Impregnation through diffusion
The 2018 International Building Code clarifies that fire-retardant-treated wood (FRTW) can be manufactured without using the pressure process; but the wood must be impregnated with chemicals. In the 2018 code, Section 2303.2.2 has been updated to state "that the wood products must be impregnated with chemicals, even if that happens by means other than the pressure process."
Chapter 2 Definitions of the IBC, which defines FRTW as, "...a wood product that is impregnated with chemicals by a pressure process or other means during manufacture, and which exhibits reduced surface-burning characteristics and resists propagation of fire." Additionally, the product is subject to testing in accordance with ASTM E84 or UL723.