Badger brushes

In terms of the soap being acid based, acidic need not describe something acidic. It can also be used for something to describe something that has a corrosive effect, and of which soap drying inside a brush knot is shown to do. My source would be the various brushmakers who make this their livelihood off this. Even without them I’ve seen enough old vintage brushes and how they were treated to realize it isn’t a good idea. 

I am not denying that routinely leaving soap to dry inside the knot of a brush is detrimental. It is just that the use of the term "acidic" is not correct. According to the Oxford dictionary, the definitions of the word acidic can be as follows:

      Having the properties of an acid, or containing acid; having a pH below 7.
      Sharp-tasting or sour
      Describing a person's tone that is bitter or cutting

      An intense or bright color- acidic yellow
      Rock, especially lava, that is rich in silica
      Steel slag that is rich in silica

None of those definitions would apply to a soap. 

Strong alkali substances are often described as caustic. However, to be considered corrosive, the pH needs to be greater than 12.5. Since soaps are normally in the range of 8-10, they do not fall into the corrosive range. However, if you use "lye soap", the pH can be high enough to be corrosive.

You can object to the terminology all you please as acidic is not the best word for it. Whatever word you want to use it destroys the hair at the base of the knot over time when not properly rinsed out. As to the length of time it takes for this, it is anybody’s guess, and as you admit there is a possibility of some damage, thus you’ve made my argument. Know that if you leave your brush overnight set with lather, it can possibly cause damage. 

The brush makers themselves make note of soap lather being left inside the knot has a detrimental affect that eats away at the hair and weakens it over time. We have had Brad, which while he does not make the knots himself, certainly is well qualified in that he procures his own knots, works with one of the finest knot makers out there, and has seen more brushes in person than even us but jobs with 100+ brush collections will see in our lifetime speaks of seeing this exact problem with a brush being lathered and left to dry.

I don’t think the risk is worth it, that there is ample evidence as to why not to do it, and I will speak up about it. I wouldn’t like for someone to buy a new brush and to do it without reading as to why not to do it.

Gentlemen, possess no empirical metric for the mechanism nor degree of degradation occurring but the anecdotal evidence all can surely agree is there some degree of degradation by sitting overnight in soap (otherwise the softening of the knot all acknowledge would be absent).  Perhaps the debate better focused on whether the degradation is beneficial or deleterious if intentionally conducted under time constraints. As is often stated, what is glorious on a macro scale is horrific on a micro scale (not my opinion but a universal consensus judging by accepted societal institutions termed governments).