well, i finally gave it a try! i decided i was only warming the soap- sort of like dishwashing- no extra pot needed. another grater was bought, but wouldn't have been needed. i found ways to avoid the need to buy a hand mixer. i decided to go ahead and give my regular bar soap choice a try.
liquid soap (from a bar of soap)- adapted from bea johnson and the farmer's nest
1. grate the bar of soap (castille soap and mrs.meyers soap also work- but not all soap does)
2. add the grated soap to 1 gallon of water in a pot and warm over medium heat, stirring until soap dissolves
3. let sit overnight. (i poured my soap into two glass casserole dishes. a couple of hours after i'd poured my soap into the casserole dishes, it had set. i put the lids on and went to bed.)
4. set, it is more like set jello consistency than liquid soap. here's where you could use your mixer, but i just dove in and mushed it between my fingers…fun! :) the farmer's nest said it would end up a snot-like consistency- gross, but true.
5. pour into containers (i used glass quart jars and an empty shampoo bottle.)
i'd love to hear if you make liquid soap and what tweaks you make!
update: april 15, 2015 this zero-waste liquid soap is not a success for us. it was fun to start with a bar of soap and see it come out as liquid soap…plus there is so much of it. like i said in this post, it did not work as dish soap or shampoo for us, as i'd originally hoped. we've been trying to use it for liquid hand soap for a while and don't really like it for that either. it doesn't lather, so we don't really feel like it's cleaning well.
i have found a natural, low-waste alternative that i do really like for hand soap: a little bit of castile soap + lots of water in a foaming soap dispenser. i had a dispenser that came with soap in it, and now i just refill it with my own mixture of soap and water. castile soap is pricey, so this very diluted mixture will make it last.
update: March 30, 2018 I no longer buy castile soap, because it is quite expensive + I don't use it in any other ways. I've tried it as dish soap, which I don't like because it is oily, expensive, + doesn't lather. I don't like it for my hair for similar reasons. Now we just use a drop of dish soap to wash our hands. I refill a smaller soap dispenser about 1/4 full at a time, because I find that a full bottle dispenses too much soap + we go through it too fast. This way, I buy the largest bottle of dish soap that I can find at the grocery store (I consider it bulk) + it lasts a long time. One bottle of dish soap works for dishes, hands, + hand washing intimates too.