I think it has to do with consistency of tense in the sentence.
Verbs ending in "ing" generally tend to be part of the continuous tense. So in the option you selected, the sentence reads, "forming..., and airing and drying, ... to lay them.." Because of the change in tense between "forming" and "to lay them", the sentence makes it sound like you are forming the mud or clay into bricks IN ORDER to lay them in the wall. That is not actually the case and forming the bricks and then laying them in the wall are just progressive steps in the process of building solid walls.
So it is important to keep the tense consistent when describing a process. Saying "the most consistent method is to form bricks, and after some drying and airing, to lay them into the wall" or something like that, conveys the steps much better.
Does that make sense?
