Soap is made up of lye, oils and water.

Lye is alkaline.  Alkali are basic salts that dissolve in water. (Remember the pH scale?  Water’s a 7.  Lower is acidic, higher basic.  Lye is high on the pH scale, and since it’s far from neutral it will burn you just like an acid will.  Wear gloves.)

When lye, water and oil combine (insert chemistry here) they create a molecule that will dissolve with both oil and water.  Think about how hard it is to make oil and water combine, or to get oil off of something with water alone.  Soap grabs both water & oil molecules, grabbing the grime and washing it away with the water.

Humans have been making soap since 2800 BC, the time of Babylonians.  Like lye, wood ash is alkaline, so can be mixed with oil & water to make soap.  Props to the Babylonians.
Additional props to the chemists who figured out how to make cleaner types of lye, such as sodium and potassium hydroxide.  Sodium hydroxide is used for hard bar soaps and potassium hydroxide for liquid soaps.

There’s two ways to make hard soap – hot and cold process.

When you mix up the lye, water & oil, you end up with a pudding-like texture that will harden up in time.  For cold process, you put your soap pudding into a mold and let things harden up & the pH mellow out naturally.  This is called curing and takes up to 4-6 weeks.
Hot process starts out the same way, but instead of curing the soap at room temperature, you heat the soap mixture up.  This way, you can use your soap the same or next day.
Hot process requires a bit more management, and the temperature increase can make things go wrong.  Heating it up too quickly can create a soap volcano that slowly tries to escape up out of your pot and will take the stain clean off your cabinets (trust me on that one).  On the flipside, cleanup is much easier with hot process. With cold process, the soap pudding you’re putting into molds is still a caustic gel, so you need to remember your gloves and be careful when cleaning up.  With hot process, you’ve already made soap.  Nothing to fear.

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