One of the things that can make stovetop cooking a bit challenging is that there are times when the whole cooktop is too hot for things that you want to cook very slowly or gently. A stovetop which is too hot can be caused by a number of reasons. The first and probably most common reason is that your fire is serving multiple functions. For example, if you are baking something that needs a hot oven--say 400-425 degrees or more--or if your cookstove is doing double duty as the sole means of heating your home on a cold winter's night, your fire might be hot enough that the whole cooktop is running somewhere between what would be considered high or medium high heat on a modern range.
In his 1978 book entitled Wood Heat, John Vivian writes, "When there is a good baking fire going, there will be live flame under all lids and the entire cooktop will be hot enough to boil water with the lids in place." In my experience, the "live flame under all the lids" part isn't true, but the part about the entire cooktop being able to boil water certainly is.
Sometimes you also need a hot fire because of what you are doing on the cooktop itself. If you are canning--especially in the case of water bath canning--or heating large amounts of water for laundry, you might need a raging fire beneath just part of the cooktop. What you are trying to cook on the rest of the stovetop might not benefit at all from such an intense heat.
Furthermore, in the case of small cookstoves (see the pic of my brother's), two-lid kitchen heaters, boxwood stoves, or combination ranges where the wood heated cooktop is abbreviated, the heat of the cooktop might be uniformly hot when you need a variety of heat levels.
This is where trivets and simmering pads come in handy. By raising a cooking vessel enough that it no longer is making direct contact with the stovetop, they reduce the amount of heat which is being transferred from the stove to the food. These wood cookstove accessories can be found in just about any shape or form and are generally inexpensive. I added to my collection of these over the summer, so I thought I would show them to you.