There are many ways to incorporate various clays in handmade soap. I use them in cold process soap making, as that is the kind of soap that I make. They may be able to be used in melt and pour soap making, but the way they are used may be different. Please consult another source for this to make sure, because I'm not an authority on melt and pour soap making.
Clay in cold process soap can add color, as well as other skin benefits such as a gentle exfoliation. It can give the finished soap a nice "glide," such as in a shaving soap, helping the razor to glide over the skin smoothly and hopefully reducing nicks and cuts. There are several different types of clay available, so let's take a look at a few of them.
Bentonite clay usually forms from the weathering of volcanic ash. There are different types of bentonite clay, named after the main consistituent element: sodium, aluminum, potassium, or calcium. Usually light gray in color, it is reputed to assist in absorbing toxins from the skin. It also helps to hold scent in a bar of soap. This is one of my favorite clays to use in soap, especially in an activated charcoal bar to further purify the skin.
Rhassoul clay typically comes from Morocco, and contains higher percentages of silica and magnesium than other types of clay. Some people claim that this makes it more effective than other types. It is usually a light reddish gray in color. It is also called Red Moroccan Clay.
Kaolin clay can come in many different colors, including white, pink, blue, yellow, and more. It is a very mild clay, and is used in many different applications. You can find it in cosmetics, deodorants, toothpaste, and some digestive aids. White kaolin clay is also used to make porcelain.
Illite clays are sometimes called a "clay mica," because it has properties of both a clay and a mica. Because of this, illite clays do not swell when hydrated with water, like other clays. As such, this means that illite clays may be better used in different products than the other types of clay we have looked at so far.
For more information on clays, these are some great resources that really delve into the chemistry behind them: