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DURABILITY AND RESILIENCE- Each wool fiber is a molecular coil spring making the fiber remarkably elastic. Nature has folded the chemical polypeptide chains back upon themselves in such a way that they act like a coiled spring which elongates when it is extended and retracts when it is released. This molecular crimp, along with the 3-dimensional fiber, allows wool fibers to be stretched up to 50% when wet and 30% when dry, and still bounce back to their original shape when stress is released. But be careful: When wool is wet the fibers are weaker. Recovery from stress takes place faster when the fiber is in a humid environment; that’s why steaming a wool garment will freshen the fabric and why a steam iron is recommended for pressing wool. The flexibility of the wool fiber also makes it more durable. A wool fiber can be bent back on itself more than 20,000 times without breaking, compared to about 3,000 times for cotton and 2,000 times for silk.

FIBER ABSORBENCY - Wool is a hygroscopic fiber; it takes up moisture in vapor form. Tiny pores in the epicuticle make the fiber semi-permeable, allowing vapor to pass through to the heart of the fiber. Wool can easily absorb up to 30% of its weight in moisture without feeling damp or clammy.

DYED EASILY - Wool absorbs many different dyes deeply, uniformly and directly without the use of combining chemicals. Wool is amphoteric, which means it reacts with both acids and bases; thus it accepts both acid and basic dyestuffs. Dyes penetrate into the inner medulla core of the fiber where a chemical reaction occurs making the color change permanent except under extreme and prolonged fading conditions.

FLAME RESISTANT- Because wool contains moisture in each fiber, it resists flame without chemical treatment. Instead of burning freely when touched by flame, wool chars and stops burning when it is removed from the source of the fire. Wool is self-extinguishing. It will not support combustion; this is why wool carpet and wall covering is used in airplane interiors.

Wools of New Zealand (www.woolsnz.com) is an international association of wool product producers and manufacturers. The video clips used in this online course are the property of Wools of New Zealand and are used with permission. Special thanks to WNZ for their support.

Wools of New Zealand

Wool is a natural protein fiber that grows from the follicles of the sheep’s skin. It is like human hair in that it is composed of keratin-type protein. Chemically these proteins contain 5 elements: carbon, hydrogen, oxygen, nitrogen and sulfur. These 5 elements are combined into 19 amino acids linked together in ladder-like polypeptide chains.

Fleece Terminology3

Before discussing the characteristics of different sheep breeds, it is first necessary to understand some simple terms that are important in classifying and describing different wools. After all, it should be obvious that different breeds of sheep will produce wool that will be different. Different breeds of dog have very specific hair characteristics – consider a short-haired Dachshund and a Collie, for example. And just as human beings differ in hair characteristics (e.g., length, thickness, curl), so also do sheep.

There are a number of different terms that apply to wool fiber.

Staple length

First, unlike man-made fibers that are extruded as continuous filaments - many miles (kilometers) long - the length of a wool fiber is far shorter. The length is determined by the characteristics of the sheep breed and the frequency of shearing. If sheep are not shorn, they moult when their fleece reaches a certain length. The typical length at which the fiber is "harvested" is known as the staple length. Depending on the breed and the clip, this may vary, typically between one inch (25 mm) for lambs' wool, to 12 inches (300 mm) for Lincoln wool.



Fiber diameter is measured in microns (micrometers or mm), which is one millionth of a meter or 1/25,400th of an inch.

Wool from different sheep breeds varies in fiber diameter. The finest may be as low as eight microns (8 mm), and the coarsest is over 50 microns. The finer wools, typically from the Merino breed, are used for apparel fabrics, such as fine worsted suits and high-quality knitwear, and the coarser wools find their way into chunky knits, upholstery - and carpet.

The average carpet wool is usually about 35 microns in diameter. The term fiber fineness is often used interchangeably with fiber diameter. Thinner wool fiber makes a softer textile for clothing use. Thicker wool fiber is more durable for use in carpet pile.

Fiber fineness is determined by fiber diameter.



Different breeds produce wools of different crimp, or waviness. Crimp gives the characteristic of bulkiness or cover to carpet and rugs. It also traps air to insulate, and provide warmth and resilience – the ability to spring back when crushed - to finished wool products.

Wool's natural crimp provides resilience and insulation.


Finally, vegetable matter, an important characteristic of the cleanliness of the wool, is used to signify the typical contaminants of the fleece. This is discussed in greater detail in a later section.

Body Fluids Marking Ink




Wools also differ in color and luster. Selection depends on how the wool is to be used (e.g., clothing, carpet or rugs). Typically in carpet or rugs, color ranges from light cream to dark black and this natural color range can be seen in true "Berber" carpet.

Staple Length  Typically 1”-6” (24-150mm)
 Diameter  25-40 µm (micron=1/1000th mm)
 Crimp Straight to very ‘wavy’
 Strength/damage e.g., presence or absence of weak spots
 Cleanliness amount of contamination, wool grease, etc.


cream, grey, brown, black

Table 1- Definition of the quality of wool


Grading of fleeces for sale takes into account all of these characteristics.

Wool Fiber Grades

Wool is also separated into grades based on the measurement of the wool's diameter in microns. These grades may vary depending on the breed or purpose of the wool. For example:

  • 17.6-18.5 - Superfine Merino
  • 19.6-20.5 - Fine medium Merino
  • 20.6-22.5 - Medium Merino
  • 22.6 and greater - Strong Merino


  • 24.5-31.4 - Medium
  • 31.5-35.4 - Fine crossbred
  • 35.5 or greater - coarse crossbred
In general, anything finer than 25 microns can be used for garments, while coarser grades are used for outerwear or rugs. The finer the wool, the softer and less durable it will be, while coarser grades are more durable and less prone to pilling.


The physical structure of the outer scaly layer of the wool fiber contributes to wool’s unique property of felting. Under the mechanical action of agitation, friction and pressure in the presence of heat and moisture, the scales on the edges of the wool fibers interlock, preventing the fiber from returning to its original position. Felting shrinkage is irreversible.

The felting property of wool is both an advantage and disadvantage. In a controlled situation the felting quality is called fulling or milling and creates a softer finish for woven wool fabric. Felting is also an advantage because it provides for a wide variety of non-woven felt fabrics for hats and for industrial uses. Felting is a disadvantage because it makes the washing of untreated wool fabrics difficult. Felted wool yarns have become popular for use in area rugs and even carpet.  Cleaning these may become a real challenge to the professional cleaner.


felted wool yarns are used in rugs and carpet

Endnotes:  3W J Onions, Wool, Ernest Benn Ltd, 1962.

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