Er... Uh... Have you ever felt like someone was asking the world of you and they didn't realize it?
The appeal of many soaps is a wonderfully fluffy lather! However, lather is a tricky subject, which is where the intricate dance of art and science comes into play for developing a soap recipe.
First, let's talk about skin. There are three layers of skin -- the epidermis, dermis, and subcutaneous layer. The epidermis (top layer) is then divided into another four layers (five if we're talking about the soles of feet). Basically, the very top layer of the epidermis is sparsely populated by skin cells about to be shed. In between all those skin cells on the top layer is our own oil that acts as our own natural moisturizer. This oily layer creates a barrier that prevents water from evaporating out of our bodies. When our skin (specifically the epidermis) lacks water, it feels "tight" and looks "flaky."
Soap works in two ways mostly. It removes oil, but it also deposits oil (based on "superfat"). "Superfat" is when there are extra oils in soap that have not been saponified (turned into soap).
The amount of lather contributes directly to the drying effect of soap. The more lather, the more drying the soap will be. Bubbles are the result of soap dissolving in water. The more soap molecules that are free to create bubbles in the water, the more oils are removed from the top layer of skin exposing the skin to more water evaporation. This layer of oil contains bacteria, dirt, and other toxins.
However, the soap that is "superfatted" is also leaving behind a layer of oil to replace the dirty oils that have just been removed from the skin.
Tallow, being nearly identical to our human oils, makes a great substitute barrier. So, more bubbles can be achieved in a tallow based soap rather than a vegetable based soap because even though it's removing a critical layer of oils, it is also replenishing that layer with a nearly perfect substitute.
Does that mean soaps without lather, like olive oil, are moisturizing? Well, not necessarily. Olive oil soaps, while lacking lather (theoretically not as drying), don't quite add the layer of oils that the skin needs to protect the body from water evaporating from the dermis layer of skin. So to some people (like myself) olive oil soaps can feel more drying despite the lack of lather which should signify conditioning properties.
Soap with glycerin also feels more moisturizing. Glycerin, a humectant, draws water up from the dermis below and from the air above making the upper layer of skin feel plump and not "tight." Glycerin is present in virtually all handmade soap, while commercial companies remove glycerin to sell on the side.
However, that being said, there are even more factors that come into play regarding the dryness of skin. Not even the most moisturizing soap (or lotion) in the world can prevent dry skin if you are not drinking adequate water. Superfatted soap can leave a barrier on the skin to prevent water-loss, but it cannot add water to your skin.
During the winter (especially where it snows), the humidity in the air freezes creating "dry air" which pulls water out of the body. Not to mention, every breath into the lungs is humidified only to lose that water when one exhales. Being dehydrated makes the skin have less water and thus feels tight, dry, and flaky.
A hot shower feels luxurious during the winter, but the heat causes more water to evaporate leaving the skin dryer than before. Likewise, after getting out of the pool, laying out in the sun to dry off causes more water to evaporate out of the skin as well as the chlorine that strips away the natural oils of skin and hair.
Holistically speaking, not only is it beneficial to replace the layer of oils that were removed from skin by using soap with extra fat, but also make sure to replenish the amount of water in the skin by drinking adequate water. By doing so, you are preventing the problem of dry skin as well as maintaining proper balance within the epidermis.
Thank-you for reading!