Salt soap is soap made with high content of salt, usually range from 50~100% of soaping oil. It can be done in cold process and also oven process. The first time I tried making salt soap I did it in oven process, meaning using oven heat to force it to saponify in 90 minutes then cut right the way when it's cool enough. If it's left uncut for hours you won't be able to cut it anymore, it would turn rock hard, that's the salt working. Since then I've been soaping salt bars cold process. I make my salt soap the same way I make all my other soap, cold process, because I use milk to soap. Without using the oven heat to force saponification I need to avoid cutting because that high of salt content would harden the soap and make it too crumbly to cut. My solution is using individual silicone molds. My favorite shape is oval, easy fit in my palm and easy to tumble under water.
Besides being rock hard, another obvious characteristic of salt soap is it doesn't produce bubbly suds. It makes rather creamy lather almost lotion like. What's salt soap good for if it doesn't lather well? High concentration salt is very effective in exfoliating dead cells off your skin leaving you clean but smooth after shower. Think about the reason why people take bath with salt soak, it softens your skin and exfoliates away dead skin cells. If you don't have time to take a long luxury bath or simply don't have a tub at home, using salt soap in the shower would be the only alternative.
I have tried a few different salt to soaping oil ratio in the past, and finally settled with 80%, meaning if I use 100 g oil to soap, I would add 80 g of salt. Almost any kind of salt can be used to make salt soap, even table salt. The only salt that's not good for salt soap is Dead Sea Salt. Dead Sea Salt has a property of attracting moisture in the air, if used in big quantity it would cause the soap to sweat and weep if it's at a humid area. I like using sea salt and Himalayan pink salt for their richness in mineral. I love using salt soap to wash my combination skin type face. It is not suitable for sensitive skin or wounds, think about how much it hurts when your wound touches salt... ouch.