Bruce the Chef

Soap is made by mixing lye and
water with fats or oil.  Through a complex chemical reaction lye which
is also called sodium hydroxide (a base) converts Fats or Oil (an acid)
to soap.  This process is called saponification.  When oil or
fats (acid) come into contact with the lye or sodium hydroxide (base) the
saponification process begins with the lye turning the oil/fat into a completely
new substance that we know as soap.  The process changes the two substances
into a thicker and more uniform solution.  The point at which the
solution begins to thicken is called a trace.  Any  oils/fats
or other ingredients added at this point will not be substantially converted
and will basically remain in the soap in their original form.  At
the trace stage the soap can be poured into molds where it will continue
to harden.  The chemical reaction will continue for about 3 weeks
at which time the soap is hard and ready to use.

The process that the home soapmaker
will use is referred to as "cold process soapmaking".  Even though
it is called a cold process, heat is required for the chain reaction to
take place.  This heat is provided by the chain reaction of the water
and the lye and their incorporation into the oil or fat.  (You will
notice that when the lye is poured into the water it heats up immediately)
Stirring the oil/fat/lye mixture helps this process to continue and to
be uniform.