Many people buy bar soap in bulk, but the money you save often gets tossed when you stop using the scraps. Everyone does it. I got sick of throwing money in the trash and decided to start saving the scraps of bar soap. I melt them down once a quarter or so in a small saucepan and form a few new bars of 'mystery soap' that I then use just like normal soap. Beats ending up in a landfill. Here's how I did it.
Step 1: Collect the Soap
Save the soap scraps. I toss them into a drawer in my bathroom and wait until I have about 20 slivers of soap scraps. Personally, I use Ivory and Irish Spring, but you'll also find some Lava scraps in there too. Your preferences may vary, and you may want to separate them out if you're looking for consistency in the bars you make. Me, I don't really care.
Step 2: Break the Soap Slivers Into Small Chunks, Add Water
Break them into small pieces, about the size of a quarter, and then add about enough water to be visible at the level that the soap slivers are at. Turn on the burner to medium heat.
Step 3: Boil, Gently
Avoid boilovers, but simmer for a few minutes. The goal here is to get the water and the soap hot enough that they begin to stick together. They will, trust me. Stir often.
Step 4: Pour Into a Steel Colander
I put a frying pan underneath the colander so that the soap dribblings are captured and not wasted. Use a wooden spoon to push the soap mess around the colander so that the majority of the excess water is drained. You're trying to get a molten soapy glob, basically.
Step 5: Grease Two Small Glass Bowls
This makes it much easier to extract the soap 'bars' afterwards. Continue to let the soap mess cool and drain for a few minutes.
Step 6: Form Bars in the Bowls
Using a spoon, gather half the warm soap matter from the colander and stuff into a bowl. Repeat with other greased bowls as needed. I dump the captured soap dribblings from the frying pan into one of the bowls that needs more soap matter - this one usually ends up being the 'fluffiest' soap. Clean up (this part takes the longest, in my experience), and place in the refrigerator overnight.
Step 7: Liberate the Soap From Bowls
Use a steak knife - slip it between the cooled soap matter and the glass bowl - it should pop right out with a little twisting. Use the recycled soap immediately or store it for the next time you (or mankind) runs out of soap.