydrated.

It is important to rehydrate all of the fingers as only rehydrating one finger may not be enough to make an identification if the antemortem records aren't good enough. It will also expedite the identification process when more fingerprints are recorded. Make sure to support both shoulders before adding any liquid to the buckets so no chemicals are spilled. While it is not necessary to soak the entire hand, doing so is okay. The main goal is to focus on the end joints of the fingers, which will be used for fingerprinting. If the whole hand is soaked, then the palm prints will also rehydrate. At a minimum, the end joint of each finger should be fully submerged in the solution. Also, it is common that the thumbs may be positioned in a way that requires the whole hand to be submerged to keep them in solution. The crucial fingers to rehydrate for identification are the thumbs and indexes since they are the most commonly found antemortem records. The solution should always be premixed to ensure an even mixture.

The sodium hydroxide solution is made from dissolving sodium hydroxide pellets in water. Typically, a mixture of 3 to 5 percent sodium hydroxide will yield best results, with the higher concentration going faster but also potentially causing more damage if left too long. Sodium hydroxide does have the ability to dissolve both the epidermis and eventually the dermis, rendering the fingers unprintable if left long enough unattended. The solution can just be poured gently into the bucket, ensuring all of the fingertips are submerged. Because it is premixed, there is no need to stir or mix the solution once it is in the bucket. Because of the thumb positioning for this particular decedent, the whole hand is covered to make sure the fingers stay under solution. Once the solution is poured, it is important to double check that all levels and positions are good and will be stable. A towel or sheet or something of that nature can be placed around the bucket just to ensure the bucket doesn't easily move should it get bumped.

Lastly, a little more solution is added to top it off to make sure the hand stays submerged. Next, the ammonium hydroxide solution is added to the other bucket to soak the other hand. Normally only one kind of solution is necessary, but this demonstration should help each office understand which one they wish to use. The same method is employed for both solutions, and the liquid should be added until all the digits are submerged. It is important to do a final check to make sure the fingers are submerged and the arms are stable. The sodium hydroxide soak needs to be monitored every two to four hours to make sure that the skin does rehydrate, but it doesn't dissolve the hand completely. If fingers were removed to be soaked, this will decrease the time needed to soak and increase the frequency in which it should be checked.

Ammonium hydroxide will typically take longer in general but will be gentler on the skin and should not dissolve the skin if left unmonitored. At this point, the body can be stored in an isolated area, or it can be returned to the refrigerator with the understanding that it will drastically slow down the rehydration process. This case will next be checked at two and then four hours.

It has now been four hours as the two-hour mark didn't show sufficient rehydration progress. The hand in sodium hydroxide will be examined first. Looking at the arm as a whole, the mummified skin tone and consistency hasn't changed, and the skin is dry and resembles leather. Above the arm, the arm is still completely dry and unchanged. Next, the hand is gently lifted out of solution, and the dissolved epidermis actually be seen dropping back into the solution. This method does not allow for the epidermis to be printed. Rather, the chemicals dissolve the epidermal layer and rehydrate the dermis so it may be printed or photographed.

Once removed, the fingers can be gently wiped to the paper towel or cloth towel to remove the epidermis that has dissolved but not yet fallen into the solution. The fingers should also be more pliable now and can be carefully opened up to see how far along the ridge rehydration has progressed. Now that the thumb can be opened up and viewed, it can be monitored to see when it is printable or ready to be photographed. The hand has not yet reached peak hydration time, but it is functional compared to where it started. It's important to note that the friction ridges can now be seen with the sodium hydroxide method. Before there was very little ridge detail and nothing usable could be seen, but the ridges are now visible. Due to the basic pH nature of this solution, the hand must be fully cleaned potentially with acetone or dish soap before trying to print it.

It may take up to three days to dry enough to be printed as a solution will continue to exude out of the pores. The faster option would be to have it photographed with a scale now that the capture can be one-to-one at a 90-degree angle. This is also the one time where the vinyl stickers and powder will not work as it will be too slippery for it to stick.

Now that the epidermis has been cleaned off and removed, what remains is the dermis. Even though it's still quite wrinkled, it is actually rehydrated quite a bit in four hours. The ridges are visible and can be captured for the first time. Additional soaking time would allow for more hydration but caution should be taken as the process will eventually dissolve the dermis as well.

Next, the hand soaking in ammonium hydroxide will be examined. Unlike the sodium hydroxide solution, the solution does not dissolve either layer nor does it cause the skin to be very slippery. It simply rehydrates it.

The sodium bicarbonate in the solution also acts as a buffer to stabilize the skin. The fingers are much more pliable and can be opened now to be viewed. When working to open the fingers, be cautious of the condition of the fingertips. If the end joints start to feel squishy, take care not to use the fingertip to bend it back. The hand that was originally rock hard and mummified can now be manipulated easily access the fingers and skin. As stated before, the solution does not dissolve the epidermis, which can be seen still intact. However, it is sloughing and separating from the dermal layer underneath, which may allow for it to be degloved and potentially printed separately. Longer soaking time may be needed for the remainder of the epidermis to finish sloughing off.

One other trick that can help improve the quality of the ammonium hydroxide-soaked digits would be to employ a shorter version of the boiling method at the end. Simply boiling the exposed dermis for no more than 10 to 15 seconds can drastically improve the ability to print the fingers with vinyl lifters and powder. While both hands have benefitted from additional soaking time, each has exemplified ridge detail that could be captured. If a technician is unsure if the prints are good enough, talking to local law enforcement or contacting the FBI Special Processing Center will help decide if additional work is needed.

After thoroughly drying the skin, it is possible to attempt to print it even before optimal soaking time has concluded. Often times, the first lift of a powder print will not look great due to excess moisture in the pores. In this case, simply print again without adding more powder or use an alcohol wipe to dry once more and then print again. If the recordings are still low quality, additional soaking time or boiling may be necessary. A light coating of powder is applied to the dried finger and a vinyl lifter is used to take the recording. Even though it does not look good, it should be saved and included on the acetate with any other recordings, as occasionally multiple recordings will show different ridge arrangement areas of the finger. It is always better to err on the side of too many prints than not enough. Also, after the first print is taken, just spreading the powder evenly on the finger with the glove instead of reapplying may give a good enough distribution.

Here are a few examples of fully rehydrated fingers. In this first photo, the hypodermis layer of the skin can be seen on the majority of the finger which does not contain friction ridges. But the small portion of dermis still on the outside edges of the finger were able to be rehydrated and recorded and the print later identified. This is a great example of why not to give up on a case if the hands are still present.

This is an example of a set of hands that were from a case found in the desert. After 24 hours of rehydration in sodium hydroxide, the epidermis completely dissolved, the hands rehydrated and were able to be printed. An identification was easily made and alleviated extensive DNA work. Prior to this rehydration, the fingers were not even accessible.

In this video, we discussed two possible types of rehydrating mummified skin. Hopefully after watching the video, you will have a better understanding of what can be done to help obtain prints from even the most difficult of cases. The chemicals used can be purchased from any chemical supplier, but waste should be disposed in accordance with local and national regulations. Being able to identify a case without a tentative name commonly relies on the use of fingerprints since there are vast antemortem databases to search against. Without a tentative name, a DNA or a dental search will likely be unsuccessful. By having these tools and others available to your office, you can ensure that no one goes without a name.

We encourage you to watch the remaining videos in this series so that you may learn additional rehydration and reconditioning techniques that can further assist in obtaining the prints needed to identify the deceased. We also encourage you to reach out to the FBI Laboratory with any questions you may have regarding this content or any case related questions we may assist you with