Cotton and linen fabrics are two natural-content materials that can withstand higher heat settings in the ironing process. These fabrics tend to wrinkle faster but are also much easier and less fussy to iron.
When ironing cotton and linen fabrics you will most likely be able to use the highest heat setting provided by your iron. A protective cloth is not needed unless the fabric has a surface finish that can be damaged by direct iron plate contact- as always, it is a good idea to test a piece of your fabric first.
Cotton and linen fabrics can be steamed at the highest setting as the fibers can withstand it quite well without compromising their durability. Not only that, since both cotton and line fabrics tend to wrinkle quite easily, steaming provides a faster way to iron providing a smoother result. Steam is especially important when ironing out cotton/linen stubborn creases that result from the fabric being left folded in a the same position for an extended period of time.