Mother Earth News Blogs > Homesteading and Livestock

Homesteading and Livestock

Self-reliance and sustainability in the 21st century.

HOMEGROWN 101: How To Make Cold-Process Lye Soap

By Caroline, HOMEGROWN Flock-tender

At first glance, the art of soapmaking seems that it should be left to the chemists in a lab. By chemical definition, soap is the salt of a fatty acid, a triglyceride, which are three molecules of fatty acids attached to a single molecule of glycerol (glycerin). The bars we use for cleaning are simply animal or vegetable oils treated with a strong alkaline solution called lye (sodium hydroxide). Lye catalyzes the saponification process, the hydrolysis of fats into free fatty acids, which then combine with alkali to form crude soap. The liberated glycerin by-product is either left in the soap or collected, depending on which soap-making process is used.


(Photo by Type F)

Have I lost you yet? Because I’m a bit confused myself … Let’s break down the soap-making process and start scrubbin’ with homemade bars!

There are three basic batch processes that can be used in soap-making at home: the cold-process, the semi-boiled process and the fully boiled process. Each process is defined at the temperature in which the saponification reaction occurs.  Soap can also be made through the melt-and-pour and rebatching processes, where pre-made glycerin bars are melted down, colors and additives are added, and they are then molded

Here is a more detailed explanation of the different soapmaking techniques.  Let’s focus on the old-fashioned cold-pressed lye soaps:

Cold-Process Lye Soaps  


-More control over your ingredients to create unique recipes

-Soap is made from scratch


-Lye must be stored and handled safely

-Numerous materials and tools are required

-More time-intensive

-Lye to fat rations must be computed to ensure a mild product using saponification charts and lye calculators

This is the process most often used by artisan and hobby soapmakers.  The glycerine by-product is not extracted from the soap, and the reaction takes place over many days, even after the soap has been molded.  The remaining glycerin works as a moisturizing agent and keeps the soap soft.  Handmade cold-pressed soaps often use excess fat to consume the lye, which is known as superfatting.

While the reaction takes place mostly at room temperature, some heat is reaquired to get the saponification going.  The temperature of the batch is raised enough that the fat being used is completely melted, and is kept warm after mixing to ensure that the lye is completely used up. 

The cold-process requires exact measurements, using saponification charts to be sure that the finished soap product does not have excess hydroxide or unreacted fat.  Also, the use of the alkaline substance lye, can be potentially dangerous.  Lye has a pH of 13, meaning its can cause chemical burns and is highly corrosive.  Things to keep in mind when handling lye:

The first step in soap-making is to dissolve the lye in cold water – half of your water can be ice!  The oils are then melted.  Oils/fats can be animal or plant-derived.  Animal fats include lard or tallow, and vegetable oils include avocado, coconut, olive, castor, palm, peanut, soybean, sweet almond, jojoba, and kukui nut.  See this saponification table for more information. 

Once both the lye and oils have cooled to about 100-110 degrees Fahrenheit, and are no more than 10 degrees Fahrenheit apart in temperature, they are combined and stirred with a stick blender, until trace is achieved.  Trace refers to the point at which the fats and oils have mixed with the lye.  When trace is achieved, the soap appears thick and viscous.  Trace is affected by the heaviness of the fats used; heavier fats speed up the trace process.  Mixing method also affects trace.   When essential oils and additives are added depends on the viscosity of the additives - light, medium, or heavy trace  (essential oils, fragrances, herbs, oatmeal, and others are added at a light trace as the mixture starts to thicken).  Check out these recipes!


(Photo by Soap Queen)

The batch is then poured into molds and kept warm by wrapping the molds in blankets or towels, and left to continue saponification process for another 18 to 48 hours.  Milk soaps do not require insulation, which may cause the milk to scald.  The soaps may go through a gel phase before finally hardening.  Most soaps are ready to be cut and used after the insulation process, but it is safer to let the bars harden for another few weeks (2-6) before use. 

More information on cold-process lye soap making: