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 but because your lye is at room temperature, you could wind up at 95°F after mixing. Just record the actual time and temperature. You will want to record the temperature frequently (about once per minute) at the beginning when the temperature is changing rapidly. You can then record less frequently as the soap slowly cools. It is not important to hit any particular time or temperature. Just record the actual time and temperature as close to one another as possible. If you notice the soap moving into gel phase, be sure to make a note of it.
Figure 1 shows a temperature profile for a medium-water soap. The horizontal line at 71°C (160°F) marks the division between gel phase and solid soap. The soaps labeled 40°C and 60°C never get warm enough to reach gel phase. They gradually warm as the exothermic saponification reaction proceeds. Eventually, the heat produced by the reaction is less than that lost to the surroundings, and the soaps slowly cool. The soap labeled 65°C gradually warms until it reaches gel phase. Then the reaction takes off, climbing well above the gel temperature. The two hot soaps start above the gel temperature and the saponification reaction proceeds very rapidly. They hit peak temperatures in a matter of minutes, and once saponification is complete, they cool more rapidly than the other soaps because no further heat is coming from the reaction.
Figure 2 shows a temperature profile for a low-water (water discounted) soap. With a lye concentration of 40%, this soap gels at 87°C (189°F), much higher than the medium- water soap does. While the soaps labeled 40°C, 60°C, and 65°C get hotter than the corresponding medium-water soaps, they do not reach this higher gel temperature. Only the two hottest soaps reach gel phase, and they do so very quickly.
The specific temperature for the gel phase transition
depends on the oils used as well as the lye concentration, but the general trend is that the gel temperature increases as the lye concentration increases. At the same time, soaps made with high-concentration lye get hotter than those with low-concentration lye, and they do so more quickly. Whether or not a soap gels depends on whether the soap reaches the gel temperature. The temperature profile also gives you a clue as to how quickly the saponification reaction is completed. When the temperature reaches its peak and begins to decline, it is because heat is being lost to the surroundings more rapidly than it is being produced by the reaction.
The temperature profile can also be used to diagnose problems with ingredients that are adversely affected by high temperatures. While measuring the profile on a batch, notice the temperature at which the adverse effect takes place. In subsequent batches, you can gradually decrease the starting temperature or decrease the lye concentration to prevent the soap from getting too hot.
The temperature profile is very easy to add to a batch record. It requires only a thermometer, a stopwatch, and a few extra minutes to write down the time and temperature. You can plot the data on graph paper or using a spreadsheet, or you can just look at the data without plotting it. While you may not bother to record it for every batch, it can be useful for diagnosing problems and solving them.
1. Data in this article were collected by students John Campbell, Andrew Basinger, Tyler Bowman, and Ron Davis under the supervision of Kevin Dunn. Parts of it are documented in Chapter 22 of Scientific Soapmaking (Kevin Dunn, Clavicula Press, 2010).
Figure 2

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