Temperatures, and why they matter
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The lever that decides how fast the batch moves and whether it gels.
Two temperatures to bring together
The lye solution, which starts very hot and has to come down.
The oils, which have to be melted and then usually cooled.
Bringing them close together is the conventional approach, and how close matters less than most guides suggest.
The conventional range
Both somewhere between roughly 35 and 50 degrees, within about ten degrees of each other.
Warm enough that hard oils do not solidify back out, cool enough that the batch does not race.
Use a thermometer rather than judgment. The difference between 40 and 55 is the difference between a workable batch and one that traces before the swirl.
Hot and cold, and what each gives
Warmer: faster trace, more likely to gel, better color development, and less working time.
Cooler: slower trace, less likely to gel, more working time for intricate designs, and a risk of false trace where hard oils solidify and look like trace while the batch has not emulsified.
Which is why a design-heavy batch is made cool and a plain one can be made warm.
Milk and sugar change it
Anything containing sugars, milk, honey, beer, purée, generates its own heat on top of the reaction.
Those batches are made cold: frozen liquid, cool oils, no insulation, sometimes a refrigerated mold.
Ignore that and the batch overheats, cracks, volcanoes or scorches to an orange, ammonia-smelling mass.
Which is worth knowing before promising somebody a goat milk bar, because it is the least forgiving batch in the craft.