Stripping (bleaching) dyes from fibers is a common procedure in forensics labs. It is done for three reasons:
- The presence of dyes may interfere with some chemical fabric tests
- Comparing the action of various stripping agents on a questioned specimen against the action of those same stripping agents on known specimens may allow the questioned specimen to be characterized. For example, navy-blue known and questioned specimens may react similarly to one stripping agent, but very differently to a second stripping agent, which establishes that the fabrics and/or dyes in the two specimens are not identical. Conversely, if the two specimens react similarly to all stripping agents, the forensic scientist can state that the two specimens are entirely consistent within the limitations of the stripping test
- Any existing dye or dyes must be stripped before the fiber specimen can be subjected to the dyeing tests described in the following lab session.
Professional forensics labs use many different stripping agents to discriminate fibers and dyes, some of which contain very hazardous chemicals. (Modern dyes are surprisingly hard to remove from fabrics.) In this lab session, we’ll use the following readily-available stripping agents to test known and questioned fibers.
Dilute acetic acid
Ordinary distilled white vinegar is another name for 5% acetic acid. This mildly acid stripper can be used straight from the bottle to remove basic dyes from silk, wool, and other animal fibers.
Dilute aqueous ammonia
Clear household ammonia is generally 5% to 10% concentration. Dilute one part clear household ammonia with about four parts tap water to produce a 1% to 2% ammonia solution. This mildly basic stripper can be used to remove acid dyes from silk, wool, and other animal fibers.
Rit Dye Remover
Most supermarkets carry Rit Dye Remover in the laundry section, although we’ll use it in a far more concentrated solution than the label recommends. The primary active ingredient is sodium dithionite (also called sodium hydrosulfite), which is widely used in forensics labs for dye stripping. Dithionite solutions are good general strippers for most plant fibers and many synthetic fibers, although they work poorly or not at all on acetate and triacetate fibers and most animal fibers. To make up this stripper, stir one level teaspoon of Rit Dye Remover into about 100 mL of water. If any solids remain undissolved, decant off the clear solution.
Chlorine laundry bleach
Ordinary chlorine laundry bleach used undiluted is an extremely powerful basic stripper. Standard solutions are 5.25% sodium hypochlorite, which may be used in this lab session, but if possible use one of the more concentrated (~ 6% to 6.5%) solutions sold as “Ultra” bleach or similar names.
Acidified 2% chlorine bleach
Acidified chlorine bleach may be effective in stripping black or other very dark dyes from a specimen if none of the other listed stripping agents works. To make up 100 mL of this stripping agent, carefully add distilled white vinegar to 38 mL of 5.25% chlorine laundry bleach until your pH meter or pH test paper indicates the pH is about 5. (Quite a bit of vinegar will be needed to acidify the bleach, which is typically pH 10.8 or so.) Make up the solution to 100 mL with water. This solution should be made up as needed immediately before use, and any remaining fresh or spent solution should be discarded by flushing it down the drain with plenty of water.
Mixing acids with chlorine bleach is ordinarily a major no-no, because it produces toxic chlorine or chlorine dioxide gas. With a dilute solution of chlorine bleach and acetic acid, which is a weak acid, the gases produced remain (mostly) in solution, although you should still perform this activity only in a well-ventilated area.
During this lab session, you’ll heat this solution, which may cause chlorine dioxide gas to be evolved. If that happens, add 3% hydrogen peroxide solution dropwise until the evolution of gas ceases.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- loupe, magnifier, or stereo microscope
- hot water bath (see Substitutions and Modifications)
- graduated cylinder, 10 mL
- graduated cylinder, 100 mL
- test tubes (as required)
- test tube rack
- test tube holder
- stirring rod
- pH test paper (or pH meter)
- acetic acid stripper (see Introduction)
- aqueous ammonia stripper (see Introduction)
- dithionite stripper (see Introduction)
- chlorine laundry bleach
- acidified chlorine laundry bleach (see Introduction)
- fiber specimens (see Substitutions and Modifications)
Read the MSDS for each of the chemicals you use and follow the handling precautions noted. Do not mix these strippers. Chlorine bleach produces toxic gases when mixed with acids or bases. Wear splash goggles, gloves, and protective clothing at all times.
Substitutions and Modifications
- You need a hot water bath to run these tests at 100 Â°C. The easiest solution is to use a beaker large enough to hold all of your test tubes. Fill the beaker about half full of water, and place it on a hotplate. Bring the water to a boil and reduce the heat to keep it boiling gently.
- Obtain several fiber specimens that are dyed a similar dark color (navy blue is ideal). Ask someone to choose a piece of one of the known specimens as your questioned specimen.
- If you have not done so already, put on your goggles, gloves, and protective clothing.
- Bring the water bath to a gentle boil.
- Label five test tubes, one for each of the stripping solutions.
- Place a small snippet (about 5mm to 15mm square) of the questioned specimen in each of the test tubes, transfer a few mL of the corresponding stripping solution to each test tube, and place all of the test tubes in the hot water bath.
- Observe the specimens carefully, agitating the tubes occasionally. Some specimens may begin losing color very quickly, while others may appear unaffected even after several minutes in the solution. After five minutes or when no further changes are evident, remove the test tubes from the hot water bath and allow them to cool.
- Flush the solutions from the test tubes down the drain with plenty of water, being careful to retain the bleached specimens in the tubes.
- Fill each tube with tap water and use the stirring rod to agitate the fiber specimens to remove excess stripping solution. Repeat this wash several times with fresh water.
- When the specimens are thoroughly washed, remove them from the test tubes and set them aside on a paper towel until they have dried completely. (Make sure not to lose track of which specimen is which; you can use transparent tape to affix the specimens to your lab notebook, but make sure not to cover them completely with the tape.)
- When the specimens have dried thoroughly, observe each of them closely and note their appearance in your lab notebook and/or Table 6-12. Compare the stripped specimens side-by-side with the original material, and use a magnifier, loupe, or stereo microscope to examine the fabric closely to observe, for example, if the fibers have been bleached evenly or unevenly or whether fibers running in one direction reacted differently from fibers running in the cross direction.
- Repeat steps 4 through 9 for each of your known specimens.
Figure 6-15. Fabric specimen before dye stripping
Figure 6-16. The same fabric specimen after dye stripping
Table 6-12. Test Fiber specimens by Dye Stripping – observed data
|acidified chlorine bleach|
Q1: Burning and solubility tests have established that your questioned fiber is synthetic. Which stripping agents would you rule out immediately, and why?
Q2: Burning and solubility tests have established that your questioned fiber is triacetate. Which of the listed stripping agents would you use to remove dyes from this specimen, and in what order? Why?