Round brushes may have a core made of all the following materials EXCEPT?

Round brushes are commonly used in hairstyling for creating volume, curls, and smoothing the hair. The core of these brushes can be made from various materials that are suitable for heat and can withstand the styling process.

When considering the materials that can be used for brush cores, plastic, rubber, and wood are all practical options. Plastic cores are lightweight and resistant to moisture, making them durable and easy to clean. Rubber is often utilized for its flexibility and cushioning properties, which can aid in grip and comfort during styling. Wooden cores can provide aesthetic appeal and are typically more environmentally friendly than synthetic materials.

However, leather is not a suitable material for the core of a round brush. Leather is susceptible to heat and moisture, which could lead to deterioration over time when exposed to the high temperatures of styling tools or the humidity from hair. Therefore, it is not typically used in the construction of brush cores, making it the exception among the choices provided.