There are many ways to recycle those pesky soap scraps every household produces without fail, and all methods are very accessible thanks to Google (and of course right here on Instructables!).
I'm just adding another method which may work for some and not for others. But choice is good, hey!
What makes my method different from others, is that I use a meat grinder to chop up the soap scraps. This has some advantages regarding the whole process.
In a way this is a recycling instructable in most aspects, because if you use Granny's old unused meat grinder, you're in fact recycling it also. My molds are also recycled containers which had other uses before.
Step 1: The Theory
I've always tried the method of sticking old soap scraps together (1st pic), but without fail some scraps would come loose as soon as I start using it.
I realized that the problem is probably not that the pieces are not sticking together strong enough, but that they are too big. And while you can't really change the adhesive force, you can change the size of the scraps.
And following that line of thought, the smaller the pieces are, the better they will stick together. So is it logical to aim for the absolute smallest pieces possible.
Enter the meat grinder (2nd pic). If everything works as it should, it is an incredible fast and easy way of reducing soap scraps to a dusty heap of small granules.
Further on in the process the fine soap granules also mean that you can just mix it with some cold water, and press it into your soap molds. No messy heating up even necessary. If everything goes smoothly, your soap recycling will literally take minutes.
As always, however, there is at least one caveat: The soap scraps must be quite dried out, otherwise the soap won't crumble when being ground, but just be reformed into soap pellets the size of the holes of the grinder. Then the traditional method of using a kitchen grater or something similar would be better.
A note about the meat grinder: It should be one with a slicing cutter as shown in the last picture. Without the cutter, the grinder tends to clog up too easily.
Step 2: The Grind
First I group the soap scraps according to colour. If there are too few scraps of a colour, I combine colours. In the 1st picture you can see I had to combine pinks and purples, while there were enough whites to make up their own group.
Then it's time for the grinding (2nd pic). I put in a few scraps as shown, and monitor how the grinding goes. Small granules should start exiting soon at the front. If the handle gets too difficult to turn, you can do a few reverse swings, which will move the contents back and even it out, before changing to the forward direction again.
There will always be some pieces and granules left inside the grinder, so after each colour it must be opened up and the left overs added to the rest. A paint brush makes this easy (pic 3).
If everything went smoothly, you should now be left with some containers with coloured soap granules/dust (last pic).
Step 3: Mixing and Moulding
First spray your molds with cooking spray if there is any chance that the soap will stick to the sides and bottom (1st and 2nd pics). I don't have any experience with silicone molds, so don't know how they have to be treated. I just used some plastic tops which came with some cookies we bought. If you do not use spray, the soap may stick to the inside of your container as shown on the left in the 3rd picture.
In a separate vessel, add cold water sparingly while stirring (4th pic) until you have a pasty consistency (2nd last pic). This is probably also the right time to add any colouring or scent to the mix.
You can now immediately ladle the blob of soap into your molds. Press it down to remove any air gaps.
Let it stand for a day or two (last pic).
Step 4: Cut and Dried
After a day or two, the soap should be dry enough for the molds to be removed (1st pic). If necessary like in this case, it is easy to cut them into usable pieces (2nd pic), and leave them on a drying rack or plastic sheet to dry out further (last pic). Note that I put them on their ends in order to expose the largest areas to air.
How successful I have been remains to be seen because this is my first batch, but the consistency of the soap seems quite solid so I am hopeful to have a nice batch of recycled soap for future use.