astillar

To splinter — break wood, glass, or a similar material into small, sharp fragments (astillas), whether accidentally (la tabla se astilló al caer) or on purpose to make firewood or kindling. The reflexive astillarse is by far the more common form, describing something breaking apart into splinters on its own.

• La rama se astilló al partirse en dos. — The branch splintered when it broke in two.
• Astillaron los troncos para encender la chimenea. — They split the logs into kindling to light the fireplace.
• Cuidado, la madera vieja se astilla fácilmente. — Careful, old wood splinters easily.

Confusing astillar (to splinter, break into sharp fragments) with partir or romper (to break in general) — astillar specifically implies the jagged, sliver-like fragments typical of wood or similar brittle material.