TSG Chapter V. Analysis and Testing Methods for Textiles - Section C. Fiber Identification

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Contributors: Originally drafted by Denyse Montegut. Contributions from: Melanie Sanford, Elena Phipps, Teresa Knutson, Irene Karsten, Fran Mayhew, Anne Murray, Sarah Stevens, Lucy Commoner.
Editors: Kathy Francis, Mary Kaldany, Anne Peranteau, Nancy Pollak, Deborah Trupin

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Copyright: 2018. The Textile Wiki pages are a publication of the Textile Specialty Group of the American Institute for Conservation of Historic and Artistic Works.
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Contents

Fiber Identification

Introduction: Description and Limitations

Most textile conservators use polarized light microscopy as the primary tool for fiber identification. However, some fibers can be identified with a simple compound microscope without polarizing filters. Solubility and burn tests can be used as additional or corroborative tests.

Definitions

Definitions for terminology used in this chapter can be found in the Glossary of Microscopical Terms and Definitions (New York Microscopical Society 1989).

Reasons for identification

  • To make informed decisions about treatment, exhibition, and storage
  • To document the item from which the fibers are taken
  • To assist in dating or authentication

Limitations of identification techniques

  • All fiber types found in historical collections can be identified using a light polarizing microscope, confirmed when necessary by solubility or burn tests.
  • For decisions related to conservation treatment it is usually sufficient to name the generic family to which the fiber belongs. For example, it is normally sufficient to identify a fiber as bast fiber, rather than to determine whether it is flax or hemp. Likewise it is normally sufficient to identify a fiber as acrylic, rather than to test further to determine whether it is Orlon or Acrilan.
  • While it is possible to determine different types of nylon or polyester using a microscope, for more specific information it is usually necessary to submit the sample for spectrographic analysis.
  • One advantage of beginning fiber identification with microscopy is that doing so requires an extremely small sample size. The identification, if not immediately conclusive, will narrow the field of possible fiber types for further testing.
  • Chemical solubility tests often require multiple samples and the use of toxic chemicals at high temperatures.
  • Burn tests require even larger samples than solubility tests, and differentiation is only possible by category type (proteins, cellulosics, or thermoplastics), which, by the nature of the technique, makes identification of fiber blends impossible.

Identification guide features

Different guide features are used to identify natural and man-made fibers.

  • Natural fibers have specific morphological characteristics, as well as optical properties that are key to their identification.
  • Man-made fibers may or may not contain unique morphological characteristics, but their optical properties will allow for identification.

Methods of overcoming difficulties in differentiating man-made fibers

  • Use of mounting media to determine the refractive index of the fiber
  • Use of polarized light microscopy techniques and a Michel-Lévy chart of interference color and retardation to determine birefringence
  • Corroborating findings from microscopy with a chemical solubility test or a burn test

Factors to consider

Procedural matters

  • Fiber identification is destructive testing, as it requires taking samples, which cannot be returned to the object. Evaluate whether fibers can or should be taken.
  • Permission for sampling is required by some institutions and owners.

Artifact condition

  • Archaeological or highly degraded samples may be problematical in handling or loss of guide features.
  • Pristine objects may not offer adequate accessibility of yarns for sampling.

Materials for microscopy

For simple microscopy:

  • Light microscope or polarizing light microscope with 10x, 20x, and 40x objectives
  • Standard microscope slides with paper labels or wax pencil for labeling
  • Microscope cover-slips
  • Small scissors, fine-tipped tweezers, dissection picks or probes
  • Glycerin (n= 1.473) or mineral oil (n=1.478)
  • Set of commercial or home-made reference fiber sample slides
  • Reference book with images, if reference fibers are not available

For preparing fiber cross sections the following may also be needed:

  • Fiber cross sectioning sheets (sometimes called Joliff plates)
  • Razor blades, scalpel
  • Collodion (nitrocellulose)
  • Permanent mounting medium of n=1.539 or n=1.66

For identification of synthetic fibers the following may also be needed:

  • Set of refractive index liquids (range from n=1.500 to 1.700). Clove oil (n=1.532) may also be helpful as its refractive index is in the range specified on the flowchart provided by (Stoeffler (1996).
  • Michel-Lévy chart of interference colors, found in textbooks or available from manufacturers of polarized light microscopes
  • Microscope eyepiece with micrometer

For advanced techniques (hot-stage microscopy, etc.) additional supplies will be needed.

Note: Many materials are available from McCrone Microscopes)

Materials for chemical solubility tests

  • Set of appropriate chemicals as suggested by the literature. Most common among solubility chemicals are:
    acetone, acetic acid, meta cresol, phenol, sodium hypochlorite, formic acid, hydrochloric acid, sulfuric acid, dimethylformamide, and xylene.
    Health and safety note: Many of the chemicals listed here are hazardous. Practice safe handling procedures and avoid breathing fumes.
  • Microscope slides with a built-in well for holding liquids, or watch glasses or very small beakers
  • Beakers, glass stirring rods
  • Glass test tubes and test tube rack
  • Bulb droppers
  • Protective gloves and eyewear
  • Hotplate
  • Fume hood or portable exhaust unit

Materials for burn tests

  • Gentle flame (candle, butane lighter, or Bunsen burner)
  • Tweezers (inexpensive or old are suitable as they will be damaged by flame)
  • Protective eyewear
  • Beaker or glass of water (fire safety precaution)

Methods

The initial examination prior to sampling

This is necessary before proceeding with any identification technique.

Information learned from the object itself

Using a hand magnifier or ideally a stereo binocular microscope, look over the entire object.

  • Understanding the construction of the object and its weave and yarn structures is important to the identification of elements.
  • Determine whether warp and weft are distinguishable.
  • Note the location of all the fiber types present in need of separate identification – from the warp(s), weft(s), embellishments, and repairs.
  • Clues to separate selection of fiber samples are often found in differences of colors, thicknesses, sheen, etc.

Evaluate information in the object file.

  • The date or the history or culture of the object may suggest a range of fiber types possibly present.
  • If microscopy reveals the presence of inappropriate fiber types it may suggest a later addition, repair, or restoration. (For example, finding nylon lace around the cuff of a nineteenth-century jacket would open questions.)

Removing the test sample from the object

Prepare method of documentation for sampling

Evaluate the influence of yarn construction on sampling

  • Yarn construction may determine the best sampling method.
  • If in normal viewing or with slight magnification the individual yarns or plies appear to be different, then care should be taken to evaluate each part.
  • If the object history or object examination with binocular microscope suggests that a fiber blend is a possibility, then the sample must be of the whole yarn diameter, cut perpendicularly across the yarn with a scalpel or razor blade – including all plies. Remember that yarns can be blends – either with different fiber types found in various plies, or intimately blended into one ply or even into one filament.

Prevent contamination of the test samples

  • Keep the work station and tools free from fiber contaminants
  • Clean tweezers and other tools with alcohol wipes, fiber-free wipes, or clean fingers between samples to prevent contamination.

The length of the cut of the sample will be determined by end use.

Approximate lengths needed are as follows:

  • A sample mounted for immediate microscopic identification in a temporary mount requires a length of 0.25cm.
  • A burn test requires a minimum length of 1.0cm.
  • A chemical solubility test requires a length of 0.5cm.

Document the location of the fiber sample taken from the object.

When sampling at an out-of-lab venue, be sure to document both the textile object and location.

  • If possible, it is best to take a slightly larger sample in case you may have to run several corroborative tests.
  • Protect the sample, either by making a dry mount (placing it under a cover slip taped down to the microscope slide), or by using a tiny screw-top jar or small paper or plastic bag. Static will be a problem for degraded samples and will influence the method.

Microscopy

Longitudinal fiber sample, temporary mount (discarded after use)

Separate yarn into fibers
  • Use a dissecting needle or pin to tease apart the 0.25cm length sample into fibers. This can be done on the surface of a microscope slide. Alternatively, some conservators tease fibers apart on another suitable clean surface and transfer some fibers to the glass slide.
  • If a fiber blend is possible then a whole yarn diameter must be examined. Stereo binocular magnification is helpful for this step.
Select mounting medium
  • For initial viewing, select glycerin (R.I. of 1.473) or mineral oil (R.I. of 1.478) as a mounting medium.
  • Water (R.I 1.33) is also used by conservators but its refractive index is farther way from most fibers.
  • Remember that visual details are best viewed when using a mounting medium with a refractive index (R.I.) that is reasonably close, but not an exact match to the refractive index (R.I.) of the fiber. Both glycerin and mineral oil are inexpensive and non-toxic and give a good first viewing under the microscope for most fibers.
  • Refer to appendices in Identification of Textile Materials (The Textile Institute 1985) for charts of refractive indices for common fibers and other mountants for microscopy.
  • For additional information or if details are not clear, select a specific refractive index as the mounting medium. Cargille sells a set of refractive index liquids, available through the McCrone catalogue.
Make the slide
  • Pick up a cover slip and touch the dropper of the mounting medium to the top center of the cover slip leaving a small drop-sized amount.
  • Turn the cover slip over and place the hanging droplet down carefully onto the pile of loose fibers. The medium will spread out covering the teased-out fibers.
  • Label the slide to identify the specimen using a paper label, wax pencil, or frosted end on the slide.

Longitudinal fiber sample, permanent mount (for long-term storage)

Permanent mounts are inefficient for initial fiber identification (they are time-consuming to make and have a limited range of available permanent refractive indices). Once identification has been made via other means (temporary mounts, cross sections, or corroborative tests) a permanent mount can be made to accompany documentation of the textile object. Care should be taken to make a well-prepared slide, as this will become part of the object’s records. Another common use of permanent mounts is to create a reference library of fibers.

  • Take the 0.25cm length sample, tease the fibers apart on the slide with a dissecting needle, pin, tweezers, or other sharp tool until uniformly spaced out, without too many overlapping fibers.
  • Select a permanent mounting medium. Most are solid at room temperature but liquefy upon heating. A permanent medium with an R.I. near the middle of the range for fibers is preferable (e.g., Cargille Meltmount ® with an R. I. of 1.539).
  • Place the container holding the mounting medium (e.g., balsam bottle with glass dropper) onto a hotplate at a low-to-medium heat and wait for it to liquefy. If the hotplate is too hot the medium will smoke, burn, and discolor. Working in a fume hood is ideal, as this material has fumes which are odorous and possibly dangerous to breathe.
  • Once the medium has become liquid, raise the dropper up directly from the liquid and touch that one drop of liquid to a cover slip. It may take one or two drops to deliver enough medium.
  • Place the cover slip with the semi-hard drop of medium onto the pile of waiting fibers. Alternatively place the dry cover slip down onto the pile of loose fibers and then just touch the hot mounting medium drop to the edge of the cover slip, from where it will be drawn under via capillary action.
  • If necessary, move the whole microscope slide to the hot plate edge very briefly, where the drop will re-liquefy and spread out over the fibers. Remove and allow to cool.
  • Place a permanent identification sticker onto the slide, giving full documentary information for the sample. A systematic documentation method should be developed for permanent slides in a collection.

Cross section mounts

Cross Section mounts are an adjunct method that provides additional information or collaboration for fiber identification. Often they are essential in differentiating between closely related fibers (e.g., hemp and ramie) by offering additional information about fiber morphology.
If available, a larger sample may be useful for this type of mount (at least 1.0cm length of a full yarn, but arranging several yarns side-by-side is best if available).
Hardy microtome method. Adapted from Identification of Textile Materials, (The Textile Institute 1985)

  • If the top plate of the microtome is held by set screws, unscrew them to remove the top plate. Open the base of the fiber slot.
  • In order to minimize the amount of artifact test fiber needed, use of a “filler” fiber/thread is suggested (e.g., six-strand embroidery floss works well). Filler thread should be a known fiber, and distinctly different from the test fiber. The correct amount of filler thread will be determined by trial and error; it should fill the base of the slot completely and snugly when the microtome metal tongue is pushed into the slot.
  • Fill the microtome slot with filler threads.
  • Prepare the test fibers by aligning somewhat parallel using a dissecting needle or a pin.
  • At the top of the plate, separate the filler threads, fanning them out slightly, then insert the test fibers near to the center and close to the plate.
  • Pull the bundle of filler threads slightly down into the hole so that the test fibers enter the hole but are not pulled completely through. Cut the extra fiber flush against the metal plate on both sides.
  • Turning the screw knob pushes up the plunger. This raises the bundle of fibers, which are then consolidated with a small drop of nitrocellulose (or another consolidant) applied with a tiny brush or glass rod. Allow the consolidant to dry completely.
  • Use a sharp, single-edge razor blade and a slicing motion to cut flush against the metal plate to make one cross section. Mount on a microscope slide in a temporary or permanent mount.
(See also Identification of Textile Materials (The Textile Institute 1985, 145) and instructions provided with the instrument)

Method for embedding the sample in various media and cutting fiber cross-sections free-hand

Using cellulose acetate film
  • Cut two 1” squares of clear 3-5 mil cellulose acetate.
  • On top of one of the cellulose acetate squares, align the fiber sample with all fibers running parallel.
  • Put a small drop of acetone on top of the fiber sample, cover with the second square of cellulose acetate and press between your fingers for a minute until it dries. Avoid skin contact with acetone.
Diagram of trimmed cellulose acetate-embedded sample
Trimmed cellulose acetate-embedded sample
  • Trim the square into a trapezoid shape so that you have narrowed down the cellulose acetate around the end of the fiber sample. The fibers should be perpendicular to the edge of the plastic (see diagram at right).
  • Using a sharp scalpel or single-edge blade, shave off little slivers from the end with fibers onto a glass slide. Try to avoid sawing across the fibers. Also try to cut perpendicular to the fiber alignment to achieve a true cross-section. The resulting section is often most successful when the acetate sandwich is held in the air between the thumb and index finger of one hand while the shaving is done by the knife in the other hand (carefully) pulling towards the thumb. This takes practice.
  • The specimen is mounted on a microscope slide for viewing. Water can be used as the mounting medium, but glycerin is preferable since it has a refractive index of 1.47, which is slightly lower than the cellulose acetate. This minimizes the cellulose acetate and enhances the fiber. Cover with cover slip.
  • A permanent mount can be made by ringing the cover slip with PVA or some other ringing varnish.
Using low-density polyethylene film
  • Method is similar to the cellulose acetate method above but uses polyethylene as the embedding material and heat (to join the layers by melting) instead of acetone.
  • See (Palenik and Fitzsimmons (1990, 313-320) for a detailed description of this method.

Method using a fiber cross-sectioning sheet (also called a Joliff plate) This device is a microscope slide-sized thin plate with multiple tiny holes that will hold fibers perpendicular to the microscope objective.

(1) Thread a loop of sewing thread up through one of the holes in the cross-sectioning sheet.
(2) Into that loop, lay the sample of unknown warp or weft yarns (keep them in yarn form without teasing apart), along with enough of a known fiber type (e.g., embroidery thread in a contrasting color) to fill the hole properly. Pull the unknown yarns and the thread through the hole using the tails of the sewing thread loop. The proper size bundle will have enough resistance to pop into the hole and stay there without movement.
(3) Apply a small drop of collodion (nitrocellulose) to the fibers nearest the plate hole on both sides and allowed to dry completely (stiff). This will help keep the fibers in the hole while being cut.
(4) Use a straight razor held at an angle to make a cut flush with the top and bottom of the plate surface. Try to do it in one clean pass as short choppy cuts will cause fibers to be at different heights and thus fibers will not be in focus together.
(5) Place the Joliff plate on the microscope stage and move the hole with fiber section into the light path of the microscope for viewing.
(6) See (Palenik and Fitzsimmons (1990) for a full description of the Joliff Plate technique, and for an additional embedding method.

General guidelines for fiber microscopy

A well-adjusted microscope is mandatory for proper identification.

  • The stage and objectives should be centered.
  • The light should be a uniform, centered, creamy white field (not too bright or gray). To maximize image definition and cut down on stray light, adjust the field diaphragm using the condenser height adjustment knob. Focus it until the edges of the polygonal shape are sharp, and then open the field iris diaphragm until the polygonal shape is just outside the field of view.
  • Adjust the aperture iris diaphragm to maximize contrast and depth of focus. Remove an eyepiece from the microscope (typically the right eyepiece). Look down the left eyepiece and stop down the aperture diaphragm until 20% to 30% of the field of view is obscured.
  • For crossed polars, the field should be made a uniform solid black by adjusting the dial on the analyzer.

Always follow the same steps in gathering information.

For example, begin with plane-polarized light and lower magnification to make general observations, then move on to higher magnifications, use of filters, etc. for additional and more critical observations and measurements.

When gathering information for fiber identification by microscopy, always select the “best candidates” on the slide mount to observe.

  • Fibers that are not resting on or too close to other fibers in the mount
  • A fiber that lies as straight as possible, shows its full undistorted diameter, and seems to be representative of the majority of fibers on the slide.
  • Recognize what may be a contaminant
  • Handle degraded specimens with care to avoid loss of information.

Observe the sample and make clear notes on your observations.

Compare with a known sample of the suspected fiber type.


Standard Microscopy Techniques

Examination with Plane-Polarized Light (PPL, bright field)

  • View the mounted fiber first under the 10x objective. With standard 10x eyepieces this results in magnification of 100x. Survey the entire sample to make sure that there is not more than one fiber type present. Increase to the 20x or 40x objective to allow for better-detailed viewing of individual fibers. Stopping down the aperture iris helps to illuminate some surface structures. Fibers that are not well-defined will not yield clear information.
  • Evaluate the sample to see if there may be a blend of two or more fibers. If so then it may be desirable to mount the different fibers separately. Different fibers may be found:
  • In different plies of the yarn
  • As an intimate mixture in a single yarn
  • As a single fiber mixture (only for synthetic fibers). This category includes a type with a core material within a sheath of another, and a side-by-side arrangement of different types. Cross sections or solubility tests where only one part dissolves may help point out this type of blend.
  • Under PPL the presence of specific morphological characteristics can be viewed, and the fiber diameter can be measured using the optical grid in the eyepiece. Observe and make note of the morphology of the fiber (such as scales, lumen, medulla, cross-markings, etc). Compare observations with known reference slides or published texts and photomicrographs to decide whether the fiber is natural or man made.
Recommended references for photomicrographs:
  • Identification of Textile Materials (The Textile Institute 1985) for natural and manmade fibers
  • (Appleyard (1978) for differentiation of wool fibers from various animal sources and for details on fiber types and additional identification methods, such as scale casts
  • (Goodway (1987) for differentiation of wool types using a charted analytical form
  • (Petraco (2003) for a flow-chart method of identifying specialty animal fibers, with color photographs, and for a method using 550nm plates for differentiating bast fibers
  • (Catling and Grayson (1998) for more details on the identification of plant fibers, including crystal types, and additional methods such as ashing

Examination with Crossed Polars (XP, black field)

Two polarizing elements are required to achieve crossed polars. The one between the light source and the specimen is called the polarizer;the one between the objective and ocular is called the analyzer. When the vibration directions of the two polarizing elements are crossed at 90 degrees, crossed polars are achieved.
(1) Set up and examine sample
(2) Insert the analyzer to achieve crossed polars.
(3) Adjust the focus, the light source, and the dial on the analyzer for a fully black field.
(4) Study the fiber’s optical properties under XP. Depending on the two refractive indices of the fiber (nll and n$ \perp $), the fiber will either retard the light, producing a variety of exiting interference colors if the fiber is anisotropic, or will not change the black field at all if the fiber is isotropic.
(5) Rotate the stage and examine the optical changes. If a fiber is oriented in the E-W direction (parallel to the polarizer’s direction) or in the N-S direction (parallel to the analyzer’s vibrational direction) it will become dark and will show no colors. In the 45° position off any axis, the interference colors will be brightest.
(6) Observe the interference colors. Note that these are dependent upon the thickness of the fiber and its birefringence.
  • For preliminary information on the unknown fiber, the observed interference colors can be categorized into one of five groups of patterns—isotropic, low, medium, high, and super-high birefringent (anisotropic) fibers, thus limiting possible fiber types.
  • Compare observations with known reference slides or published texts and photos to match the observed color with the behavior of a known fiber type.
(7) Match interference colors observed with published reference charts (e.g., (McCrone et al. 1987, Appendix 1). Note that the charts describe expectations for fiber diameters typical for a given fiber type. If the fiber being examined varies from that range the behavior may vary.
  • Under XP, if the obliquely oriented fiber is dark and almost indistinguishable from the black field, an isotropic fiber is indicated. Common fibers in this category are triacetate, fiberglass, and vinyon.
  • Under XP, if the obliquely oriented fiber is bright with white, gray, or colored wavelengths, an anisotropic fiber is indicated. Take note of the nature and intensity of these colors, categorizing the unknown fiber into one of the above-mentioned four anisotropic categories in the McCrone chart. This will initially limit fiber type possibilities.
  • Whites, grays, with occasional dull yellows indicate a fiber from the low-birefringence category. Common fibers in this category are wool, acetate, acrylic, modacrylic, and the regenerated proteins.
  • Overall muted colors of mainly gold with touches of orange or blue indicate a fiber in the medium-birefringence category. Common fibers in this category are cotton, jute, viscose, cuprammonium, and olefins.
  • Very bright hot colors like bright blue, magenta, and hot yellow indicate a fiber in the high-birefringence category. Common fibers in this category are silk, flax, ramie, hemp, nylon.
  • Faint pastels, often with pink and green banding, but possibly moving into white if the fiber is super fine, indicate a fiber of extremely high birefringence. Common fibers in this category are polyester, Nomex, Kevlar.
(8) A useful tool is the Michel-Lévy Chart, a three-axis table that plots fiber thickness (diameter), wavelengths (shown in interference colors starting with full black and ranging through three orders (about 1600 nanometers)), and birefringence numbers. Once the diameter and the color that appears at the thickest point of the fiber are plotted, the birefringence can be found by following out the diagonal line that meets at that point, thus limiting the possibilities to only a few fiber types with that birefringence.
(9) Under XP, observe the extinction pattern. With the analyzer inserted, slowly rotate the stage. Observe the fiber’s extinction pattern as it goes from the bright oblique position to the dark position aligned with either axis. Observe where and how the fibers extinguish.
  • Man-made fibers are more highly crystalline and therefore will align well in one of those directions and become fully black.
  • The natural fibers, with their complicated internal polymer structures, may only partially or occasionally align to one of the axes, allowing for only specific parts to go dark or stay lit. For cotton, the fact that it is the only fiber that does not extinguish at any point in the stage’s rotation is definitive. For silk, its undulating pattern and the bast fibers’ cross-markings that remain bright in the dark positions are also key clues.

Examination with the 530nm compensator plate (magenta background)

Set up for examination with the 530nm compensator plate. Slide the compensator plate into position. When engaged, it adds to or subtracts from the light path one full order (530nm) of wavelength to the interference colors normally presented under XP.
  • When a fiber is oriented in the NE-SW position, 530nm is added.
  • When the fiber is oriented in the NW-SE position, 530nm is subtracted.
  • This is only informative for man-made fibers with a certain amount of crystallinity and is therefore not used for fibers that are suspected to be natural.
Observe the colors produced and match observations with published literature.

Advanced Microscopy Techniques

At times, standard microscopical methods may not be enough to fully confirm identification. It may be that the choices have been narrowed down to two possibilities, or that corroboration is needed if there is lingering doubt.

Method for determining the exact refractive indices (R.I., nll and n$ \perp $) of the unknown fiber (often the most reliable technique).

Background: Behavior of fibers in liquid mounting medium
  • When mounted in a medium with a refractive index close to their own R.I. (but not the exact number) fibers will show the sharpest and finest details for viewing.
  • When mounted in a medium with a refractive index exactly the same as their own R.I., fibers will seem to “disappear” in the mounting medium (i.e. the fiber’s outline will disappear). If there were any internal structures, dyes, or delusterants present, those will remain visible.
Health and safety note: Some of the chemicals used for determining refractive index by matching can be hazardous, especially in the higher refractive indices. Practice safe handling procedures and avoid breathing fumes.
Determining nll of the fiber
1) Adjust the microscope for plane-polarized light.
2) Stop down the aperture iris. This will sharpen definition of the edge and Becke line (see below).
3) Select an appropriate mounting medium and mount fibers according to the temporary mounting procedure above.
  • If a possible fiber type (or two) has been suggested by standard microscopy, consult a refractive index chart for the nll of that fiber type and mount the unknown in it. If it disappears in that mounting medium then you have a match.
  • If there are no clues as to the fiber type, start with a medium with index of refraction of approximately 1.535 (just as many fibers above that number as below in order to narrow the field right away).
4) Find the “best candidate” from which to collect information. (i.e., a fiber that is particularly straight, or at least straight for some short length along the fiber, that is away from the main group of fibers, and that is in focus.)
5) Once the segment of a fiber is selected, orient that straight portion of the fiber in the E-W direction.
6)Carefully observe the character of the fiber’s dark outline.
  • If the outline is thick, the fiber’s index of refraction is very different from the index of refraction of the mounting medium.
  • If the outline is thin, the fiber’s index of refraction is closer to matching the index of refraction of the mounting medium.
  • If the outline disappears, the fiber’s index of refraction matches the index of refraction of the mounting medium. Consult the nll column of the refractive index chart for fiber types having that nll.
When selecting the R.I. of the next test mounting medium, the Becke line (glowing halo around the fiber’s outline) is useful in deciding if a R.I. of higher or lower number should be chosen. The Becke line shows the relationship of the R.I. of the fiber to the R.I. of its mounting medium. See (Robinson and Bradbury (1992, 54-68) for a full discussion of the Becke line test.
Determining n$ \perp $ of the fiber
1) Follow all of the above steps for determining nll, but orient the fiber in the N-S direction for information gathering.
2) Refer to the n$ \perp $ column on the refractive index chart for matching.
3) Select an appropriate mounting medium, and mount fibers according to the temporary mounting procedure above.
  • If a possible fiber type (or two) has been suggested by standard microscopy, consult a refractive index chart for the n$ \perp $ of that fiber type and mount the unknown in it. If it disappears in that mounting medium then you have a match.
  • If there are no clues as to the fiber type, start with a medium with index of refraction of approximately 1.535 (just as many fibers above that number as below in order to narrow the field right away).
4) Find the “best candidate” from which to collect information. (i.e., a fiber that is particularly straight, or at least straight for some short length along the fiber, that is away from the main group of fibers, and that is in focus.)
5) Once the segment of a fiber is selected, orient that straight portion of the fiber in the N-S direction.
6) Carefully observe the character of the fiber’s dark outline.
  • If the outline is thick, the fiber’s index of refraction is very different from the index of refraction of the mounting medium.
  • If the outline is thin, the fiber’s index of refraction is closer to matching the index of refraction of the mounting medium.
  • If the outline disappears, the fiber’s index of refraction matches the index of refraction of the mounting medium. Consult the n column of the refractive index chart for fiber types having that n$ \perp $.
  • When selecting the R.I. of the next test mounting medium, the Becke line (glowing halo around the fiber’s outline) is useful in deciding if a R.I. of higher or lower number should be chosen. The Becke line shows the relationship of the R.I. of the fiber to the R.I. of its mounting medium. See (Robinson and Bradbury (1992, 54-68) for a full discussion of the Becke line test.

Hot stage microscopy

Requirements and potential of the technique
  • Requires a special microscope stage, which may not be available in most conservation labs
  • May use smaller samples than burn tests
  • Delivers more specific results than burn test as identification can be made on the basis of melting point or lack of melting

Chemical solubility testing

Based on its unique polymer construction, each fiber type will dissolve in only specific chemicals that affect their secondary interpolymer bonds. Unlike burn tests, chemical solubility testing can give detailed information on the identity of an unknown fiber, especially differentiating among synthetic fibers, and when there are blends present. Note that, as with burn tests, some fiber treatments and coatings can change expected solubility results. The best use of chemical solubility tests is for corroboration of a fiber choice suggested by microscopy or as an aid in the final decision when microscopy has left doubts between two final choices. While stepwise chemical solubility tests can potentially do the full job of identification alone, without the aid of a microscope, the downside is that it often requires toxic chemicals, used at raised temperatures. This should be necessary only when a polarizing microscope is not available.

General solubility test procedure

  • Take a 0.5cm long unknown fiber sample and tease it apart until individual fibers are freed from the yarn twist.
  • Place the fibers into the well of a microscope depression slide.
  • Again, as in the creation of microscope slide mounts, make sure all the fibers from the sample are in the well; otherwise quantitatively small components of blends will be missed.
  • Place the slide under a stereo binocular microscope and get the well area in focus. A black non-reflective paper is best placed underneath the slide so as to allow the best view of the fibers in the well. Set the lights in advance for the best view and magnification.
  • Based on some previous knowledge about the unknown, select from a solubility chart a liquid chemical that will give some information about the fiber. Refer to one of the many solubility charts in the literature; a useful one is found in (Kroschwitz (1990, 500-501). For example, if microscopy has narrowed the choice down to either nylon or polyester, select 20% hydrochloric acid or 85% formic acid as the test chemical. Both dissolve nylon but not polyester. As a further example, identifying acetate with cold acetone is often a first step in chemical solubility testing, and is a simple test for identifying acetate, using a chemical found in many conservation labs. Note: before using any chemical, read the MSDS sheet, follow safety precautions for ventilation and handling, and wear protective gloves and eyewear. Keep corrosive chemicals away from microscope parts.
  • Place about 4 or 5 drops onto the few fibers that are in the well of the slide (enough to set them floating).
  • According to the solubility chart, there will be a temperature, time, and concentration associated with the selection of a specific chemical. Confirm and follow these conditions.
  • Observe the outcome of the effect of the chemical on the fibers. Does it completely dissolve, swell and partially dissolve, or does it have no effect at all on the fiber? Again, compare test results using a known fiber in the same test chemicals.

Note: Chemical tests can be performed either in depression slides, as described above, or with small amounts of each chemical in small beakers. In the latter case, fiber samples are placed in each chemical and stirred with a glass rod until dissolution is observed (or not). A single sample may be used in multiple chemicals if it can be retrieved and cleaned safely. If the test is to be done in a hot or boiling solvent, the beaker is placed on a hot plate and boiling beads are used in the solvent.

Burn testing

Information on a fiber’s family group (cellulosic, protein, thermoplastic or mineral) can be gathered by the way a fiber reacts to heat and flame. Yarns containing more than one fiber type will not give useful burn test results, so microscopy should always the first step to insure that only one fiber type is present. (If the different fibers can be separated, as in separate plies, then a burn test can be performed on each.) Also, certain dyes and finishes, such as flammability or wrinkle-resistance treatments, may affect burn test results.

Knowing the fiber category can be useful. For example, if microscopy has narrowed down the choices to either cuprammonium rayon or nylon, a burn test will be helpful in identifying a cellulose-based fiber (rayon) vs. a synthetic thermoplastic fiber (nylon). There are many burn test result charts in the literature. See (Kroschwitz (1990, 499), and (Hall (1982, 31-32) for charts on the burning characteristics of fibers.

General burn test procedure

  • Hold an unknown fiber sample of at least 1cm in length in a pair of tweezers, and slowly move it toward a small flame (candle, Bunsen burner, cigarette lighter).
  • Observe the fiber’s reaction to the flame (shrinks away, ignites before touching the flame, melts, etc.).
  • Observe the fiber’s reaction to being ignited in the flame (starts to burn but self-extinguishes, starts to burn but immediately extinguishes, melts and burns, melts but burns with difficulty, simply chars, etc.).
  • Observe the odor generated by the burning procedure (there are many descriptions listed in various publications – but the main ones are burning hair (proteins), burning paper (natural cellulosics and rayons), acrid vinegar (acetates), and other various forms of plastic-like smells). Words used such as celery, sweet, sharp, etc., may be too subjective and rely too much on individual personal experiences to be useful.
  • Observe the remaining end of the fiber under magnification where it was burned (no bead or ash, easily crushable, soft, brownish-black, puffy bead, hard round or irregular bead of various grey to black colors, etc.). Touching the (cooled) bead end is required to determine its degree of hardness.
  • As for many other tests, the results should be compared with those of burn tests of a known reference fiber.

References

Appleyard, H.M. 1978. Guide to the Identification of Animal Fibres, second edition. Leeds: Wira.

Catling, D.and J. Grayson. 1998. Identification of Vegetable Fibres. London: Archetype Publications.

Greaves, P.H. and B.P. Saville. 1995. Microscopy of Textile Fibers. Microscopy Handbook 32. Oxford: Bios Scientific Publishers, in association with the Royal Microscopical Society.

Goodway, M. 1987. Fiber Identification in Practice. Journal of the American Institute for Conservation 26:27-44.

Hall, D. M. 1982. Practical Fiber Identification. 2nd ed. Auburn: Auburn University Printing Services.

Kroschwitz, J. I., ed. 1990. Polymers: Fibers and Textiles, A Compendium. Encyclopedia Reprint Series. New York: John Wiley & Sons.

McCrone, W. C., L. B. McCrone and J.G.Delly. 1987. Polarized Light Microscopy. Chicago: McCrone Research Institute.

New York Microscopical Society. 1989. Glossary of Microscopical Terms and Definitions. 2nd ed. New York:New York Microscopical Society.

Palenik, S. and C. Fitzsimmons. 1990. Fiber Cross-Sections: Part II, A Simple Method for Sectioning Single Fibers. The Microscope 38: 313-320.

Petraco, N. and T. Kubic. 2003. Color Atlas and Manual of Microscopy for Criminalists, Chemists, and Conservators. Boca Raton: CRC Press.

Robinson, P.C. and S. Bradbury. 1992. Qualitative Polarized-Light Microscopy. Royal Microscopical Society Microscopy Handbooks 09. Oxford: Oxford Science Publications.

Stoeffler, S.F. 1996. A flowchart system for the identification of common synthetic fibers by polarized light microscopy. Journal of Forensic Sciences 41(2):297-299.

The Textile Institute. 1985. Identification of Textile Materials. 7th ed. Manchester:The Textile Institute

Further reading

Commoner, L. 2005. Method for making cross-sections using cellulose acetate. Unpublished document, shared with TSG Catalogue by author’s permission.

Delly, J.G. 2003. The Michel-Lévy Interference Color Chart – Microscopy’s Magical Color Key. Web document: Modern Microscopy. http://www.modernmicroscopy.com/main.asp?article=15&page=1

Fiber Reference Image Library (FRILL), a web-accessible image database of textile fibers. Website developed with support from the National Park Service and the National Center for Preservation Technology and Training, and additional funds from the Ohio State University Historic Costume & Textiles Collection. https://fril.osu.edu

Hatch, K. L. 1993. Textile Science. Minneapolis: West Publishing Company.

Murray, A. 2005. Splitting hairs: a sampling technique for fibers. In Textile Specialty Group Postprints. American Institute of Conservation 33rd Annual Meeting, Minneapolis,MN.

Phipps, E. and M. S. Hwang. 2009. Thirty Years of Microscope Imaging Technologies in the Textile Conservation Department, Metropolitan Museum of Art. Textile Specialty Group Postprints. (digital) 19:77-88.

Rochow, T. G. and E. G. Rochow. 1980. An Introduction to Microscopy by Means of Light, Electrons, X-Rays, or Ultrasound. New York: Plenum Press.

Schwartz, E. R. 1934. Textiles and the Microscope. New York: McGraw-Hill.

Timar-Balazsy, A. and D. Eastop. 1998. Chemical Principles of Textile Conservation. Oxford: Butterworth Heinemann.




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