The temporary wooden scaffolding over which roman arches were constructed is called

The scaffolding was called centering

Basically, it was a wooden construction that was used as an imitation of the arch and and would hold it until it was built and strengthened after which it was removed. The centering was a key component since it would not be able to build it so perfectly or at all if it weren't for the centering. Such centering is not used anymore due to other contemporary technology.