I was chatting last night on an Etsy board, and the question came up, "what's the difference between cold process and hot process?" Another soaper claimed that hot process is milder and has a better lather. I avoid the drama llama, so I kept my fat mouth shut, but that wasn't entirely accurate, and I thought I might bring my soapy experience, and my science degree here and shine some light on this. Most soapers think of soap making as: lye + fatty acids = soap + glycerin. But there's another ingredient, and that is heat. Heat is a product of this chemical reaction, but heat can also be added or conserved by insulation to speed up this reaction. In cold process soap, no extra heat is applied to the emulsified lye, water, and oils. This process can take a very long time. A cold process soap with milk, for example, will darken if heat is allowed to stay in the process, because the milk cooks, caramelizing the milk sugars. For that reason, many milk soapers will even refrigerate the soap to prevent this. But this soap may take as long as six weeks to fully saponify.
With a hot process soap, heat is added, and saponification may be complete within an hour. It doesn't make this soap any more gentle, it just makes it gentle faster. Much faster. Also, it evaporates most of the water from the product, making the bar harden faster. A fresh hot process soap, cured for one week, will lather better than a six week old cold process bar. The reason for this is that this fresh hot process bar still has a higher water content than the fully cured cold process bar. Soap salts have to dissolve in water before they will lather, and the fresh HP bar is already partially dissolved. A six week old HP bar and a six week old CP bar made with the same recipe will have the same lather - and the CP bar, being more porous may even lather better, as it will dissolve in water easier.
So, HP is not more gentle than CP, and does not lather any better. These are properties of your formula. Gentleness is kind of a vague term - it may indicate pH, superfatting percentage, or how much the soap dissolves the oils on the surface of your skin. If a soap has a high percentage of coconut oil, it may feel like it's harsh, because it does remove your skin's oils very well. A lard soap may feel very gentle, if it's been superfatted up to 8%.
And what about melt & pour, what makes it different? Well, melt and pour soap is prepared with a hot process method, with an added alcohol such as sorbitol (which is a long chain alcohol that does not evaporate and dry the skin) used to dissolve the soap salt into a solution which will solidify at room temperature, but will remelt at a relatively low temperature because of the added glycerin, with no additional fluid added. To melt a regular HP or CP soap, you have to heat it to over 140 degrees, and have added liquid to dissolve fully. A melt and pour soap will melt at temps around 100 degrees, with a nice pourable consistency. Making melt and pour soap from scratch is frankly, a pain in the ass. Many high quality melt and pour bases are available, and they are relatively inexpensive, and very safe to work with - no lye! Some bases even have added detergents, making their lather very bubbly, but a bit more drying.
So that's the truth. The process you decide to use for soap making will highly affect what additives you can use, how much and what type of fragrances to add, and what pigments as well. But it's all soap, and the formula is what makes it lather, clean, or condition - not the process.
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