Procedure:

FIRST PUT ON YOUR PROTECTIVE GLOVES, APRON AND GOGGLES

1. It's advisable to do this first step outside. Stirring continuously, gradually add the sodium hydroxide carefully to 200cc (6.75 fl. oz.) of cold distilled water in one of the mixing bowls. Never add the water to the pellets! Avoid inhaling the fumes while doing this. The temperature of the solution will rise as you continue stirring, until all of the solid has dissolved. Let this solution cool to around 45° C (113°F).

2. Next weigh out the vegetable fat, olive oil and coconut oil into one of the mixing bowls. Warm the mixture gently by standing the bowl in hot water, stirring all the time, until it reaches 45° C (113°F). Don't let the temperature of this mixture rise above 50°C (122°F).

3. When the sodium hydroxide solution and the mixture of fat and oils are both at the required temperature, pour the former slowly into the latter and stir thoroughly for 15 minutes with a spoon.

4. Saponification doesn't happen instantly! With some vegetable oils it can take weeks to complete, but you should see something happening after 5 or 10 minutes; the mixture should start to thicken up. The process involves several stages, the first of which is called "tracing". If you let the mixture run off the back of the spoon into the mixing bowl, you should see that the soap that's formed floats on top; in other words, it leaves a "trace" on the surface. The time it takes to trace depends on the particular fats and oils you use; the lighter the fats or oils, the longer they'll take to trace.

Stir the mixture gently for 5 minutes or so, then leave it for 15 minutes, before stirring for a further 5 minutes... and so on, until after about an hour the mixture thickens to the consistency of custard.

All this may just seem like cooking, but there's some serious chemistry going on here. As saponification proceeds, the oils, fat and caustic soda react together, heat is generated, water evaporates and the soap begins to harden.

5. If you want fragrant soap, stir in the essential oil or lemon juice at this stage.

6. Pour your traced mixture into the plastic molds and put them in a warm, dry place.

7. Clean up your work area immediately and thoroughly wash all of the utensils you've used in hot soapy water.

8. After a week or so, your soap should be hard enough to remove it from the molds. Put your rubber gloves on to do this! At this stage, it will still contain some unreacted sodium hydroxide. To make it safe to use, stand it on newspaper in the open air at room temperature for two to three weeks, after which saponification should be complete. The longer you leave it, the better your soap will be.

UNDER NO CIRCUMSTANCES SHOULD YOU USE YOUR SOAP UNTIL YOU'RE SURE THAT IT CONTAINS NO UNREACTED CAUSTIC SODA! EVEN THEN, TO BE ON THE SAFE SIDE, ONLY USE IT TO WASH YOUR HANDS; NEVER USE IT TO WASH YOUR FACE.

You might like to try to and improve on the above recipe by using different oils and fats, or different reaction times. You could also add food colours to your soap to improve its appearance. This is the way practising synthetic chemists work, experimenting with different starting materials and reaction conditions, until they get the result they want. So experiment away and have fun!

Resources 

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