HOMEGROWN 101: How To Make Cold-Process Lye Soap

Reader Contribution by Farm Aid And Homegrown.Org
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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 rebatchingprocesses, 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

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:

  1. Always add lye to cold water to dissolve (it may contain ice); and never the reverse – it is combustible when wet!
  2. Always use plastic or stainless steel containers with lye
  3. Keep your skin protected with sleeves and gloves
  4. Use powdered lye or granules
  5. Keep vinegar and milk handy in case of burns.  Eyes can be doused with milk and vinegar applied to skin in event of an accident
  6. Never inhale fumes
  7. Store lye in a cool, dry place with good ventilation and CLEAR labeling

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:

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