What is Medullation and Why Does it Matter?

What is that big, scary word we’re not ready to tackle until we have another cup of coffee?  Before your eyes glaze over and you run screaming away from your device, let’s sum this up simply.

Medullation is a fancy way of saying a hair has a medulla, or a hollow space in the cortex. Look at the fibers below. You’ll notice the human hair and dog fur both have a darker color running down the center of the shaft. This is the medulla. Shown below, the Bluefaced Leicester, a longwool breed of sheep often referred to as BFL, does not show any medulla at all. And this is one of the ways we differentiate wool from other fibers. Wools are generally solid.

Human Hair

Dog Fur

BFL Wool





This is great news for dyers! A solid core means that wool takes dye better than hair! Dye molecules attach to the fiber itself, either directly or with the help of a mordant, so any hollow space means not only is there less hair for dye to attach to, but the air pocket makes it more difficult to fully saturate the fiber. This is why angora rabbit and alpaca fibers can be tricky — though not impossible — to dye in rich or dark colors. On the other hand, I suppose this is not-so-great news for those of us trying to cover up our gray hairs!

Surprise! Alpaca isn’t a wool at all! It is actually hair, just like the hair on your head! Despite it being a hair, Alpaca still shares many of wool’s desirable qualities like crimp, fineness, warmth, and density. It is one of the fibers that rides the line between wool and hair and could be described as a wool-like hair. For example, it is medullated like hair, but has a density of around 25,000 follicles on average. Medullation generally means a thicker hair. This is why alpaca is more suitable to spin and wear than your own hair; it still usually retains the fineness of a wool, despite its medullation.  

A fiber with a medulla will be warmer than a fiber without. That is because pockets of air add an insulating layer. Air is a poor conductor of heat when compared to most other mediums. This is because molecules are not in direct contact with each other. Heat causes atoms to vibrate. When a vibrating atom comes in contact with another atom, it causes that atom to vibrate as well. This is called induction. In an air pocket, there are many fewer atoms than in a denser medium, so heat transfer is greatly reduced. Thus, a fiber with air space provides better insulation and warmth than a solid fiber. That is why many fur & hair fibers, like angora and alpaca are considered to be warmer than wool.

Consider the two fibers in the photo.  These are both from the same Jacob fleece.  One shows partial medullation (the dark lines in the center of the fiber).  The fiber that is medullated is almost twice as thick as the fiber which is not.

Once again, the primitive breeds just don’t want to fit into a nice, easy category. Many primitive breeds will show partial medullation, such as this sample of Jacob fleece. Rather than having no medulla or one continuous medulla, the cortex is divided into several uneven chambers. 

 Fibers with a medulla tend to have a higher micron count and feel coarser and more prone to producing the “prickle-factor.”  This is why it is often considered an undesirable trait in wool.  



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