Methods for analyzing fibers/fabrics as evidence

Teacher's notes:

Supplies needed (per team):

enough of each known fabric (I use white or off-white samples of cotton, wool, acetate, silk, and polyester)
unknown fabric
stereomicroscope
candle, matches
forceps
6 small test tubes/rack per team
acetone
6 microscope slide/cover slip
microscope

It is very important to keep the flames and the acetone away from each other!

Student instructions

Fiber Identification Lab In this lab, you will be exploring the characteristics of various fabrics and their fibers. You will then attempt to identify an unknown fabric based on your initial observations. You will work with five different fabric samples, numbered as follows: 1. wool; 2. silk; 3. acetate; 4. polyester; and 5. cotton

Be sure to create a data table and record all of your results!

Exercise A. Initial Observation

1. Look carefully at each sample without any magnification. Describe what you see, in detail. This could include descriptions of the weave pattern, roughness, size of fibers, thickness of material, sheen, etc.

2. Observe the sample using a stereomicroscope and describe what you see.

3. Feel and smell the samples, and describe.

Exercise B. Burn Test

CAUTION: Use all due caution. All flammable materials (including the acetone supplied with this lab) should be kept far away from the work area or in another room. Do not perform burn tests without your teacherÕs supervision.

1. Light a small candle and place it on a table in a safe place.

2. Use forceps to pull one thread of wool from the sample.

3. Holding the thread with the forceps, slowly move the end of the thread toward the flame from the side. Note the reaction of the fibers in the thread as they approach the flame (e.g., curl away from the flame or melt).

4. Move the end of the thread into the flame and pull it back out. Notice the manner in which the fibers ignite and burn (e.g., quickly or slowly). Do the fibers continue burning when removed from the flame? If the burning fibers give off a noticeable odor, try to describe it. Allow the thread to burn itself out.

5. After the flame has been dead for several seconds, examine any ash or residue left. Note the color and form of the residue. Press a finger tip on the cooled residue to see if it is hard or brittle.

6. Record your observations.

7. Repeat the flame test for each of the samples.

Exercise C. Solvent Test

1. Obtain 5 small test tubes and a test-tube rack.

2. Use a glass-marking pencil to number the tubes with the numbers 1 through 5.

3. Carefully add about 3 ml of acetone to each tube.

4. Use forceps to pull a thread of wool from the fabric.

5. Use scissors to cut three pieces of the thread each about 5 mm long. Discard the rest of the thread. Use forceps to drop a piece of thread into tube #1.

6. Repeat Steps 4 and 5 for the other fibers. Place the pieces of silk in tube #2, acetate in tube #3, polyester in tube #4, and the cotton in tube #5.

7. Observe the test tubes for 10-15 minutes. Note if any of the threads is dissolved or affected by the solvents. Record your observations.

Exercise D. Microscopic Examination

1. Use forceps to pull a thread of wool from the fabric.

2. Unravel the thread.

3. Obtain a clean microscope slide and place a small drop of water on it.

4. Use forceps to tease a few fibers gently from the unraveled thread. Place the fibers on the drop of water. Discard the rest of the thread.

5. Obtain a clean coverslip and carefully lower it over the drop of water.

6. Examine the slide under the low and high power objectives of a microscope. Examine the fibers closely, noting any distinct features. Pay close attention to the presence of striations (longitudinal or cross markings), folds, or twists. Note whether the fibers are transparent or opaque. Record your observations.

7. Repeat steps 1 through 6 for each of the other four types of fibers.

Repeat the above procedures to identify your unknown sample.

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