Okay, so here's more...
The word "saponification" is often used to describe the chemical reaction that makes soap, but soap making is just one form of saponification. Saponification by a strict definition is any chemical reaction that splits a triglyceride (fat) into its component parts -- three fatty acid molecules and a glycerin molecule. Many chemicals are able to saponify, not just the strong bases (aka NaOH and KOH) that we modern soapers use to make soap.
Even water can be used to break up the triglyceride molecules and begin the process of saponifying fats. This breakdown of fats by water happens all the time -- this is one of the methods by which fats become rancid. The water breaks the fatty acids away from the glycerin, then the fatty acids continue to break down into stinky aldehydes and ketones. These are the chemicals that cause that nose wrinkling "rancid odor" most of us have smelled at one time or another.
The problems with breaking down fats with water are that (1) the initial reaction of water with the fat doesn't acually make soap (you have to add an alkali such as NaOH to finish the job) and (2) it is really, really slow unless you add heat and or pressure. Think about the short-lived fats like hemp -- its shelf life is about 6 months or so, but that is a loooong time to wait if you were hoping to get soap as the end product! So for our purposes, breaking down fats with water is not very useful except to teach a lesson about keeping your fats dry and cool to reduce the development of rancidity.
So ... moving on to more useful methods of making soap.
The oldest form of intentional soap making used a "ley" (a very old word for "lye") made from the ashes of certain marine or seacoast plants. These ashes contain high levels of potassium carbonate with some sodium carbonate. Natron was also used as a cheaper but less satisfactory alternative to the ashes. This is a mined product that is a mixture of soda ash (sodium carbonate, Na2CO3), sodium bicarbonate (NaHCO3), and other chemicals. Natron is one of the materials used by the Egyptians for mummification. edit-- The ashes of inland plants can also be used to make ley/lye, but the amount of sodium carbonate is lower, so the lye from these plants makes a softer soap compared to lye from marine/seacoast plants. The ashes from hardwood are preferred for soapmaking purposes, for what it's worth. --end edit
In the simplest form, the ashes were mixed with water and allowed to slowly steep, much like tea. Some of the potassium carbonate and sodium carbonate from the ashes dissolved in the water. Some of these carbonates reacted with the water to transform into potassium hydroxide (KOH) and some sodium hydroxide (NaOH). This mixed hydroxide and carbonate ley was then mixed with fat, then heated and stirred until it formed a crude paste soap. Some of the carbonates in this soap batter would have reacted with the free fatty acids roaming around in the fats, but some of the carbonates would have remained intact as contaminants in the soap.
In a refinement of this process, the ashes were not used directly to make soap. They were first mixed with water and slaked lime (calcium hydroxide, Ca(OH)2). The chemical reaction of the ashes with the lime formed a pure, stronger ley of liquid potassium hydroxide and some sodium hydroxide, mixed with particles of calcium carbonate (CaCO3). The calcium carbonate particles were removed from the lye solution and the resulting clear ley was used to make a soft paste soap. People found out that adding plain salt to this soft soap would harden it somewhat. The increase in firmness was caused by the sodium in the salt replacing some of the potassium in the soft soap. Not all of the potassium could be replaced, so the resulting soap was firmer, but probably not as hard as the NaOH soap we make nowadays.
Fast forward to my great-grandmother's time. People were still making soap with ashes or lime and ashes, but folks started to use an alternative to ashes -- pure, commercial soda ash (sodium carbonate, Na2CO3). This product is also called washing soda, since it was used by housewives for laundry and general cleaning. One can add slaked lime to soda ash, boil the mixture for a few hours, and let the calcium carbonate particles settle. The resulting clear solution was lye -- sodium hydroxide in water. This could be used to directly make a hard soap. Sometimes this type of recipe is billed as making a "soap without lye", but that's really not very accurate -- it's just that the lye was formed indirectly by boiling the soda ash with lime.
This process was cumbersome and time consuming. When pure lye (NaOH) became commercially available in my grandmother's day, most housewives quickly turned to store-bought lye as a timesaving, simpler, better alternative. (The large soapmakers had had access to pure NaOH for some decades before this time, and these companies had gradually converted over from the soda ash & lime process to using commercial NaOH long before our foremothers were able to do the same.)
So now that brings me to using baking soda (NaHCO3) instead of lye. Yes, it is a mild alkali and, yes, it will create soap, although not as fast nor as efficiently as the much stronger NaOH. The saponification reaction will not be as efficient, and I'm betting the soap batter will have to be heated to force the reaction to go. Carbon dioxide (CO2) gas will be created, so the batter will foam quite a bit (think of your grade-school "volcano" made with vinegar and baking soda). This is similar to what a "wet chemical" fire extinguisher does when it is used on a grease fire in a kitchen -- the extinguisher contains potassium carbonate which reacts with the heat and fat to form a foamy soap. The foam smothers the fire.
Fat + NaOH => Soap + Glycerin
Fat + NaHCO3 => Soap + Glycerin + CO2
The availability of pure NaOH and KOH ... and more recently the invention of the stick blender ... have been real leaps forward in turning soap making into a fun hobby for many and a realistic small business venture for some. Baking soda is an interesting experiment for some to try, just like leaching ashes to make lye. But I doubt either alternative is a realistic threat to the use of NaOH and KOH.
And with that, I'm heading to bed....
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