Natural hog bristle brushes are tougher than synthetic bristle brushes and are ideal for mixing charcoal. They are reasonably priced and come in a range of sizes. Despite their stiffness, the bristles will not scratch the drawing surface. Some painters use a gentler brush, such as a watercolor brush. Charcoal is a difficult material to work with because it's so light sensitive. Once it is blended, it must be stored in a cool, dark place until it is time to apply it to your painting surface. Blending charcoal adds depth to your painting and allows you to create different values of gray by varying how much or little of the original black remains. There are two main methods for blending charcoal: wet-on-wet and dry. We'll discuss both here. Wet-on-wet means that once you have mixed some charcoal into the paint, you should add more charcoal to continue blending. This method produces a softer edge than dry blending. You can tell when you have used enough charcoal because the color will become darker where more is added and lighter where less is added. Dry-blending means that you start with all the charcoal and then add white to reduce its intensity. This creates a sharper edge than wet-blending and is useful if you want to highlight particular areas of your painting.