Incapable of being burnt or consumed by fire.

(un-, prefix1 affix 1.)

Incapable of being consumed by fire, etc.

That cannot be burnt or consumed by fire; incombustible.

(un-, prefix1 affix 1b.)

Incapable of being destroyed by fire.

= non-flammable, adj.

Not flammable, not likely to catch fire.

(un-, prefix1 affix 1b.)

= non-flammable, adj.