1.1 Stereomicroscope

The stereomicroscope is one of the simplest and easiest types of microscopes to use. It works by bouncing the light off the surface of the specimen rather than transmitting it through a slide. They are primarily suitable for observations not requiring high magnification. Its low magnification power (ranging from 2.5× to 100×) is due to its design. This microscope's illuminators can provide transmitted, fluorescence, brightfield, and darkfield reflected imagery, which allows the viewing of microscopic features that may otherwise be invisible.

With the stereomicroscope, there is a large gap between the specimen and the objective lens, which provides an upright, unreversed image. This space allows for better specimen manipulation and for a basic microscopic analysis to serve as the perfect preparation for a future, more detailed, microscopic examination and analysis. One important advantage of this scope is that the specimen does not require any special or lengthy preparation prior to observation. The specimen is simply placed under the lens and observed as needed.

The stereomicroscope is well suited for use in the preliminary identification of fibers, yarn, and weave structure when observing dated textile pieces for conservation practice. In general, textile fibers must be extracted from a yarn for proper observation and identification, but in viewing and identifying old textiles, such as tapestries or fabrics preserved for many years, removing fibers would damage the piece. With this microscope, the entire untouched, unraveled piece may be viewed without damage. In addition, this piece of equipment can be attached to a separate boom stand, allowing movement over a large object for examination. If a conservationist wants to examine the fibers of a new museum tapestry piece, a video camera may be attached to this microscope for proper record‐keeping. Later chapters will include the conservation of textiles.

1.2 Compound Microscope

The compound microscope, also known as the optical or light microscope, uses light and a series of lenses to magnify particularly small specimens. Compound microscopes were invented in the seventeenth century and vary greatly in simplicity and design. These microscopes can be very complex and are a considerable improvement from the aforementioned stereomicroscope. While stereomicroscope can only magnify up to around 100×, compound microscopes rise in resolution and magnification up to 1300×. Today, the use of reflected light in microscopy outweighs the use of transmitted light. Regarding fiber examination, light microscopes are suitable for the analysis of fiber anatomy in hair fibers, such as the different types of medulla.

1.3 Polarizing Light Microscope

The polarizing light microscope is undeniably an advanced and versatile piece of equipment. It is normally equipped with a round, rotating stage, a slot for the insertion of compensators, and a nosepiece. It stands out from other microscopes due to its preciseness in both qualitative and quantitative fiber analyses. It embodies the functionality of normal light (brightfield) microscopes while allowing the researcher to view fiber characteristics transmitted through polarized light, as opposed to reflected light. In polarized microscopes, two filters are used as an illumination technique, also known as crossed polars. One filter, an analyzer, is placed above the stage, and the second filter, a polarizer, is placed below the stage. In this polarizing technique, the filters are crossed, and an effect known as extension or black out occurs. The fibers appear bright against a black background. Polarized microscopy utilizes contrast‐enhancing technique to create a better image.

1.4 Electron Microscope

Electron microscopes are more sophisticated microscopes using electrons to form an image of the sample. SEMs are widely used in the textile laboratories. The SEM scans the surface of a sample with a focused electron beam to generate an image, converting the emitted electrons into a photographic image for display. This allows a high resolution and greater depth of focus. The SEM looks only at the surface of the specimen, which makes sample preparation simple. Instead of mounting a sample on a glass microscope slide, the specimens are placed on a strip of conductive tape that is attached to an aluminum mounting stub. SEM and the environmental SEM are primarily used in the identification of archeological textiles where detailed fiber morphological distinctions are required (Dennis Kunkel, personal communication, May 2016. Microscopy expert).

4.1 Microtome

When attempting to view a cross‐section of a fiber, the fiber must be cut into thin sections allowing light to pass through them. To cut fiber sections, you will use a fiber microtome, a tool specifically used for sampling thin cross‐sections of all types of fiber. The microtome allows better microscopic observation of the fiber tissue structure. Microtomes, used specifically in microscopy, are similar to any instrument used for sectioning thin materials. Microtomes use blades that are typically made of steel glass or diamond. Blades of steel, for light microscopy, and of glass, for light and electron microscopy, are suitable to prepare animal, plant, or synthetic tissue for viewing. Diamond blades are primarily used to slice hard materials such as teeth, bones, or plant matter, not fibers. Microtome sections can be sliced as thin as 50 and 100 μm.

4.2 Measuring Fibers Using the Metric System

Instruments such as the microscope help us to see individual characteristics on fiber materials that cannot be seen with the naked eye. To measure fibers, scientists normally use the metric system. The metric system uses meters as the standard measurement of length. One meter is equal to 100 cm, and a centimeter is about the length of a fingernail. A normal cotton fiber is 1.5 cm long. Centimeters are further broken down into millimeters; 1 cm is equal to 10 mm. One millimeter is a very small measurement, and although we can still plainly see a single millimeter, it exists as the beginning of the microscopic scale. Scientists measure the length of fibers in centimeters and millimeters and the diameter in microns. The diameter of each fiber determines the fiber fineness. For reference, the diameter of human hair is about 1 mm. Most of textile fibers are smaller than a millimeter.

4.3 Sampling

When collecting fiber samples and preparing them for microscopic examination, one must remember to obtain a representative sample of the fibers to be viewed. Obtaining a small sample size limits the observation and may not yield accurate results. A sample must contain “notoriously variable materials,” especially in examining natural fiber contents [2, p. 5]. It is important to examine multiple fiber samples to get the widest identification of its contents.

It is suggested that when dealing with blended fabrics, a preliminary sampling should be conducted to gain a truly representative sample for viewing [2]. For example, the observer should pull fibers out of both the warp and weft direction of the fabric without any selvages, treating them as separate samples. In a similar manner, yarns of varying colors should be examined by taking a separate sample of each color.

The most common specimen mountant used in textile laboratory is water. Mountants can be either temporary or permanent, water being the most temporary due to evaporation. A microscopic slide with water mountant cannot be stored. Water not only holds the fiber sample and slide cover in place, but it also improves the image quality due to water's refractive index, RI of 1.33.

Liquid paraffin, an oil, is another effective mountant with RI of 1.47, which is very close to the RIs of many textile fibers. Liquid paraffin meets many of the requirements of an effective mountant, including liquids that are colorless, nonswelling, stable, and safe [2]. These authors suggest that because liquid paraffin's RI is 1.47, “Only cellulose diacetate and triacetate fibers of RI approximately 1.46–1.47 are not clearly visible in liquid paraffin and, if their presence is suspected, a second preparation using water or cedar wood oil as the mountant should be made” [2, p. 7]. Other wet mountants used include glycerin (glyceryl) (RI 1.45) and other immersion oils (olive oil RI 1.48 and cedarwood oil 1.513–1.519).

Immersion oil may be applied in two ways: it can be placed on top of the coverslip, keeping the actual specimen from touching the oil, or the specimen can be fully submerged in the oil and then placed on the microscopic slide without the glass slide cover. The oil immersion method, including liquid paraffin, is suitable for fibers so that clearer image can be observed. It works for hair fibers (for example wool) when the anatomy of the fiber needs to be viewed. With the use of immersion oils, one can view the medulla.

4.4 Mounting

When mounting a sample, the fibers should be spread as evenly and parallel as possible in relation to the shorter dimension of the glass slide. To prepare a slide for proper examination, the examiner must check the slide and make sure both the slide and slide cover are clean and free of any impurities. Cross‐contamination is very common, especially with novice examiners. If one uses a contaminated slide, the fiber identification may yield incorrect results, as the examiner can easily mistake the contaminant for the fiber. To correctly mount a sample without contamination, the slide must first be cleaned. Then, only a few drops of the mountant should be placed in the center of the slide. Then, the fiber sample should be placed on top of the mountant. If loose fibers are difficult to handle, as they are easily lost, they could be put in a mountant before being placed on the slide. Once on the slide, they may be teased apart with a needle, and then more mountant may be added. The slide cover should then be placed onto the prepared slide. Slide covers must be placed carefully, as they can easily distort the placement of fibers on the slide. Once the slide cover is in place, any excess liquid mountant should be wiped from around the slide cover. The placement of a slide cover should also exclude any air bubbles, as they may be mistaken for fibers during the observation.

5.1 Plant Fibers

Fibers from plants are usually referred to as vegetable or cellulosic fibers. The term “cellulosic” is used because all plant fibers are composed of cellulose. Cellulose, found in all plants and trees, is a biomolecule composed of a polysaccharide consisting of chains of many D‐glucose units. It is an important structural component of the primary cell wall in plants. Cellulose in plants makes plant fibers strong; however, each plant has a different amount of cellulose. Cotton, for example is composed of 83% of cellulose, whereas linen is composed of 71% of cellulose. Cotton is believed to contain the purest form of cellulose. However, plant‐derived cellulose is usually accompanied by other substances such as hemicellulose, lignin, and pectin, amounts of which also determine the strength of cellulosic fibers. Cellulose is a building block of many textile fibers, not only natural plant fibers but also regenerated manufactured fibers (as discussed in the next section). Cellulose fibers under the microscope have an irregular shape and certain characteristics specific to a particular type fiber such as convolutions in cotton. Cellulosic fibers are easily identifiable with the use of microscope.

5.2 Animal Fibers

Animal fibers are mainly animal hair and animal secretions. Just as plant fibers are composed of cellulose, animal fibers are composed of protein molecules. Proteins are biomolecules consisting of chains of amino acid residues. A strong protein and key structural component of hair is called keratin, which gives hair its strength.

Animal hair comes from a variety of animals such as sheep, goats, rabbits. The hair from some animals, such as camels, goats, and rabbits, is called specialty or luxury fibers because it is scarce and harder to obtain than the hair from sheep. The hair from sheep for everyday wool fabrics is simply collected by sheering the animals. Other hair fibers include animal fur.

Animal secretions come from cocoons of the silk moth. These cocoons are collected and unwound to get the fibers out. Animal secretion fibers, for example silk from moth or spider webs, unlike hair fibers have proteins called fibroin. They also have a protein called beta keratin, which is responsible for waterproofing ability of the fiber. As animal fibers have irregular shapes under the microscope, a variety of animal hairs is easily distinguishable under the microscope. For example, the scales on sheep hair have a different shape than the scales on rabbit hair. Therefore, scientists have developed different ways to successfully distinguish animal hair by using the microscope.

5.3 Regenerated Manufactured Fibers

Regenerated fibers are manufactured fibers that are composed of cellulose. This is why they have a feel and exhibit some of the properties of natural fibers such as cotton or silk, i.e. comfort, absorbency, soft to the touch. These fibers are made from naturally occurring polymers in which the cellulose is broken down and reformed into a new matter. Because the cellulose is broken down, regenerated cellulosic fibers do not have the same strength as pure cellulosic fibers. Imitating much desired silk fibers, the first fiber, manufactured this way in 1924, was viscose rayon called “artificial silk” or “viscose.” It had originated in the late nineteenth century as “artificial silk” and later became known as “rayon” in the twentieth century. The name “rayon,” partially developed by the US Department of Commerce, is a combination of “ray” from the sun with “on” from cotton. The reason for this is that the first rayon fibers were bright like a ray of sun and had a hand like that of cotton. Since then there has been an evolution in the manufacture of other regenerated cellulose fibers, such as cuprammonium, high wet modulus (HWM), lyocell, and the development of at least six different types of manufacturing processes. These different rayon fibers were each manufactured to imitate a specific natural fiber, i.e. HWM (modal) was manufactured to imitate cotton.

The cellulose in manufactured regenerated fibers comes from wood also called wood pulp. The wood pulp is usually dissolved into a solution from which rayon fibers are manufactured. The solution is usually pushed through a spinneret with fibers extruded and emerging out the other end. Immediately afterward, the fibers are usually soaked in a solution resulting in fibers that look star‐like in cross‐section and have striations in a longitudinal view under the microscope. Regenerated cellulosic fibers are of more of a uniform shape because they are all manufactured via the spinneret method. Under the microscope, they are distinguishable from other natural fibers and somewhat distinguishable from other synthetic fibers. There are also regenerated protein fibers, but they are not as common as regenerated cellulosic fibers.

5.4 Synthetic Fibers

Synthetic fibers are petroleum‐based fibers, such as polyester and nylon. These fibers are synthesized from petrochemicals. Synthetic fibers are made from a synthetic polymer and are specifically engineered to have certain desirable properties. Because regenerated manufactured fibers, such as rayon, are plant‐based, while synthetic fibers are petroleum‐based, they are manufactured by different methods – wet spinning vs. melt‐spinning. Synthetic fibers, such as nylon and polyester, are chemically manufactured through melt‐spinning. The fibers are extruded from a spinneret that determines fiber shape, creating fibers that look very similar. This makes synthetic fibers difficult to distinguish under a microscope. The synthetic fibers discussed in this book include the five major fibers – polyester, nylon, acrylic, spandex, and olefin. The start of the development of synthetic fibers began with nylon. Nylon was the first synthetic fiber, and it was developed by Wallace Carothers, an American researcher who worked at the DuPont chemical company in 1930. It was a perfect time for nylon to debut as the United States was soon to be engaged in World War II. Nylon, a strong fiber, was useful for the manufacture of parachutes and ropes. Nylon replaced silk that was becoming scarce during wartime. It also became popular for women's wear such as stockings. The second synthetic fiber, polyester, came about 10 years after nylon and was introduced by the British chemists, John Rex Whinfield and James Tennant Dickson, in 1941. Because of its easy care, versatility, and affordability, polyester is the most used textile fiber in the apparel industry today.

5.5 Fiber Morphology

Fibers come in a variety of lengths and are always longer than their diameter. Fibers can be of infinite length, and these fibers are called filament, while short‐length fibers are called staple. All natural fibers come in the staple form except silk, but manufactured fibers may be made both staple and filament. One of the prerequisites of fibers to be used in textiles is its length. Fibers have to be long enough to be able to be spun into yarn. Shorter fibers are spun with a harder twist so that the fibers will not come apart, while filament fibers, given their infinite length, do not need to be spun with a hard twist. To view fibers under a microscope, fabric yarn has to be untwisted, and fibers have to be pulled out. Because of the hard twist in spun yarns, the fibers are harder to take out from the spun yarn than those from the filament yarn. When preparing a microscopic slide for viewing, a handful of fibers need to be placed on the slide. If you do not separate the fibers well enough and you put too many fibers on the slide, as students often do, you will not be able to identify the fiber characteristics because all you will see is intertwined lines, making it impossible to get a single fiber view. Care must also be taken so that the fibers on a slide do not overlap. At the other end of the spectrum, it is helpful not to have only two or three fibers on the slide because you may have difficult time finding these few fibers, especially with a higher magnification objective such as 100×.

Again, there are two main types of fiber, staple and filament (see Figure 4).

Staple fibers are short, and when spun into a yarn, they make a fuzzy yarn with protruding ends. Yarns made of staple fibers are called spun yarns, and these short fibers must be twisted hard enough to add strength so that they do not come apart.

Filament fibers, on the other hand, are smooth and long, having an infinite length. Yarns made out of filament fibers are called filament yarns. Because of their greater length compared to staple fibers, filament fibers do not need a hard twist to make a yarn. They are slightly twisted, resulting in a smooth yarn in contrast to the fuzzy yarn of staple fibers.

5.6 Fiber Shape

A part of a morphological examination of fibers under a microscope is the fiber's longitudinal and cross‐sectional view. Fiber's longitudinal (lengthwise) shape is determined by the fiber's cross‐section. If a fiber has a round cross‐section, it will have a smooth rod‐like longitudinal shape. In another example, if a fiber's cross‐section is star‐like with many ridges, these will create striations (lines) along the length of the fiber (as seen in viscose rayon). Therefore, both cross‐section and longitudinal characteristics should be considered when viewing fibers under a microscope. Natural fibers are of different shape and have distinguishing longitudinal and cross‐sectional characteristics. However, manufactured fibers do not possess as many distinguishing characteristics as natural fibers do. Many manufactured fibers share similar morphological characteristics, making fiber content difficult to identify.

5.7 Fiber Measurement

As mentioned earlier, a fiber must have a high ratio of length to thickness. The thickness or diameter of a fiber is a very important property because (i) this determines the fineness of fibers which in turn determines the end product made from the fibers, and (ii) it will determine how many fibers will be used to make a yarn. The finer the fiber, the finer the yarn that can be made from it. More fibers in the yarn's cross‐section will add strength to the yarn. There are different products made from fine and from coarse fibers [2]. When we refer to fiber size, we also use the term denier. Denier is a unit of measure that indicates the fiber thickness and weight. The denier value for microfibers (which are manufactured fibers thinner than silk) is usually less than 1 denier. There are fabrics with low denier count and with high denier count. Fabrics with low denier count are light, sheer, and soft, and fabrics with high denier count are thick, sturdy, and more opaque.