Rebatching soap

Rebatching soap is called numerous names. French milling (this term is not proper when referring to rebatch, however, is often used), remelting,  hand milling, to name a few. This simply means "remodeling" previously made cold or hot process soap. This method can be used for multiple reasons. If you aren't happy with how a batch of soap turns out, or, if you have a lot of soap scraps you don't want to waste, you can rebatch.
French milling refers to a process of passing the soap through a press to evenly distribute color and additives. It is also said to result in a more gentle, mild soap. Perhaps that is why the term "french milled" is commonly used when referring to rebatching, since rebatching is supposed to result in the same positive attributes.
Rebatched soap is thought to be even more gentle and mild than the original soap. In many cases, once it is fully dry, it will also be a harder bar of soap, lasting longer in the shower.
It is best to rebatch soap when it is still "young". Preferably within 2 weeks of making it. If you rebatch when the soap is very fresh, you can often get away without using additional liquid.
Rebatching old soap takes much longer for the soap to melt and most always requires the addition of liquid, since soap continues to dry and lose moisture as time goes on. This is why older soap is harder and lasts longer in the shower.
There is no "perfect", "exact" or "right" consistency for the soap when rebatching. It depends on what you want your final product to look like. Once the soap is soft enough to put into molds (even if you have to "glop" it in there and press it down), it is sufficiently melted. You can also melt longer to achieve a more liquid form if this is more suitable for you. Doing a partial melt will leave chunks of solid soap which looks pretty when the soap is cut, or when it is used if you are using individual molds.

As with most everything, there are a number of ways of rebatching.

Oven Method:

Grate the soap as finely as possible and place in a baking dish. Cover with just enough water or milk to get all of the soap wet. Be careful not to use too much because doing so will prolong the drying time and increase the amount of shrinkage, resulting in distorted, mishapen bars of soap.Cover. Place in the oven at 140 degrees. This takes a considerable period of time. Hours.Check the soap occasionally, stirring GENTLY! If you stir too quickly, you will make bubbles. It IS soap!! Bubbles are difficult to get rid of and will create air pockets in the soap. When the soap becomes a thick liquid, it is ready. Take it out of the oven and add your fragrances, herbs, colors. etc. Poor  into molds. Allow to harden. Placing in the freezer will speed the process. Once able, remove from molds and allow to dry. Drying time varies, depending on the amount of liquid you added at the beginning of the process.

Crock Pot Method:

To do this safely, you MUST have a crock pot that has a very low setting. Slow cookers often only have low and high and low is too hot. Crock pot brand on low is generally acceptable. As before, grate the soap as finely as possible. Put just enough liquid (milk or water) into the crockpot to cover the bottom. Add soap and cover the pot. Let it sit for an hour. Check the soap's consistency. Don't stir too much! Continue to check every hour or so, more frequently once it begins to melt. DO NOT leave it heating overnight or go for long periods of time unattended. One of two things can happen. It can burn, or, bubble up and overflow! As soon as the soap becomes liquid or soft enough to incorporate other ingredients, it is ready to do so. Pour into a mold. Place molds in the freezer until the soap is firm enough to remove. Allow to dry.

Zip Lock Baggie Method:

Put finely grated soap in zip lock baggie. Add desired amount of either milk or water. Close baggie, pressing out as much air as possible. Place inside a second zip lock baggie, closing the same way as the first one. Place baggie in pot of SIMMERING water. Cover pan. Flip the baggie over occasionally. Gently knead the soap occasionally after it begins to melt. (Be careful, it's HOT!)
Once the soap reaches the desired consistency, carefully open the baggies and add any additional ingredients, mashing the soap around to incorporate. Snip the corner off the baggie and pipe the soap into the molds. Allow to harden. Remove from mold and dry.

Below are pictures of a rebatch I did today. I didn't like the design of the soap, so, decided to rebatch. You can see the original design in the first picture and will see the end product after rebatch a few pictures later.

This was a partial melt. I used the baggie method. It took a couple of hours for the soap to melt sufficiently because the original soap was at least a year old. I tried to rebatch without any liquid at all but it just wouldn't melt. I ended up adding approximately 1 Table spoon of heavy cream. I sprinkled some of the grated soap on the top of the rebatch and poked it down in a bit with my fingers, just to make it pretty.