Hand milling has received bad press, and I think this is mostly due to being lumped in the same same category with rebatch, which is actually a method for reclaiming cold process soap that isn't up to par. Despite what you read elsewhere, I assure you hand milled soap can be smooth and pretty. The effects you can get with hand milling are only limited by your imagination. Here are some tips for making beautiful hand milled soap.
1. Start with quality cold process soap. You are not going to produce superior soap with an inferior base. You need a properly cured basic cold process soap. I've recently started buying shredded castille (olive oil) soap from brambleberry.com to save myself the time of making the soap first. This base is wonderful and melts down very smoothly.
2. Work in small batches. I work in one pound batches. When I'm soaping bulk quantities, I just make one pound batches is rapid succession. One pound of soap flakes fits neatly in a medium saucepan. I have noticed when I move to a larger pot with more flakes, the process is slowed down, and it becomes more difficult to control the heat in the pot.
3. Control the heat in the pot! I do not use the oven or the crockpot - those are for melting down large quantities which I just advised against. I'm not a fan of that method. I tried the crockpot once and scorched my soap. I use a saucepan over low heat. A double boiler might be a good idea if you have one, but I haven't found it necessary. It takes me 15 min to make a batch. Through the cooking time, I sometimes turn the heat off. There is a sweet spot where the soap will melt smoothly. Try to fill your molds with the soap too cool, and it will be too lumpy. Get the pot too hot, and it will scorch. Temperature control is, in my opinion, the make or break factor in producing a beautiful hand milled soap. I mess it up when I get in a hurry. Like with any art, you need to time to work free of distraction.
4. Use milk. Milk does not form suds in the pot the way water does. It makes the soap very smooth and silky. I use 4-5 oz milk to 1lb soap flakes. More than that and the soap will be very soft out of the mold and contract as it dries. I like to heat my soap flakes gently, being careful not to scorch, then take the pot off the heat to add the milk to avoid boiling off my milk. Another method is to add the milk to the flakes first and let the soap absorb the milk, then turn on the heat. Unscented soap will sometimes have a slight sour smell for a few weeks, but that smell goes away as the soap cures. I've never had any trouble with the soap going bad, but if you are concerned you can add a preservative.
5. Use the electric mixer. Once my soap is heated and my milk is added, I turn off the heat and use the electric hand mixer to whip the soap. This produces a smooth pudding like consistency. You don't want to over whip. If you get too much air in the soap, it will contract as it dries and won't be as pretty. You just want to whip it enough to combine everything. I spoon the "pudding" into my molds and tap them gently to get out any air holes. If you really want to be particular, use clear molds and use a toothpick to take out any little air bubbles that might be on the surface. I've decided they aren't that big of deal, so I don't worry about it. Tiny air bubbles can often be removed by rubbing the spot with a wet finger after un-molding.
6. Use your imagination! The effects you can get are truly only limited by your imagination. I like to use individual molds. I fill the molds then pop them in the freezer. When they are frozen through they come out of the molds easily. I like a rustic look and love to put fresh and dries flowers and herbs in my soap. Flower petals can be mixed into the "batter." I also lay herbs and flowers in my molds, then spoon the soap gently on top of them. Spices and clays can be used as colorants. This is the real fun of the method. It is fun to think up new ways to make decorative soap. Happy soaping!