The Identification of Natural Fibres - Canadian Conservation Institute (CCI) Notes 13/18

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CCI Note 13/18 is part of CCI Notes Series 13 (Textiles and Fibres)

Introduction

Fibre identification is an important first step in predicting the behaviour of a textile artifact in various environments. Knowing the identity of the fibre(s) is also helpful in planning appropriate conservation treatment(s) and/or storage methods. This Note describes two simple and practical ways (the burn test and microscopic examination) to identify cotton, flax, silk, and wool — the most common natural fibres in Western artifact collections (for more information about the characteristics of natural fibres, see CCI Notes 13/11 Natural Fibres). Other qualitative and quantitative tests are described in the referenced literature, but these may require more sophisticated methods, expertise, and instrumentation. However, they may be available from textile testing services or local university scientific laboratories.

Factors to Consider

Several factors can complicate the identification of fibres within a textile artifact:

  • The poor condition of a degraded historic textile may make it difficult or even impossible to identify the characteristic diagnostic features of fibres.
  • Yarns that are blends of two or more fibre types, including man-made fibres, can complicate the interpretation of the results of burn tests. Consequently, another test method such as microscopic examination of fibres should be used to confirm the results of the burn test.
  • Neither a burn test nor a microscopic identification is regarded as conclusive on its own, but either can be used as a confirmatory test. Further tests by specialists may be needed to make a positive identification.

Before carrying out tests on artifacts, it is important to obtain curatorial permission. Also, identification methods should be practiced first on fibres from known, expendable textiles.

Sampling

Fibres can be identified from very small yarn samples. Typically, 0.4 cm (1/8") or less is clipped from an exposed yarn end in good condition in an inconspicuous area. Both warp and weft yarns should be sampled because some fabrics are made of one fibre type in the warp and another in the weft. As much as possible, samples should be taken from all of the different fabrics making up a textile or costume, including stitching threads. It is important to document the sample location(s) in the written records, accompanied by a photograph. Once taken, fibre samples should be placed in the fold of a white or coloured paper that has been labelled (this makes it easy to see and manipulate them).

Burn Test to Identify Fibres

A burn test can be used to distinguish plant fibre (cellulose) from animal fibre (protein). The burn test is not usually the sole method of identification, but complements other techniques.

Materials

  • Fine scissors
  • Fine stainless steel tweezers
  • Unscented candle or other source of flame
  • Lighter or matches

Precautions

  • Burn tests should be carried out in an area safely away from artifacts and combustible materials.
  • Fibre samples produce little smoke but, before carrying out multiple tests, verify that doing so will not accidentally activate smoke detectors.
  • If using matches to light the candle, let the odour of the lit match dissipate before beginning the burn tests.
  • Practice the technique with known samples from an expendable textile before doing the test on yarns from an artifact. It is important to gain experience in making observations by sight and smell at each stage of the burning process.
  • Be aware that the flame will consume the fibres extremely quickly and, because the samples are very small, the burning fibres may be difficult to see.

Procedure

Holding the fibres or small yarn sample with the tip of the tweezers, note the following:

  • Visual observations as the sample is brought near the side of the flame, in the flame, and out of the flame
  • Odour as the fibres burn
  • Characteristics of the ash including the colour and texture

Compare the results with those in Table 1.

Table 1. Burn characteristics of cellulose and protein fibres in good condition
Type of FibreBurn Test
Near
Flame
In FlameOut of
Flame
OdourAsh
Cellu-
lose, e.g. cotton and flax
  • does not
    shrivel away
    from flame
  • ignites
    imme-
    diately
    with
    contact
    to flame
  • burns readily in flame
  • conti-
    nues
    to burn
  • has an
    "after-
    glow"
    when remov-
    ed from flame until fibre ex-
    pended
  • smells likes burning paper
  • fluffy
  • small
  • white to grey-
    coloured
Protein, e.g. silk and
wool
  • shrivels away from flame
  • burns slowly
  • self-
    extin-
    guishes
  • smells like burning hair
  • very small
  • dark
  • bead-
    like mass that breaks apart easily

Weighted silk

Some silk fabrics are weighted by the addition of metallic salts in the dye bath or in other finishing processes. The weight added to silk compensates for the weight lost when silk is degummed in preparation for processing. However, in the late s, processors began using metallic salts in excessive quantities. The salts accelerated the deterioration of the silk by increasing its sensitivity to light and, eventually, causing extensive weakening and fracturing of the fabric. A burn test can be used to determine if a silk fabric is weighted, because only weighted silks leave behind a very fine ash residue in the shape of the yarn or fabric structure.

Microscopic Identification of Fibres

Microscopy can be used to confirm the results of burn tests, and will make more sense of the results from burn tests on blended yarns. Microscopic examination also helps to distinguish cellulose fibres from protein fibres. Fibres in poor condition, very dark fibres, and those with surface finishes may be difficult to identify with a microscope.

Materials

  • Compound light microscope (up to 400× magnification: 10× eyepiece plus 10×, 20×, and 40× objectives); minimum recommended magnification is 100×
  • Microscope slides
  • Microscope cover glasses
  • Microscope slide trays or holders
  • Small beaker of water or mineral oil, to use as a mounting medium for temporary slides
  • Fine tweezers
  • Eye dropper
  • Fine scissors
  • Permanent marker and/or pencil, for documentation

Commercially available reference slides of the most common natural fibres — cotton, flax, silk, nd wool — are invaluable aids in becoming familiar with the surface and morphological features of fibres (Suppliers). These can be examined at various magnifications and compared to the characteristics of fibres presented in 'Longitudinal features of cellulose fibres in good condition'. As with the burn test, samples extracted from known, expendable textiles should be used to gain experience before the procedure is carried out on fibres from an artifact.

Procedure: Longitudinal view of fibres

The sampling procedure described previously can be used to obtain fibres from the textile. Once obtained, the fibres should be mounted on the microscope slide with water or mineral oil (water enables the surface features to be seen more readily; mineral oil shows the interior of the fibres better).

Using fine tweezers, place the fibres in the centre of a glass slide that is free of ontaminants such as other fibres. Tease the sample fibres apart with the tips of the tweezers. To simplify identification, the fibres should be separated from one another rather than being in a tightly twisted mass or overlapping one another. Introduce a small drop of mounting medium to the slide. Gently lay a microscope cover glass over the fibres, so as not to disturb their lacement. The mounting medium will quickly distribute itself under the cover glass and the sample can then be examined under the microscope. Natural fibres will swell slightly as they react to the mounting medium.

Place the slide on the stage and adjust to the lowest magnification, typically the 10× objective (with a 10× eyepiece, magnification is 100×). Low magnification is useful for "finding" the fibres on the slide by adjusting the coarse and fine focus knobs, and for bserving convolutions and surface features such as nodes. To observe morphological features of fibres in detail, use a higher magnification (e.g. a 20× or 40× objective). The entire length of several fibres should be observed. To do this, locate one cut end and follow the length of the fibre by moving the stage in small increments. Reference books may be helpful for comparing unique surface features that distinguish fibres from one another (Bibliography). Microscopic identification can be straightforward due to unique, easily identified morphological features. Unusual or confusing features may require expert assistance.

Mercerized cotton

Mercerization, a process discovered in the s, improves the lustre, dye affinity, and strength of cotton fibres. This textile process causes the fibre to swell and become more cylindrical in shape, thus losing much of the twist so characteristic of cotton ('Longitudinal features of cellulose fibres in good condition').

Documentation

It is important to keep a record of all findings. Record the date, number and name of the artifact, description of sample, sampling location, warp or weft threads, colour of fibre, magnification, fibre features, and fibre type. Sketch the fibres observed. Some microscopes feature standard film or digital cameras, from which a photographic record can be obtained.

Longitudinal features of cellulose fibres in good condition

Cotton.
Figure 1–
Cellulose Fibres: Cotton
Longitudinal Features: looks like a ribbon with twists (convolutions) at intervals along length of fibre, interior central canal or lumen may look like a striation (a minute groove running the length of the fibre), lumen is large, typically more than half the full width of the fibre.
Mercerized Cotton.
Figure 2–
Cellulose Fibres: Mercerized Cotton
Longitudinal Features: mercerized fibres have fewer convolutions, lumen may look like a striation.
Flax.
Figure 3–
Cellulose Fibres: Flax
Longitudinal Features: single fibres or ultimates have nodes at intervals along fibre length in the form of I, V, or X, similar to the appearance of bamboo, irregular width, interior central lumen is quite small, typically less than half the full width of the fibre, often seen as a bundle of fibres tightly packed in the lengthwise direction, rather than as individual fibres.

Longitudinal features of protein fibres in good condition

Cultivated Silk.
Figure 4–
Protein Fibres: Cultivated Silk
Longitudinal Features: looks like a cylindrical, smooth rod with periodic bulges, may sometimes have faint striations.
Wild Silk.
Figure 5–
Protein Fibres: Wild Silk
Longitudinal Features: flattened rod, like a ribbon, with irregularities in fibre diameter, may have more pronounced striations than cultivated silk, may have perpendicular cross-markings, similar to flax.
Fine Wool.
Figure 6–
Protein Fibres: Fine Wool
Longitudinal Features: outer surface and edges rough, due to overlapping surface scales, no medulla (the dark central solid line or row of interrupted dots, depending on the animal).
Coarse Wool.
Figure 7–
Protein Fibres: Coarse Wool
Longitudinal Features: outer surface and edges rough, due to closely spaced, zig-zag or jagged-edge surface scales, medulla visible.

Suppliers

Note: The following information is provided only to assist the reader. Inclusion of a company in this list does not in any way imply endorsement by the Canadian Conservation Institute.

Fibre reference sets in prepared slides, microscopy supplies, microscopes

The McCrone Group

Microscopes and microscope accessories:

SDL Atlas – Textile Testing Solutions

Microscopy supplies

Scientific suppliers

Bibliography

  • American Association of Textile Chemists and Colorists (AATCC). AATCC Technical Manual, . Research Triangle Park, NC: AATCC, .

  • Catling, D., and J. Grayson. Identification of Vegetable Fibres. London: Chapman and Hall Ltd., .

  • Cook, J.G. Handbook of Textile Fibres — Natural Fibres, 5th edition, 2 vols. Shildon: Merrow, .

  • DeGruy, I.V. The Fine Structure of Cotton — An Atlas of Cotton Microscopy. New York: Marcel Dekker Inc., .

  • Hudson, P.B., A.C. Clapp, and D. Kness. Joseph's Introductory Textile Science, 6th edition, New York: Harcourt Brace College Publishers, .

  • Textile Institute of Manchester. Identification of Textile Materials, 7th revised edition Manchester: Textile Institute, .

  • The Fiber Reference Image Library (FRIL).

  • United States National Park Service. Development of a Web-Accessible Reference Library of Deteriorated Fibers Using Digital Imaging and Image Analysis: Proceedings of a Conference, (edited by J. Merritt). Harpers Ferry Center: U.S. National Park Service, .


By the staff of the CCI Textile Lab

Également publié en français.
Also available in French.

©Minister of Public Works and Government Services Canada,
Cat. Nº NM95-57/13-18-2010E
ISSN 0714-6221


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