Heatless curling is the safest option when you do not know whether a synthetic wig is heat-resistant. The tradeoff is time.

Roller set

Mist the wig lightly, wrap small sections around rollers, and let it dry fully. Smaller rollers create tighter curls.

Braid waves

For waves, braid damp sections and let them set overnight. This works best on longer wigs.

Pin-curl shape

Roll sections by hand, pin them flat, and let them dry. This gives softer vintage bends.

Bottom line

To curl synthetic hair without heat, use moisture, tension, and time. Do not remove rollers until the fiber is fully dry.