A quality paint brush will have hairs that form a point and have a good spring to them - they bend while painting but return quickly to their original shape. A good brush will also hold lots of paint and deliver that paint evenly throughout the stroke.
Traditional glass painting brushes include many different types. Paintbrushes can have a number to indicate their size - the larger the number, the larger the paintbrush. Other brushes are sold by size in inches (2.54 cm per inch).
Some of the most common types of brushes for glass and ceramic painting include:
Red Sable hair.
The sable hair comes from the tail of the sable marten, a sort of weasel. The hairs taper naturally to a point. These relatively expensive brushes are soft and flexible, hold their paint well and can make an expressive thick-thin line. Kolinsky sable from Siberia is considered to be the finest sable.
Cheap brushes will normally be of poor quality and the painted result will reflect that fact. High quality brushes, when properly taken care of, will give you years of friendly use and be well worth the investment.