Shaving Brushes and the Scientific Method

He’s at it again! thebigspender has weighed in with another zinger, this time at The Straight Razor Place. The subject: Custom brush maker Tom Rakowski. At least thebigspender admitted up front that he didn’t know what he was talking about… You can look here to see what I think of thebigspender's poor logic. He is consistent.

I’ve never seen one of Tom’s brushes in person either. I have seen his photos, and he does a very nice job turning his handles. Even without holding one of Tom’s brushes though, I can see a potential problem. And you will see it too when you finish this post.

Tonight, I offer you some direct knot size and characteristics comparisons. This is your chance to look at real photos and be your own knot scientist.

First, lets talk about knot size. Most knots are classified by their diameter. I use 22mm, 25mm, and 28mm silvertip badger knots. This allows me to make a range of brushes from small through pretty large. I have made some 30mm brushes -- those guys were giants!

The following picture shows you a comparison of the three knot sizes I use. All three knots are set in the board with a depth that will give them what I call a medium feel. (Here’s what I mean by medium: They should work well for both creams and soaps. If you’re a dedicated soaper, go firmer, but if you use both and only want one brush, medium is the ticket.)



So, you can see that by increasing the diameter, the knots (and brushes) get much larger. Math alert. This is because the number of bristles increases with the square of the knot diameter. For equally dense knots, a 25mm brush is 29% larger than a 22mm knot. A 28mm knot is 62% larger than a 22mm knot and 25% larger than a 25mm brush. In addition to these differences, the lofts vary. For equal lofts, larger diameter knots are firmer. So for the same feel, a larger diameter knot can support a longer loft.

This also means that we can change the firmness of a brush by changing the loft. A shorter loft on equally dense knots results in a firmer brush. There is some physics behind this, but I won't bore you with that. The empirical evidence is there. This next picture below shows three different lofts for a 25mm knot.

Quite a difference isn’t it? My experience is that many people seem to prefer a medium to firm knot. Even a densely packed knot is going to be floppy if the loft is too long.

Now back to Tom Rakowski’s brushes. Beautiful turnings. Based on his photos, his bristle loft also appears to be a pretty high loft. So if there is a potential problem here, it is that the knots are not set to create the feel that (in my experience) many people seem to prefer.

Finally, if you are a flexible brush person, then a Rakowski brush will probably work pretty well. If you ask Tom to set your knot to a specific loft, you are likely to be satisfied too. After all, that is the point of a custom brush – get it the way you want it.