Antonyms for "boils away" would be related to the opposite of evaporation or heating. Some examples could include condenses, solidifies, freezes, cools, or solidifies. These terms indicate a process where the substance becomes more compact or dense, rather than dissipating or becoming less substantial. For instance, when milk boils away, it turns into a condensed or thickened form, while if it were cooled, it may ultimately solidify as a result of the change in temperature. Therefore, antonyms for "boils away" might convey a sense of preservation or retention rather than a reduction in mass or substance.