Throughout my experiments with oil and varnish, I regularly used solutions of potassium hydroxide to clean glassware. This forms a highly soluble soap (potassium resinate) with pine rosin, and saponifies linseed oil even after it has oxidized. Interestingly, the resulting solution often formed a lather when stirred, and generally left the glassware absolutely spotless. Since I have now completed my exploration of wood and metal coatings, I have decided to investigate soaps in more detail; however, linseed oil is unsuited for this purpose. As a drying oil, it is fairly reactive, and has a tendency to go rancid when used in soap. Olive oil, on the other hand, is the quintessential non-drying oil, and has been used to make soap for thousands of years.
To begin, I first added 30mL of water to a beaker, to which I added 30g of potassium hydroxide flakes, very slowly and pausing when necessary to avoid boiling. Once the flakes had dissolved and the solution had cooled to room temperature, I then added 150g of olive oil, which formed a layer on top of the water. I then stirred the mixture vigorously to form an emulsion, so that the two immiscible components could react with each other. The mixture slowly thickened, and after four hours of intermittent stirring (every 30 minutes until homogeneous), the emulsion had become thick enough to support itself, and the oil no longer had a tendency to separate out. I then left the mixture to react on its own, and over the next day it slowly became a transparent gel. I left it for a further day and there was no change, indicating that the reaction was complete. I verified this with a strip of pH paper; it gave a result of around 9, which is ideal for hand soap.
At this point the soap was usable in paste form, and was neither greasy nor drying, further indicating that the oil and alkali were properly balanced. It produced a small amount of lather, had a neutral smell, and was otherwise entirely usable as soap. However, I found the idea of paste soap to be somewhat unsanitary, so I decided to dilute it to a liquid for use with a pump dispenser. I added water to the paste in small increments over the course of two days, which slowly caused it to expand and become softer; however, it still retained its paste consistency and would not readily flow or pour. Ultimately, it took three parts of water (by volume) for every one part of paste to reach the desired consistency. I then added it to an ordinary pump dispenser, where it behaved identically to commercial liquid soap.
Overall, this process was straightforward and inexpensive, if somewhat tedious. The simple ratio of ingredients (1:1:5 by weight) is easy to remember, and the soap produced is useful and non-irritating. I will continue using this soap daily for hand-washing, and will also experiment with it in other contexts to determine its versatility.
The use of a handheld blender shortens this process significantly, forming a thick emulsion in less than a minute; however, this causes the subsequent reaction to be rapid and exothermic, so care must be taken when preparing larger batches. I have also found it advantageous to add a small amount of turpentine to the oil prior to blending (0.1% by weight, or 3 drops per 100g of oil). This adds a sweet pine scent to the soap, appears to improve its lathering properties, and may have additional benefits as a preservative and antiseptic. This improved soap is excellent for hand-washing, but I have also experimented with it on dishes and laundry, and it handles both of these tasks well. This soap has greatly simplified my collection of household cleaning materials, and at roughly $6 per gallon, it is considerably less expensive than the products it has replaced.