Soap is a product of the saponification process, a chemical reaction between lye (strong alkali) and fat (derived from fatty acids). As chemical processes rely on exact proportions of materials and particular temperatures, soap making is more like a chemistry class than cooking.

For a garden-scented soap you can use oil in which aromatic plant parts have been immersed. For details of the maceration process and suggested plants see Bottling Plant Fragrances

Recipe ingredients for a soap made from olive oil (this is the “cold” soapmaking process):

• 1000 grams (not millilitres!) of olive oil (plain or scented with garden plants – see the link above)
• 135 grams caustic soda crystals (sold as 98% Sodium Hydroxide). If you want more moisturising soap, add 5% less
• 380 grams water (= 38% of oil weight)

Optional – for fragrance and texture:

• Essential oil 2-3% of carrier (olive) oil weight (20-30 grams)
• 20 ml of Sodium Lactate which makes the soap more moisturising and aids the removal of soap cakes from patterned molds
• Dried additives such as oatmeal, lavender buds, flower petals, spices, clay. Please note they will leave behind a residue in your bath and on the shower floor.

Equipment:

• Tempered glass bowl or measuring cup to dissolve lye
• Stainless steel pot to warm up the oil. It needs to be large enough to add the lye solution to. If you use a microwave, then large glass jug with a spout would work NEVER USE ALUMINIUM POTS OR UTENSILS
• Two food thermometers (I use the ones for cappuccino froth)
• Long-handled silicone, stainless steel or wooden spoon
• A handheld mixer, stick blender
• Apron, gloves, protective glasses and mask
• Soap molds: for individual soaps this could be either special molds or a silicone muffin tray. You can use empty 1 L milk cartons to fill them up halfway to make soap blocks. Such block will be soft enough to slice it after 36 hours into soap cakes to be cured individually.

Please pay attention to the important information below for safe and successful soap making.

Recipe/formula: As with every chemical reaction, it is important to use correct measurements (including liquids – BY WEIGHT, NOT VOLUME!). Soap making is not like cooking – a small change in proportions or type of oils used means the soap will be too caustic or too greasy and you cannot adjust the components afterwards.

Different oils need different amounts of lye. To find out how much caustic soda is needed, multiply the weight of the chosen oil by the saponification value*

For example, the saponification value for olive oil is 0.1353; multiply this value by 1000 grams of olive oil. 0.135 x 1000grams = 135 grams caustic soda.

/* saponification values of various oils can easily be found on the Internet, for example on Soap Making Resource website.

Wear protective clothes: lye (a solution of caustic soda) can burn. Cover yourself with an apron over a long-sleeved top, wear gloves and enclosed shoes. Protect your eyes and face with goggles and a face mask. If splashed on, rinse immediately with vinegar.

Be careful when preparing the lye solution: always ADD THE CAUSTIC SODA (sodium hydroxide) CRYSTALS TO THE WATER and NEVER THE OTHER WAY AROUND!.  Pour the crystals into cold water slowly and steadily, mixing with a long-handled spoon. DO NOT INHALE THE FUMES, they are very irritating for the eyes and lungs. The solution will heat up to near boiling temperature (to reduce the heat you can refrigerate water before using it to make lye).

Temperature is crucial: both oil and lye have to be between 40-50 degrees Celsius before you mix them. While the two main ingredients can be within 5 degrees C of each other, never combine them if the heat is over 60 degrees. Prepare the lye, then slowly warm up the oil. Watch the temperature of the lye solution so it doesn’t fall too low – it cannot be reheated if it becomes too cold.

Combining oils and lye: ADD THE LYE SOLUTION TO THE OIL, again in one steady slow stream. Start with stirring with a spoon, then switch to using a hand-held mixer. This will speed up the process (10-15 min compared to 50 min when mixing by hand).

When is it ready to pour? When the mixture begins to look creamy and thickens, lift the spoon/mixer and drag a thin thread of the mixture on the surface, making patterns (this is called “tracing”). If the thread stays there for a moment, before dissolving back, this is the moment soap begins to set. If you use essential oils, herbs or other additives, mix them in now and quickly pour the soap into moulds.

After about 36 hours remove the soap from moulds. Spread individual soaps on a flat basket or drying rack. The chemical reaction of saponification continues for up to 8 weeks – be patient, do not use the soap too early, as it will be too caustic and irritate your skin. I wait for 3 months to half a year (the thicker the soap cake, the longer it takes).

Important: turn over the pieces of soap every day, particularly in the first month, so they cure evenly inside.