Did you know that baking soda (sodium bicarbonate) is amphoteric (react with both acid and bases)? I didn't until recently. I know by combining baking soda with some kind of acid will set off fizz, that's how we make bath bomb (or fizzy) to self disperse in bath tub filled with water. A quick kitchen sink drain cleaning is to flush down some baking soda with vinegar. But, I've never tried reacting baking soda with a base. I just learned recently that reacting with a strong base baking soda would turn into washing soda which is a water softener.
Since baking soda reacts with lye which is a strong base, making cold process soap with baking soda is somewhat tricky. I have seen soap makers failed by adding too much baking soda into the soap batter. Too much baking soda added would end up off setting the normal saponification process and resulting in mushy soap, which is not soap at all.
As curious as I am, I just have to try this. I live in a hard water county, majority of our water source is from the underground. We did install a heavy duty water softener tank in our garage where the water enters the house, but it's never as good. In fact I cannot bring myself to drink the tab water, it tastes funny, yucky. I can never have a luxurious bubble bath with 10 inch high foam because my water is hard. If adding baking soda in soap can help combat the hard water and cut down soap scum I think I would feel like I hit the jackpot.
To prevent baking soda off setting normal saponification I decided to use lower lye discount and only add 5% of the oil weight. Since all I heard from other soap makers' horrified story is that baking soda reverses trace, I decide to take a water discount. With 5% lye discount, 5% baking soda, 33% lye concentration, my first baking soda soap didn't go too well. It went way pass plan A, totally skipped plan B straight to plan C which is just trying my best to squish all I can into the mold before it started to harden.
I was not surprised at all when I unfolded this small 5 bar test log, there were potholes EVERYWHERE.
It was very bad. This photo was taken after I tried to patch up some potholes using the scrap soap from the soap pots. There wasn't enough scrap to fill all that holes. I had to sacrifice 1/2 bar of soap to fill the rest of the holes. These are the 4 bars left:
How did I fill those potholes?! Soap is still considered soft and pliable when first unmolded. All I have to do it smash it like clay and push it into the holes with a knife (or scrapper). This only works the first couple hours of the soap out of the mold, else it gets too dry to work for seamless patching.
2 days later I took a sample piece to the sink for lather testing. I can still feel the baking soda, it smells powdery and a little chalky, but not scratchy. So far I'm not seeing a noticeable different in lather, maybe I'll wait a few weeks to wash again.