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 Taking a look at temperature profiles for cold and hot process soap.
Figure 1
 Last month we looked at the possibility of using very high temperatures to shorten the time needed to completely saponify an oil. In extreme cases, this may
the solid soap remelts, a condition called “gel phase” by handcrafted soapmakers, and “neat soap” by commodity soap chemists. The temperature at which this phase transition happens depends on the oils used and the lye concentration.
Consider, for example, our standard four-oil blend: coconut oil 28%, palm oil 28%, olive oil 39%, castor oil 5%. We can make soap from this oil using lye with a wide range of concentrations. A medium-water soap, typical of what many people use, might use lye with a concentration of 33% NaOH. A low-water soap, (often referred to as a “water discount”) might use lye with a concentration of 40% NaOH. The temperature profiles of these soaps depend on the starting temperature when the oils are mixed with the lye.
A temperature profile is a useful way to characterize a soap formula. Simply heat the oil to the starting temperature you wish to explore and, after mixing with your lye, measure the temperature periodically over the course of a couple of hours. An infrared thermometer is particularly convenient for this task, since you just point and click to take a reading. A stopwatch (perhaps on your smartphone) is convenient for measuring time.
You don't have to hit an exact starting temperature, and the lye need not be at the same temperature as the oil. For example, you might intend to start at 100°F,
happen in as little as 5 minutes. One might assume that “hot process” (HP) and “cold process” (CP) soaps differ in the temperatures used, but the designations have more to do with technique than temperature.
In the hot process, oils and lye are mixed in a container capable of applying heat to the mixture. The source of heat might be a crock pot, microwave oven, conventional oven, or hotplate. The soap batter is typically heated until it reaches gel phase and the saponification reaction is complete. Interestingly, it is possible to make HP soap at low temperature by using only gentle heat.
In the cold process, oils and lye may be heated, but no external heat is applied after they are mixed. The bulk of the saponification reaction happens in the mold. Also interestingly, it is possible to make a CP soap at high temperature just by heating the oil, as described last month. Here we will explore the relationship between time and temperature.
Typically when lye is mixed with oil, the mixture becomes gradually more viscous until it reaches trace. The batter then continues to thicken until it no longer flows. At this point it is essentially a solid. But if conditions are right,
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