We followed this tutorial, based on this recipe for transparent soap. We were slightly short of castor oil (since 250 ml oil does not weigh 250 g...), but we substituted olive oil for the missing part. This German soap calculator was handy to get the substitution amount right.
Some equipment and ingredients. For alcohol, we used a bottle of Stroh 80 rum. This gives our soap a hint of red-brown color and a nice rummy smell as well, at least during the soap-making. On the right, some sand cake molds, pudding cups, and a plastic heart-shaped bowl, to be used as soap molds.
We heated the oils and dissolved the lye in water. Here we pour the lye solution into the oil, then mix with a blender. This is where the chemistry happens, the lye and the fat react, producing soap and glycerine. The mixture quickly became opaque and quite thick. The thickened mixture was kept in our small oven on 80 C for one and a half hour.
After staying in the oven the soap was quite hard and dry. Here we add solvents - first alcohol and glycerine, then sugar solution. We had lots of small soap pieces floating around, but after sitting in the oven again, the solution cleared up miraculously.
Time to add pigments and fragrance oils! For color we used food colorants and some of my fluorescent pink pigment, dissolved in alcohol. For fragrance, we had rose geranium and grape fruit essential oils. The grape fruit smelled very nice, but was much weaker than the rose oil in the finished soap.
Then the soap was poured into forms, and put in the freezer to harden. Ice cube trays from IKEA gave nice little soaps.
Some of the finished soaps! We were apparently a bit heavy-handed with the food colorants, and some of the soaps are really too dark to be transparent. The green and the fluorescent pink (see top of post) were just perfect.