A natural fiber, and a very inexpensive brush. It has a few faults, such as bristles decaying once they get wet, whether you use the brush or not, and bristle shedding. This brush fiber is mostly obsolete in favor of synthetic bristles, such as polypropylene.
Other sizes can be special ordered , lead time runs about one week. We keep 15" and 18" blocks in inventory.
Please note that the price list shows actual block diameter which means inside the skirt of your floor machine. Simply measure inside the skirt and order the whole inch diameter that will fit inside the skirt ( ie you measure 15 1/4" inside your machine, then a 15" block would be a correct fit).