DIY: Testing fiber specimens by dye stripping

Introduction

A stripping (Bleaching) dye test is commonly used in forensic labs for removing dyes from the colored fiber specimen. The presence of dyes interferes with fabric testing, so they must be stripped before dyeing tests. This test also helps us in identifying the characteristics of questioned fiber specimen. 

Each fiber reacts differently with stripping agents, and the reaction discriminates against the fibers and dyes. Forensic scientists use different stripping agents, including: Dilute acetic acid, dilute aqueous ammonia, sodium dithionite, Chlorine laundry bleach, and acidified 2% chlorine bleach.

Apparatus 

• Goggles
• Gloves
• Beaker 250 ml
• Forceps
• Magnifier or microscope
• Paper towels
• Scissors
• Stirring rod
• Test tubes
• Tap water
• Sodium dithionite
• Acetic acid
• Aqueous ammonia
• Specimens: known and questioned specimen fiber 
• Chlorine laundry bleach

Procedure

• Wear goggles, gloves, and other protective clothing.
• Take a 250 ml beaker and fill it about half full with boiling tap water. Your hot water bath is ready. 
• Take 5 test tubes and label each one for dye-stripping solutions. 
• Take your first questioned fiber specimen and cut it into 10 small swatches (about 5 mm square). 
• Add two swatches to each of the test tubes.
• Make sure that the swatches are fully immersed in the dye-stripping solution. 
• Choose the corresponding dye-stripping solution and transfer 5 ml of it to each of the test tubes. 
• Now place all 5 test tubes in the hot water bath.
• Agitate the tubes in the hot bath occasionally and observe each specimen carefully for several minutes.
• If you see any visible change in the specimen, continue to observe it until no further changes are evident.
• After the changes have stopped, remove the test tube from the hot water bath and keep it in a cool and dry place.
• Record your observations in the lab notebook.
• Carefully discard the solutions from the test tubes while retaining the bleached specimens in the tubes. 
• Now fill each test tube with tap water to remove the excess stripping solution from the fiber specimens. Use a stirring rod to agitate the specimens in the water. 
• Wash the fiber specimen with freshwater several times to make them free of stripping solution.
• After washing with fresh water, take the specimens out from the test tube and put them on a paper towel to allow them to dry completely. 
• Repeat all the above steps for your remaining questioned specimen fibers. 
• Once all the questioned fiber specimens have dried, observe each one of the fibers carefully by using a magnifier, stereo microscope, or loupe. 
• Record the changes in their appearance in your lab notebook. 
• Compare the original fiber material with the stripped specimens for final results. 

Strengths and limitations

Strengths

Stripping dyes from fabric is crucial as it removes any dyes from the fabric which would otherwise interfere with the chemical testing of fabric. Moreover, the stripping agent’s action also helps us determine the characteristics of the questioned specimen. 

Limitations

This test does not accurately identify the questioned specimen and requires further chemical testing.

Precautions

• Wear gloves, goggles, and other protective clothing while performing the experiment.
• Carefully read the material safety data sheet for all the chemicals and follow the handling precautions. 
• Do not mix the dye strippers.
• Be extremely careful while mixing acids or bases with chlorine bleach as it emits toxic chlorine or chlorine dioxide gas. 
• Always perform the experiment in a well-ventilated area.
• Do not inhale any gas, as it can cause irritation. 
• Dye stripping solutions work much faster in hot conditions as compared to room temperature. 

Summary

• Forensic scientists do dye stripping to remove stains from the fiber specimen. 
• The presence of dyes in fiber specimens interferes with their chemical testing.
• It tells us about the characteristics of the questioned specimen. 
• Different types of chemicals are used as stripping agents for different fibers.