Heating the two phases

The two phases are heated separately and combined hot. Most first-batch failures are traceable to something in this step rather than to the emulsifying that follows.

Why heat at all

To melt the solid fats and waxes so the oil phase is genuinely one liquid, to dissolve components that will not dissolve cold, and to lower viscosity so the phases can be dispersed into each other with the energy you actually have.

An emulsion made from a partly melted oil phase will have solid particles in it that behave as defects from the first day.

Both phases, same temperature

This is the rule people break. The phases must be at the same temperature when combined, because adding a cooler phase to a hotter one causes immediate local solidification of fats and sets up an emulsion with structure defects built in.

Heat them in separate vessels, check both with a thermometer rather than by eye, and wait for the laggard.

How hot and how long

Hot enough to melt the highest-melting component with a margin, and held there long enough for everything to dissolve. Longer than necessary costs you: heat degrades actives and accelerates oxidation of the oil phase, so the aim is the shortest hold that does the job.

Water will evaporate during a long hold, which changes the percentage composition of the batch. Cover the vessel, or weigh at the end and correct back with water before combining.

What must not be there yet

Anything heat-sensitive. Most preservatives, most actives, fragrance, and many extracts belong in the cool-down rather than in a hot phase.

Adding a preservative to a hot water phase is a common and expensive error: it can degrade the preservative and leave a product that reads as preserved on the label and is not preserved in the jar.

References