A couple of my yarn bases include BFL, and at events, I'm frequently asked "What is BFL?" I want to respond with "BFL is awesome!" and leave it at that (because it is), but I give a quick answer like "it's a breed of sheep" and sometimes provide a little bit of a comparison between it and Merino. I thought this might be a good time to give you a more extended answer.
So, what is BFL?
BFL stands for Bluefaced Leicester (pronounced like the name Lester), a breed of sheep in the English Longwool family (along with Leicester Longwool and Border Leicester). Although not certain, speculation says this larger family of sheep was brought to Britain when the Romans reigned there. The Leicester family's development specifically can be traced to Robert Bakewell, who bred sheep in the eighteenth century. The Bluefaced Leicester itself, however, is a breed that developed from the Leicesters more recently, in the early twentieth century. BFL is primarily a British breed, but there are BFL sheep in the U.S. and other countries. (Note: the supplier I use for my yarn uses BFL solely from farms in the UK.)
In appearance, you might expect the Bluefaced Leicester to have a blue face and at first glance, it does not. However, if you look more closely, you'll notice that the skin on their face is actually black, with white hair over it, which gives a slightly blue appearance. You can do a Google search for BFL sheep pictures, but I rather like this one of a mother with her lambs (and at the bottom of that page, you can see a basket of fleece).
Staple length is the length of the individual fibers in a fleece. When the fleece is spun, the ends of each length of fiber are "tucked in" by the twist. If the staple length is shorter, there are ends more frequently than a longer staple length fiber. That leaves room for those ends to come "loose" more often, resulting in a more worn look (i.e., pilling) more quickly. BFL is classified as a longwool (though it's on the shorter side of the longwool category), meaning that it has a longer staple length compared to breeds in other categories.
Micron count measures the diameter of the fiber, and in general, the smaller the micron count, the "softer" the fiber. This measurement comes into play when we think about the "prickle factor" of a yarn--basically how it feels next to our skin. The idea is that fiber with a larger micron measurement (thus thicker) doesn't bend as easily, so it is more likely to feel like it is poking your skin.
Crimp is the curl/waviness found in a lock of fiber. If a fleece has a fine crimp (meaning really wavy), then it is going to be a really bouncy yarn, with a good amount of stretch and springiness back to its original shape. BFL locks look like little springs, so it is very bouncy.
Luster means how well a fiber reflects light. Silk, for example, has a high luster, which gives it a shiny appearance. BFL also has a high luster.
Comparison to Merino
Strictly getting down to basics, BFL is not as soft as Merino but it produces a stronger, better wearing fabric. Looking at some of the technical specifics gives you some background for why this is so.
If for some reason I ever have to choose between using only Merino or only BFL for the rest of my knitting life, I would choose BFL as I feel it is a more middle-of-the-road fiber, good for a lot of different purposes.
Best Uses for BFL
Most people are able to wear BFL next to their skin (though if you've been spoiled by an extremely low micron count Merino, then BFL might feel scratchy by comparison). Last year, it was quite chilly inside the building at the fiber festival I was attending, so I grabbed one of my samples: a crocheted scarf made from my Cornwall base, which is 100% BFL. I wore it all day with only the slightest sense of "prickliness" on my neck, and it kept me nice and warm.
Because BFL has a longer staple length, it won't pill like Merino does. That's good news for high wear items such as socks and gloves/mittens. Personally, where I think BFL excels is as a sweater yarn. A local-to-me knitter, Erin, has made two sweaters from my Cornwall base (the Amna Cardigan and the Blackberry Cabled Cardigan); both turned out amazingly. If you live in the DFW area, I hope you have a chance to see one or both of these projects in person at some point. (Thank you, Erin, for doing such amazing work!)
Blended with other fibers, BFL can bring out positives and counteract some negatives. For example, silk is a shiny, lustrous fiber, but it doesn't have much stretch to it. BFL itself has a rather lustrous appearance, so it only enhances that quality in silk. But BFL is very sproingy (due to the spring-like crimp in its locks), so when added to silk, the resulting yarn has more bounce than just silk alone. (That's what is going on in my laceweight Isolde base.)
If you've never worked with BFL before, I encourage you to give it a try. Like I mentioned above, I currently have two bases with BFL: Cornwall and Isolde. If you want to get some of this yarn (or if you already have some in your stash) and you need suggestions for projects, I have two bundles on Ravelry set up with pattern recommendations:
Recommendations for Cornwall
Recommendations for Isolde
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