“as if burned into lime.” This is a powerful simile in the text because the limestone that was quite common in Palestine was quarried into building stones of various sizes and used to build many things from small buildings to large Temples. But the limestone was also burned into lime and used to fertilize the fields. The analogy is that the enemy that might seem so large and imposing, like a huge building, will be burned into nothing just like limestone is burned into lime. But if the fire is too hot, some 900 degrees, even the lime decomposes.
“like thorns.” The thorns could seem like an impenetrable barrier and/or be a serious trouble, but they could be cut down and burned, and be no more. Some people think this section of Isaiah, especially Isaiah 33:14, refers to people burning forever in the Lake of Fire, but both the context and the wording of the text militates against that. For example, here in Isaiah 33:12, God is speaking of things that burn up, not things that burn forever. For example, limestone burns down to lime, but even lime itself decomposes at about 900 degrees. And thorns burn up completely in a fire. In the context, God is speaking of the destruction of His enemies and that they will come to nothing. In the future God’s enemies will not burn forever but like the thorns will be burned up in the Lake of Fire. [For more on the death of the wicked, see Appendix 5, Annihilation in the Lake of Fire”].